This page contains a description of all fields in the snapshots, group catalogs, merger trees, and supplementary data sets.
There are 136 snapshots stored for every run. These include all particles/cells in the whole volume. The full snapshot listings, spacings and redshifts can be found in the API. A partial listing is provided in the following table:
Snap  Scale factor  Redshift 

0  0.020932  46.773 
32  0.090937  9.9966 
45  0.14264  6.0108 
49  0.16678  4.9959 
54  0.19968  4.0079 
60  0.24949  3.0081 
68  0.33311  2.002 
85  0.50068  0.9973 
103  0.66531  0.50305 
135  1  0 
Every snapshot is stored in a series of "chunks", i.e. more manageable, smallersize files. The number of chunks per snapshots is different for the different runs, and is:
Run  Alt. Name  Total NumPart (DM)  Chunks per Snapshot  Chunks per Groupcat  Avg Snapshot Size  Avg Groupcat Size  Total Data Volume 

L75n455FP  Illustris3  94,196,375  32  2  22 GB  100 MB  3 TB 
L75n455DM  Illustris3Dark  94,196,375  8  2  3.2 GB  50 MB  0.4 TB 
L75n910FP  Illustris2  753,571,000  256  4  176 GB  500 MB  24 TB 
L75n910DM  Illustris2Dark  753,571,000  32  4  26 GB  320 MB  3.5 TB 
L75n1820FP  Illustris1  6,028,568,000  512  8  1.5 TB  3.6 GB  204 TB 
L75n1820DM  Illustris1Dark  6,028,568,000  128  8  203 GB  4 GB  28 TB 
Note that the snapshot data is not organized according to spatial position. Rather, particles within the snapshot files are sorted according to their group/subgroup memberships, according to the FoF or Subfind algorithms. Within each particle type, the sort order is: GroupNumber, SubgroupNumber, BindingEnergy, where particles belonging to the group but not to any of its subgroups ("fuzz") are included after the last subgroup. The following figure provides a schematic view of the particle organization within a snapshot, for one particle type. Note that the truncation of a snapshot in chunks is arbitrary, thus halos may happen to be stored across multiple, subsequent chunks. Similarly, the different particle types of a halo can be stored in different sets of chunks.
Caption. Schematic diagram of the organization of particle/cell data within the snapshots for a single particle type. Within a type, particle order is determined by a global sort of the following fields in this order: FoF group number, Subfind subhalo number, binding energy, nearest FoF group number. This implies that FOF halos are contiguous, although they can span file chunks. Subfind subhalos are only contiguous within a single group, being separated between groups by an "inner fuzz" of all FOF particles not bound to any subhalo. Here $N_c$ indicates the number of file chunks, $n_F$ the number of FOF groups, and $N_{S,j}$ the number of subhalos in $j^{\rm th}$ FoF group.
Every HDF5 snapshot contains a "Header" and 5 additional "PartTypeX" groups, for the following particle types (the DM only runs have a single PartType1 group):
The most important fields of the header are given in the following table.
Header  

Field  Dimensions  Units  Description 
BoxSize  1  $ ckpc/h $  Spatial extent of the periodic box (in comoving units). 
MassTable  6  $ 10^{10} M_\odot/h $  Masses of particle types which have a constant mass (only DM). 
NumPart_ThisFile  6    Number of particles (of each type) included in this (sub)file. 
NumPart_Total  6    Total number of particles (of each type) across all (sub)files of this snapshot, modulo $ 2^{32} $. 
NumPart_Total_HighWord  6    Total number of particles (of each type) across all (sub)files of this snapshot, divided by $ 2^{32} $ and rounded downwards. 
Omega0  1    The cosmological density parameter for matter. 
OmegaLambda  1    The cosmological density parameter for the cosmological constant. 
Redshift  1    The redshift corresponding to the current snapshot. 
Time  1    The scale factor a (=1/(1+z)) corresponding to the current snapshot. 
NumFilesPerSnapshot  1    Number of file chunks per snapshot. 
The complete snapshot field listings, including dimensions, units and descriptions, are given for all the particles types in the following large table.
PartType0 (gas)  

Field  Dimensions  Units  Description 
Coordinates  N,3  $ ckpc/h $  Spatial position within the periodic box of size 75000 ckpc/h. Comoving coordinate. 
Density  N  $ (10^{10} M_\odot/h) / (ckpc/h)^3 $  Comoving mass density of cell (calculated as mass/volume). 
ElectronAbundance  N    Fractional electron number density with respect to the total hydrogen number density, so $n_e = \rm{ElectronAbundance} * n_H$ where $n_H = X_H * \rho / m_p$. Use with caution for starforming gas (see comment below for NeutralHydrogenAbundance). 
GFM_AGNRadiation  N  $ erg / s / cm^{2} * (4\pi) $  Bolometric intensity (physical units) at the position of this cell arising from the radiation fields of nearby AGN. One should divide by $4\pi$ to obtain the flux at this location, in the sense of $F = L / (4 \pi R^2)$. 
GFM_CoolingRate  N  $ erg\,\,cm^{3} / s $  The instantaneous net cooling rate experienced by this gas cell, in cgs units (e.g. $ \Lambda_{\rm net} / n_H^2 $). 
GFM_Metallicity  N    The ratio $ M_Z / M_{total} $ where $ M_Z $ is the total mass all metal elements (above He). Is NOT in solar units. To convert to solar metallicity, divide by 0.0127 (the primordial solar metallicity). 
GFM_WindDMVelDisp  N  $ km/s $  Equal to SubfindVelDisp. 
InternalEnergy  N  $ (km/s)^2 $  Internal (thermal) energy per unit mass for this gas cell. 
Masses  N  $ 10^{10} M_\odot/h $  Gas mass in this cell. Refinement/derefinement attempts to keep this value within a factor of two of the targetGasMass for every cell. 
NeutralHydrogenAbundance  N    Fraction of the hydrogen cell mass (or density) in neutral hydrogen, so $ n_{H_0} = \rm{NeutralHydrogenAbundance} * n_H $. (So note that $n_{H^+} = n_H  n_{H_0}$). Use with caution for starforming gas, as the calculation is based on the 'effective' temperature of the equation of state, which is not a physical temperature. 
NumTracers  N    The number of child tracers residing within this gas cell. 
ParticleIDs  N    The unique ID (uint64) of this gas cell. Constant for the duration of the simulation. May cease to exist (as gas) in a future snapshot due to conversion into a star/wind particle, accretion into a BH, or a derefinement event. 
Potential  N  $ (km/s)^2 / a $  Gravitational potential energy. 
SmoothingLength  N  $ ckpc/h $  Twice the maximum radius of all Delaunay tetrahedra that have this cell at a vertex in comoving units ($s_i$ from Springel et al. 2010). 
StarFormationRate  N  $ M_\odot / yr $  Instantaneous star formation rate of this gas cell. 
SubfindDensity  N  $ (10^{10} M_\odot/h) / (ckpc/h)^3 $  The local total comoving mass density, estimated using the standard cubicspline SPH kernel over all particles/cells within a radius of SubfindHsml. 
SubfindHsml  N  $ ckpc/h $  The comoving radius of the sphere centered on this cell enclosing the 64±1 nearest dark matter particles. 
SubfindVelDisp  N  $ km/s $  The 3D velocity dispersion of all dark matter particles within a radius of SubfindHsml of this cell. 
Velocities  N,3  $ km\sqrt{a}/s $  Spatial velocity. The peculiar velocity is obtained by multiplying this value by $ \sqrt{a} $. 
Volume  N  $ (ckpc/h)^3 $  Comoving volume of the Voronoi gas cell. 
PartType1 (dm)  
Field  Dimensions  Units  Description 
Coordinates  N,3  $ ckpc/h $  Spatial position within the periodic box of size 75000 ckpc/h. Comoving coordinate. 
ParticleIDs  N    The unique ID (uint64) of this DM particle. Constant for the duration of the simulation. 
Potential  N  $ (km/s)^2 / a $  Gravitational potential energy (note: not available in DMonly runs). 
SubfindDensity  N  $ (10^{10} M_\odot/h) / (ckpc/h)^3 $  The local total comoving mass density, estimated using the standard cubicspline SPH kernel over all particles/cells within a radius of SubfindHsml. 
SubfindHsml  N  $ ckpc/h $  The comoving radius of the sphere centered on this particle enclosing the 64±1 nearest dark matter particles. 
SubfindVelDisp  N  $ km/s $  The 3D velocity dispersion of all dark matter particles within a radius of SubfindHsml of this cell. 
Velocities  N,3  $ km\sqrt{a}/s $  Spatial velocity. The peculiar velocity is obtained by multiplying this value by $ \sqrt{a} $. 
PartType3 (tracers)  
Field  Dimensions  Units  Description 
FluidQuantities  N,13  Various  Thirteen auxiliary quantities stored for each tracer with differing significance. See Tracer Quantities below. 
ParentID  N    The unique ID (uint64) of the parent of this tracer. Could be a gas cell, star, wind phase cell, or BH. 
TracerID  N    The unique ID (uint64) of this tracer. Constant for the duration of the simulation. 
PartType4 (stars / wind particles)  
Field  Dimensions  Units  Description 
Coordinates  N,3  $ ckpc/h $  Spatial position within the periodic box of size 75000 ckpc/h. Comoving coordinate. 
GFM_InitialMass  N  $ 10^{10} M_\odot/h $  Mass of this star particle when it was formed (will subsequently decrease due to stellar evolution). 
GFM_Metallicity  N    See entry under PartType0. Inherited from the gas cell spawning/converted into this star, at the time of birth. 
GFM_StellarFormationTime  N    The exact time (given as the scalefactor) when this star was formed. Note: The only differentiation between a real star (>0) and a wind phase gas cell (<=0) is the sign of this quantity. 
GFM_StellarPhotometrics  N,8  $ mag $  Stellar magnitudes in eight bands: U, B, V, K, g, r, i, z. In detail, these are: Buser's X filter, where X=U,B3,V (Vega magnitudes), then IR K filter + Palomar 200 IR detectors + atmosphere.57 (Vega), then SDSS Camera X Response Function, airmass = 1.3 (June 2001), where X=g,r,i,z (AB magnitudes). They can be found in the filters.log file in the BC03 package. The details on the four SDSS filters can be found in Stoughton et al. 2002, section 3.2.1. 
Masses  N  $ 10^{10} M_\odot/h $  Mass of this star or wind phase cell. 
NumTracers  N    Number of child tracers belonging to this star/wind phase cell. 
ParticleIDs  N    The unique ID (uint64) of this star/wind cell. Constant for the duration of the simulation. 
Potential  N  $ (km/s)^2 / a $  Gravitational potential energy. 
SubfindDensity  N  $ (10^{10} M_\odot/h) / (ckpc/h)^3 $  The local total comoving mass density, estimated using the standard cubicspline SPH kernel over all particles/cells within a radius of SubfindHsml. 
SubfindHsml  N  $ ckpc/h $  The comoving radius of the sphere centered on this star particle enclosing the 64±1 nearest dark matter particles. 
SubfindVelDisp  N  $ km/s $  The 3D velocity dispersion of all dark matter particles within a radius of SubfindHsml of this cell. 
Velocities  N,3  $ km\sqrt{a}/s $  Spatial velocity. The peculiar velocity is obtained by multiplying this value by $ \sqrt{a} $. 
PartType5 (black holes)  
Field  Dimensions  Units  Description 
BH_CumEgyInjection_QM  N  $ 10^{10} M_\odot/h (ckpc/h)^2 / (0.978Gyr/h)^2$  Cumulative amount of thermal AGN feedback energy injected into surrounding gas in the quasar mode, total over the entire lifetime of this blackhole. 
BH_CumMassGrowth_QM  N  $ (10^{10} M_\odot/h) $  Cumulative mass accreted onto the BH in the quasar mode, total over the entire lifetime of this blackhole. 
BH_Density  N  $ (10^{10} M_\odot/h) / (ckpc/h)^3 $  Local comoving gas density averaged over the nearest neighbors of the BH. 
BH_Hsml  N  $ ckpc/h $  The comoving radius of the sphere enclosing the 64 nearestneighbor gas cells around the BH. 
BH_Mass  N  $ 10^{10} M_\odot/h $  Actual mass of the BH; does not include gas reservoir. Monotonically increases with time according to the accretion prescription, starting from the seed mass. 
BH_Mass_bubbles  N  $ 10^{10} M_\odot/h $  Accreted mass in current duty cycle for AGN "radio mode" bubble feedback. When this value reaches a critical fraction of BH_Mass_ini, the bubble energy is released. 
BH_Mass_ini  N  $ 10^{10} M_\odot/h $  BH mass at the start of the current duty cycle for AGN "radio mode" feedback; reset after each duty cycle. See BH_Mass_bubbles. 
BH_Mdot  N  $(10^{10} M_\odot/h) / (0.978Gyr/h)$  The mass accretion rate onto the black hole, instantaneous. 
BH_Pressure  N  $ (10^{10} M_\odot/h) / (ckpc/h) / (0.978Gyr/h)^2 $  Physical gas pressure (in comoving units) near the BH, defined as $ (\gamma1) \rho u $, where $ \rho $ is the local comoving gas density (BH_Density, as above) $u$ is BH_U (defined below). If this pressure is lower than the reference gas pressure, $ P_{ref} $, the BH accretion rate is lowered by $ (P_{ext}/P_{ref})^2 $. 
BH_Progs  N    Total number of BHs that have merged into this BH. 
BH_U  N  $ (km/s)^2 $  Thermal energy per unit mass in quasarheated bubbles near the BH. Used to define the BH_Pressure. Not to be confused with the "radio mode" bubbles injected via the unified feedback model. 
Coordinates  N,3  $ ckpc/h $  Spatial position within the periodic box of size 75000 ckpc/h. Comoving coordinate. 
HostHaloMass  N  $ 10^{10} M_\odot/h $  Mass of FoF group that hosts the BH. 
Masses  N  $ 10^{10} M_\odot/h $  Total mass of the black hole particle. Includes the gas reservoir from which accretion is tracked onto the actual BH mass (see BH_Mass). 
NumTracers  N    The number of child tracers residing within this BH. 
ParticleIDs  N    The unique ID (uint64) of this black hole. Constant for the duration of the simulation. May cease to exist in a future snapshot due to a BH merger. 
Potential  N  $ (km/s)^2 / a $  Gravitational potential at the location of the BH. 
SubfindDensity  N  $ (10^{10} M_\odot/h) / (ckpc/h)^3 $  The local total comoving mass density, estimated using the standard cubicspline SPH kernel over all particles/cells within a radius of SubfindHsml. 
SubfindHsml  N  $ ckpc/h $  The comoving radius of the sphere centered on this blackhole particle enclosing the 64±1 nearest dark matter particles. 
SubfindVelDisp  N  $ km/s $  The 3D velocity dispersion of all dark matter particles within a radius of SubfindHsml of this cell. 
Velocities  N,3  $ km\sqrt{a}/s $  Spatial velocity. The peculiar velocity is obtained by multiplying this value by $ \sqrt{a} $. 
The general unit system is ${\rm kpc}/h$ for lengths, $10^{10} {\rm M}_\odot/h$ for masses, ${\rm km/s}$ for velocities. The frequently occurring $(10^{10} {\rm M}_\odot/h) / (0.978 {\rm Gyr}/h)$ represents massovertime in this unit system, and multiplying by 10.22 converts to ${\rm M}_\odot/{\rm yr}$. Comoving quantities can be converted in the corresponding physical ones by multiplying for the appropriate power of the scale factor $a$. For instance, to convert a length in physical units it is sufficient to multiply it by $a$, volumes need a factor $a^3$, densities $a^{3}$ and so on. Note that at redshift $z=0$ the scale factor is $a=1$, so that the numerical values of comoving quantities are the same as their physical counterparts.
Each Monte Carlo tracer particle stores 13 auxiliary values. These are updated every timestep where the tracer parent is active. Many are reset to zero immediately after they are written out to a snapshot, such that their recording duration is precisely the time interval between two successive snapshots. Some are only relevant when the tracer resides within a parent of a specific particle type (e.g. gas or star). The following table describes these fields. Also note that tracers are exchanged (and can therefore change their parents) in the following ways:
Number  Name  Reset Each Snapshot?  Units  Description 

0  TMax  Y  Kelvin  The maximum past temperature of the parent gas cell, back to the previous snapshot. Only updated when parent is a gas cell. 
1  TMax_Time  Y    Scalefactor of the above TMax event. Only updated when parent is a gas cell. 
2  TMax_Time_Rho  Y  $ (10^{10} M_\odot / h) / (ckpc/h)^3 $  Density of the parent gas cell when the most recent TMax was recorded. Only updated when parent is a gas cell. 
3  RhoMax  Y  $ (10^{10} M_\odot / h) / (ckpc/h)^3 $  Maximum past density of the parent gas cell, back to the previous snapshot. Only updated when parent is a gas cell. 
4  RhoMax_Time  Y    Scalefactor of the above RhoMax event. Only updated when parent is a gas cell. 
5  MachMax  Y    Maximum past mach number of the parent gas cell, as set in the Riemann solver. Only updated when parent is a gas cell. 
6  EntMax  Y  $ P / (\rho / a^3)^\gamma $  Maximum past entropy of the parent gas cell, back to the previous snapshot. Only updated when parent is a gas cell. 
7  EntMax_Time  Y    Scale factor of the above EntMax event. Only updated when parent is a gas cell. 
8  Last_Star_Time  N    Scale factor, set only when this tracer exchanges from a (real) star particle to a gas cell,
or from a wind particle to a gas cell. The first case sets LST = a and the
second case sets LST = a corresponding to the scale factor when this last
exchange occured. If a tracer has never yet been in a particle of type 4, then
LST = 0 . Tracers currently in a (real) star particle have a constant
LST = 2 , and tracers currently in a wind particle have a constant
LST = 3 .

9  Wind_Counter  N  int32  Integer counter initialized to zero, increased by one each time this tracer is moved from a gas cell to a wind particle. 
10  Exchange_Counter  N  int32  Integer counter initialized to zero, increased by one each time this tracer is exchanged, regardless of parent type. 
11  Exchange_Distance  N  $ ckpc/h $  Cumulative sum of the spatial distance over which this tracer has moved due to Monte Carlo exchange between gas cells. In particular, the sum of the parent gas cell radii when either the originating parent or destination parent is of gas type. 
12  Exchange_Distance_Error  N  $ ckpc/h $  Cumulative sum of $r_{\rm cell} \times ( \sqrt{N_{\rm exch}  \sqrt{N_{\rm exch}1} )$, when either the originating or destination parent is of gas type. 
Four separate "subbox" cutouts exist, for each full physics run. These are spatial cutouts of fixed comoving size and fixed comoving coordinates. They are output at each highest timestep, that is, their time resolution is significantly better than that of the main snapshots. This may be useful for some types of analysis or particular science questions, or for making movies. Two notes of caution: first, the time spacing of the subboxes is not uniform in scale factor or redshift, but scales with the time integration hierarchy of the simulation, and is thus variable, with some discrete factor of two jumps at several points during the simulations. Second, the subboxes, unlike the full box, are not periodic.
Run  Number of Subbox Snapshots 
Chunks per Snap 
Time Resolution (at z=6) 
(at z=2)  (at z=0) 

455_FP  1426  1  ~7 Myr  ~12 Myr  ~33 Myr 
910_FP  2265  16  ~4 Myr  ~6 Myr  ~17 Myr 
1820_FP  3976  512  ~2 Myr  ~3 Myr  ~8 Myr 
The four subboxes sample four different areas of the large box, roughly described by the environment column in the following table. The particle fields are all identical to the main snapshots. However, the ordering differs. In particular, particles/cells in the subboxes are not ordered according to their group membership, as no group catalogs are available for these cutouts.
Subbox #  Environment  $\Omega_m^{sub}$  XYZ Center  BoxSize  Volume Fraction 

0  Crowded, including a $5 \times 10^{13} {\rm M}_\odot$ halo  1.47  (9000, 17000, 63000)  7.5 ${\rm cMpc}/h$  0.1% 
1  Less crowded, including several $\gt 10^{12} {\rm M}_\odot$ halos  0.16  (43100, 53600, 60800)  8.0 ${\rm cMpc}/h$  0.12% 
2  Less crowded, including several $\gt 10^{12} {\rm M}_\odot$ halos  0.29  (37000, 43500, 67500)  5.0 ${\rm cMpc}/h$  0.03% 
3  Least crowded, including several $\sim 10^{12} {\rm M}_\odot$ halos  0.25  (64500, 51500, 39500)  5.0 ${\rm cMpc}/h$  0.03% 
There is one group catalog associated with each snapshot, which includes both FoF and Subfind objects. The group files are split into a small number of subfiles, just as with the raw snapshots. Every HDF5 group catalog contains the following groups: Header, Group, Subhalo, Offsets. The IDs of the members of each group/subgroup are not stored in the group catalog files. Rather, particles/cells in the snapshot files are ordered according to group membership. Each group contains its total length, allowing IDs and all other fields of member particles/cells to be accessed using an offset table type approach. This applies to subhalos as well, e.g. the subhalos belonging to group 0 are listed first.
In order to reduce confusion, we adopt the following terminology when referring to different types of objects:
The Group fields are derived with a standard friendsoffriends (FoF) algorithm with linking length $b=0.2$. The FoF algorithm is run on the dark matter particles, and the other types (gas, stars, BHs) are attached to the same groups as their nearest DM particle. The fields for the FoF halo catalog are described in the following table (all fields are float32 unless otherwise specified):
Field  DataType  Dimensions  Units  Description 

GroupBHMass  float32  N  $ 10^{10} M_\odot / h $  Sum of the BH_Mass field of all blackholes (type 5) in this group. 
GroupBHMdot  float32  N  $(10^{10} M_\odot/h) / (0.978Gyr/h)$  Sum of the BH_Mdot field of all blackholes (type 5) in this group. 
GroupCM  float32  N,3  $ ckpc/h $  Center of mass of the group, computed as the sum of the mass weighted relative coordinates of all particles/cells in the group, of all types. Comoving coordinate. (Available only for the Illustris3 run) 
GroupFirstSub  int32  N    Index into the Subhalo table of the first (i.e. central/primary) Subfind subhalo within this FoF group. The subhalos of a group are ordered in descending total number of bound particles/cells. The first/central subhalo is usually, but not always, the most massive. Note: This value is signed (or should be interpreted as signed)! In this case, a value of 1 indicates that this FoF group has no subhalos.

GroupGasMetallicity  float32  N    Massweighted average metallicity (Mz/Mtot, where Z = any element above He) of all gas cells in this FOF group. 
GroupLen  int32  N    Integer counter of the total number of particles/cells of all types in this group. 
GroupLenType  int32  N,6    Integer counter of the total number of particles/cells, split into the six different types, in this group. Note: Wind phase cells are counted as stars (type 4) for GroupLenType. 
GroupMass  float32  N  $ 10^{10} M_\odot / h $  Sum of the individual masses of every particle/cell, of all types, in this group. 
GroupMassType  float32  N,6  $ 10^{10} M_\odot / h $  Sum of the individual masses of every particle/cell, split into the six different types, in this group. Note: Wind phase cells are counted as gas (type 0) for GroupMassType. 
GroupNsubs  int32  N    Count of the total number of Subfind groups within this FoF group. 
GroupPos  float32  N,3  $ ckpc/h $  Spatial position within the periodic box (of the particle with the minimum gravitational potential energy). Comoving coordinate. 
GroupSFR  float32  N  $ M_\odot / yr $  Sum of the individual star formation rates of all gas cells in this group. 
GroupStarMetallicity  float32  N    Massweighted average metallicity (Mz/Mtot, where Z = any element above He) of all star particles in this FOF group. 
GroupVel  float32  N,3  $ km/s/a $  Velocity of the group, computed as the sum of the mass weighted velocities of all particles/cells in this group, of all types. The peculiar velocity is obtained by multiplying this value by $1/a.$ 
GroupWindMass  float32  N  $ 10^{10} M_\odot / h $  Sum of the individual masses of all wind phase gas cells (type 4, BirthTime <= 0) in this group. 
Group_M_Crit200  float32  N  $ 10^{10} M_\odot / h $  Total Mass of this group enclosed in a sphere whose mean density is 200 times the critical density of the Universe, at the time the halo is considered. 
Group_M_Crit500  float32  N  $ 10^{10} M_\odot / h $  Total Mass of this group enclosed in a sphere whose mean density is 500 times the critical density of the Universe, at the time the halo is considered. 
Group_M_Mean200  float32  N  $ 10^{10} M_\odot / h $  Total Mass of this group enclosed in a sphere whose mean density is 200 times the mean density of the Universe, at the time the halo is considered. 
Group_M_TopHat200  float32  N  $ 10^{10} M_\odot / h $  Total Mass of this group enclosed in a sphere whose mean density is $\Delta_c$ times the critical density' of the Universe, at the time the halo is considered. $\Delta_c$ derives from the solution of the collapse of a spherical tophat perturbation (fitting formula from Bryan+ 1998). The subscript 200 can be ignored. 
Group_R_Crit200  float32  N  $ ckpc/h $  Comoving Radius of a sphere centered at the GroupPos of this Group whose mean density is 200 times the critical density of the Universe, at the time the halo is considered. 
Group_R_Crit500  float32  N  $ ckpc/h $  Comoving Radius of a sphere centered at the GroupPos of this Group whose mean density is 500 times the critical density of the Universe, at the time the halo is considered. 
Group_R_Mean200  float32  N  $ ckpc/h $  Comoving Radius of a sphere centered at the GroupPos of this Group whose mean density is 200 times the mean density of the Universe, at the time the halo is considered. 
Group_R_TopHat200  float32  N  $ ckpc/h $  Comoving Radius of a sphere centered at the GroupPos of this Group whose mean density is $\Delta_c$ times the critical density of the Universe, at the time the halo is considered. 
The Subhalo fields are derived with the Subfind algorithm, with modifications to add additional baryonic properties to each subhalo entry. Descriptions of all fields in this subhalo catalog are given in the following table. Note that for all mass calculations by type, wind phase cells are counted as gas.
Field  DataType  Dimensions  Units  Description 

SubhaloBHMass  float32  N  $ 10^{10} M_\odot/h $  Sum of the masses of all blackholes in this subhalo. 
SubhaloBHMdot  float32  N  $(10^{10} M_\odot/h) / (0.978Gyr/h)$  Sum of the instantaneous accretion rates $\dot{M}$ of all blackholes in this subhalo. 
SubhaloCM  float32  N,3  $ ckpc/h $  Comoving center of mass of the Subhalo, computed as the sum of the mass weighted relative coordinates of all particles/cells in the Subhalo, of all types. 
SubhaloGasMetallicity  float32  N    Massweighted average metallicity (Mz/Mtot, where Z = any element above He) of the gas cells bound to this Subhalo, but restricted to cells within twice the stellar half mass radius. 
SubhaloGasMetallicityHalfRad  float32  N    Same as SubhaloGasMetallicity, but restricted to cells within the stellar half mass radius. 
SubhaloGasMetallicityMaxRad  float32  N    Same as SubhaloGasMetallicity, but restricted to cells within the radius of $V_{max}$. 
SubhaloGasMetallicitySfr  float32  N    Massweighted average metallicity (Mz/Mtot, where Z = any element above He) of the gas cells bound to this Subhalo, but restricted to cells which are star forming. 
SubhaloGasMetallicitySfrWeighted  float32  N    Same as SubhaloGasMetallicitySfr, but weighted by the cell starformation rate rather than the cell mass. 
SubhaloGrNr  int32  N    Index into the Group table of the FOF host/parent of this Subhalo. 
SubhaloHalfmassRad  float32  N  $ ckpc/h $  Comoving radius containing half of the total mass (SubhaloMass) of this Subhalo. 
SubhaloHalfmassRadType  float32  N,6  $ ckpc/h $  Comoving radius containing half of the mass of this Subhalo split by Type (SubhaloMassType). 
SubhaloIDMostbound  int64  N    The ID of the particle with the smallest binding energy (could be any type). 
SubhaloLen  int32  N    Total number of member particle/cells in this Subhalo, of all types. 
SubhaloLenType  int32  N,6    Total number of member particle/cells in this Subhalo, separated by type. 
SubhaloMass  float32  N  $ 10^{10} M_\odot/h $  Total mass of all member particle/cells which are bound to this Subhalo, of all types. Particle/cells bound to subhaloes of this Subhalo are NOT accounted for. 
SubhaloMassInHalfRad  float32  N  $ 10^{10} M_\odot/h $  Sum of masses of all particles/cells within the stellar half mass radius. 
SubhaloMassInHalfRadType  float32  N,6  $ 10^{10} M_\odot/h $  Sum of masses of all particles/cells (split by type) within the stellar half mass radius. 
SubhaloMassInMaxRad  float32  N  $ 10^{10} M_\odot/h $  Sum of masses of all particles/cells within the radius of $V_{max}$. 
SubhaloMassInMaxRadType  float32  N,6  $ 10^{10} M_\odot/h $  Sum of masses of all particles/cells (split by type) within the radius of $V_{max}$. 
SubhaloMassInRad  float32  N  $ 10^{10} M_\odot/h $  Sum of masses of all particles/cells within twice the stellar half mass radius. 
SubhaloMassInRadType  float32  N,6  $ 10^{10} M_\odot/h $  Sum of masses of all particles/cells (split by type) within twice the stellar half mass radius. 
SubhaloMassType  float32  N,6  $ 10^{10} M_\odot/h $  Total mass of all member particle/cells which are bound to this Subhalo, separated by type. Particle/cells bound to subhaloes of this Subhalo are NOT accounted for. Note: Wind phase cells are counted as gas (type 0) for SubhaloMassType. 
SubhaloParent  int32  N    Index into the Subhalo table of the unique SUBF host/parent of this Subhalo. This index is local to the group (i.e. 2 indicates the third most massive subhalo of the parent halo of this subhalo, not the third most massive of the whole snapshot). The values are often zero for all subhalos of a group, indicating that there is no resolved hierarchical structure in that group, beyond the primary subhalo having as direct children all of the secondary subhalos. 
SubhaloPos  float32  N,3  $ ckpc/h $  Spatial position within the periodic box (of the particle with the minium gravitational potential energy). Comoving coordinate. 
SubhaloSFR  float32  N  $ M_\odot / yr $  Sum of the individual star formation rates of all gas cells in this subhalo. 
SubhaloSFRinHalfRad  float32  N  $ M_\odot / yr $  Same as SubhaloSFR, but restricted to cells within the stellar half mass radius. 
SubhaloSFRinMaxRad  float32  N  $ M_\odot / yr $  Same as SubhaloSFR, but restricted to cells within the radius of $V_{max}$. 
SubhaloSFRinRad  float32  N  $ M_\odot / yr $  Same as SubhaloSFR, but restricted to cells within twice the stellar half mass radius. 
SubhaloSpin  float32  N,3  $ (kpc/h) (km/s) $  Total spin per axis, computed for each as the mass weighted sum of the relative coordinate times relative velocity of all member particles/cells. 
SubhaloStarMetallicity  float32  N    Massweighted average metallicity (Mz/Mtot, where Z = any element above He) of the star particles bound to this Subhalo, but restricted to stars within twice the stellar half mass radius. 
SubhaloStarMetallicityHalfRad  float32  N    Same as SubhaloStarMetallicity, but restricted to stars within the stellar half mass radius. 
SubhaloStarMetallicityMaxRad  float32  N    Same as SubhaloStarMetallicity, but restricted to stars within the radius of $V_{max}$. 
SubhaloStellarPhotometrics  float32  N,8  $ mag $  Eight bands: U, B, V, K, g, r, i, z. Magnitudes based on the summedup luminosities of all the stellar particles of the group. For details on the bands, see snapshot table for stars. 
SubhaloStellarPhotometricsMassInRad  float32  N  $ 10^{10} M_\odot/h $  Sum of the mass of the member stellar particles, but restricted to stars within the radius SubhaloStellarPhotometricsRad. 
SubhaloStellarPhotometricsRad  float32  N  $ ckpc/h $  Radius at which the surface brightness profile (computed from all member stellar particles) drops below the limit of 20.7 mag arcsec$^{2}$ in the K band (in comoving units). 
SubhaloVel  float32  N,3  $ km/s $  Peculiar velocity of the group, computed as the sum of the mass weighted velocities of all particles/cells in this group, of all types. No unit conversion is needed. 
SubhaloVelDisp  float32  N  $ km/s $  Onedimensional velocity dispersion of all the member particles/cells (the 3D dispersion divided by $\sqrt{3}$). 
SubhaloVmax  float32  N  $ km/s $  Maximum value of the sphericallyaveraged rotation curve. 
SubhaloVmaxRad  float32  N  $ ckpc/h $  Comoving radius of rotation curve maximum (where $V_{max}$ is achieved). 
SubhaloWindMass  float32  N  $ 10^{10} M_\odot/h $  Sum of masses of all windphase cells in this subhalo (with Type==4 and BirthTime<=0). 
The following table describes the Header group of each file:
Field  Type  Description 

SimulationName  string  e.g. 'Illustris1' or 'Illustris2Dark' 
SnapshotNumber  int  snapshot number (should be consistent with filename) 
Ngroups_ThisFile  int  Number of groups within this file chunk. 
Nsubgroups_ThisFile  int  Number of subgroups within this file chunk. 
Ngroups_Total  int  Total number of groups for this snapshot. 
Nsubgroups_Total  int  Total number of subgroups for this snapshot. 
NumFiles  int  Total number of file chunks the group catalog is split between. 
Num_ThisFile  int  Index of this file chunk (should be consistent with the filename). 
Time  float  Scalefactor of the snapshot corresponding to this group catalog. 
Redshift  float  Redshift of the snapshot corresponding to this group catalog. 
BoxSize  float  Sidelength of the periodic volume in code units. 
FileOffsets_Snap  [$6, N_{\rm c}$] int array  The offset table (by type) for the snapshot files, giving the first particle index in each snap file chunk. Determines which files(s) a given offset+length will cover. A twodimensional array, where the element $(i,j)$ equals the cumulative sum (i.e. offset) of particles of type $i$ in all snapshot file chunks prior to $j$. 
FileOffsets_Group  [$N_{\rm c}$] int array  The offset table for groups in the group catalog files. A onedimensional array, where the $i^{th}$ element equals the first group number in the $i^{th}$ groupcat file chunk. 
FileOffsets_Subhalo  [$N_{\rm c}$] int array  The offset table for subhalos in the group catalog files. A onedimensional array, where the $i^{th}$ element equals the first subgroup number in the $i^{th}$ groupcat file chunk. 
FileOffsets_SubLink  [$N_{\rm c}$] int array  The offset table for trees in the SubLink files. A onedimensional array, where the $i^{th}$ element equals the first tree number in the $i^{th}$ SubLink file chunk. 
The following table describes the fields in the Offsets group. Note that we simply store the various useful offsets here, they relate to all types of data files and not solely to the group catalogs.
Field  Dimensions  Description 

Group_SnapByType  Ngroups_Total,6  The offset table for a given group number (by type), into the snapshot files. That is, the global particle index (across all snap file chunks) of the first particle of this group. A twodimensional array, where the element $(i,j)$ equals the cumulative sum (i.e. offset) of particles of type $i$ in all groups prior to group number $j$. 
Group_FuzzByType  Ngroups_Total,6  Offset into the "outer fuzz" (at the end of each snapshot file) for this group. 
Subhalo_SnapByType  Nsubgroups_Total,6  The offset table for a given subhalo number (by type), into the snapshot files. That is, the global particle index (across all snap file chunks) of the first particle of this subhalo. A twodimensional array, where the element $(i,j)$ equals the cumulative sum (i.e. offset) of particles of type $i$ in all subhalos prior to subhalo number $j$. 
Subhalo_LHaloTreeFile  Nsubgroups_Total  The LHaloTree file number with the tree which contains this subhalo. 
Subhalo_LHaloTreeNum  Nsubgroups_Total  The number of the tree within the above file within which this subhalo is located (e.g. TreeX). 
Subhalo_LHaloTreeIndex  Nsubgroups_Total  The LHaloTree index within the above tree dataset at which this subhalo is located. 
Subhalo_SublinkRowNum  Nsubgroups_Total  The SubLink global index of the location of this subhalo. 
Subhalo_SublinkSubhaloID  Nsubgroups_Total  The Sublink ID of this subhalo. 
Subhalo_SublinkLastProgenitorID  Nsubgroups_Total  The SubLink ID of the last progenitor of this tree (all the subhalos contained in the tree rooted in this subhalo are the ones with IDs between SubhaloID and LastProgenitorID). 
Merger trees have been created for the various Illustris simulations using SubLink (RodriguezGomez+ 2015) and LHaloTree (Springel+ 2005). The LHaloTree are essentially identical to the primary trees of the Millennium simulations, Aquarius, and Phoenix, but in HDF5 format. In the population average sense the different merger trees give similar results. In more detail, the exact merger history or mass assembly history for any given halo may differ. For any given science goal, one type of tree may be more or less useful, and users are free to use whichever they prefer. These codes are all included in the Sussing Merger Trees comparison project (Srisawat+ 2013).
The following figure shows a schematic of the structure of both the SubLink and LHaloTree merger trees. It is not necessary to understand the complete details of the trees to practically use them. In particular, the only critical links are the 'descendant' (black), 'first progenitor' (green), and 'next progenitor' (red) associations. These are shown for all tree nodes in the diagram. For their exact definitions, see the tables below. Walking back in time following along the main (most massive) progenitor branch consists of following the first progenitor links until they end (value equals 1). Similarly, walking forward in time along the descendants branch consists of following the descendant links until they end (value equals 1), which typically occurs at $z=0$. The full progenitor history, and not just the main branch, requires following both the first and next progenitor links. In this way the user can identify all subhalos at a previous snapshot which have a common descendant. Examples of walking the tree are provided in the example scripts.
Caption. Schematic diagram of the merger tree structure for both SubLink and LHaloTree. Both algorithms connect subhalos across different snapshots in the simulation. Rows indicate discrete snapshots, with time increasing downwards towards redshift zero (the horizontal axis is arbitrary). Green circles represent subhalos (the nodes of the merger tree), while beige boxes indicate the grouping of the subhalos into their parent FoF groups. The most important links are for the descendant (black), first progenitor (green), and next progenitor (red), which are shown for all subhalos. The root descendant (purple), last progenitor (blue), and main leaf progenitor (orange) links exist only for the SubLink trees, and for simplicity these last three link types are shown only for subhalos 5, 7, and 19 (darker striped circles).
The number inside each circle from the figure is the unique ID (within the whole simulation) of the corresponding subhalo, which is assigned in a depthfirst fashion. Numbering also indicates the ondisk storage ordering for the SubLink trees, which adopt the approach of Lemson+ (2006). For example, the main progenitor branch (from 57 in the example) and the full progenitor tree (from 513 in the example) are both contiguous subsets of each merger tree field, whose location and size can be calculated using these links. The ordering within a single tree in the LHaloTree is not guaranteed to follow this scheme.
The 'root descendant' (purple), 'last progenitor' (blue), and 'main leaf progenitor' (orange) links exist only for the SubLink trees. For simplicity, these last three link types are shown only for nodes 5, 7, and 19 (darker striped circles). Using these links is optional, but allows efficient extraction of main progenitor branches, subtrees (i.e., the set containing a subhalo and "all" its progenitors), "forward" descendant branches, and other subsets of the tree. For their full definitions, see the following table.
Each subhalo spans a "subtree" consisting of the subhalo itself and all its progenitors. As an example, the subhalos belonging to the subtree of subhalo 5 are shown in darker green in the figure. Other subhalos not belonging to this subtree are shown in lighter green, and their links are indicated with dashed arrows. In the SubLink trees, the subtree of any subhalo can be extracted easily using the 'last progenitor' pointer. As shown in the figure, since subhalo 13 is the 'last progenitor' of subhalo 5, the subtree of subhalo 5 consists of all subhalos with IDs between 5 and 13. Similarly, the main progenitor branch of any subhalo can be retrieved efficiently using the 'main leaf progenitor' link.
Both SubLink and LHaloTree contain the links 'first subhalo in FoF group' (light brown dotted arrow) and 'next subhalo in FoF group' (dark brown dotted arrow), which connect subhalos that belong to the same FoF group. The FoF groups do not play a direct role in the construction of the merger tree. However, in SubLink, subhalos that belong to the same FoF group are also considered to be part of the same tree. As a result, two otherwise independent trees (based on the progenitor and descendant links) are considered to be the same tree if they are "connected" by a FoF group. This is exemplified in the figure by the FoF group containing subhalos 12, 16, and 20. This FoF group acts as a bridge between the left and right trees.
The SubLink algorithm constructs merger trees at the subhalo level. A unique descendant is assigned to each subhalo in three steps (see RodriguezGomez+ 2015). First, descendant candidates are identified for each subhalo as those subhalos in the following snapshot that have common particles with the subhalo in question. Second, each of the descendant candidates is given a score based on a merit function that takes into account the binding energy rank of each particle. Third, the unique descendant of the subhalo in question is the descendant candidate with the highest score. Sometimes the halo finder does not detect a small subhalo that is passing through a larger structure, because the density contrast is not high enough. {\sc SubLink} deals with this issue by allowing some subhalos to skip a snapshot when finding a descendant. Once all descendant connections have been made, the main progenitor of each subhalo is defined as the one with the "most massive history" behind it.
Note that this merger tree comes in two varieties: "darkmatter based" and "baryonic based". The darkmatter based tree is the fiducial choice, and has been publicly released as "SubLink". The baryonic based tree is called "SubLink_gal" and has not been publicly released for simplicity, although it is available upon request. While the two will largely give identical results, depending on scientific application, one might be more useful than the other.
The SubLink merger tree is one large data structure split across several sequential HDF5 files named
tree_extended.[fileNum].hdf5
, where [fileNum]
goes from e.g. 0 to 9 for the Illustris1 run. These
files store the data on a per tree basis, and therefore are completely independent from each other. More
specifically, any two subhalos that are connected by any of the pointers described in the SubLink table are
guaranteed to belong to the same tree, and, therefore, their data is found in the same file. The following
table lists the fields which are present in each file.
Field  DataType  Dimensions  Units  Description 

SubhaloID  int64  (N)    Unique identifier of this subhalo, assigned in a "depthfirst" fashion (Lemson & Springel 2006). This value is contiguous within a single tree. 
SubhaloIDRaw  int64  (N)    Unique identifier of this subhalo in raw format (= SnapNum*10^12 + SubfindID). 
LastProgenitorID  int64  (N)    The SubhaloID of the last progenitor of the tree rooted at this subhalo. Since the SubhaloIDs are assigned in a "depthfirst" fashion, all the subhalos contained in the tree rooted at this subhalo are the ones with SubhaloIDs between (and including) the SubhaloID and LastProgenitorID of this subhalo. For subhalos with no progenitors, LastProgenitorID == SubhaloID. 
MainLeafProgenitorID  int64  (N)    The SubhaloID of the last progenitor along the main branch, i.e. the earliest progenitor obtained by following the FirstProgenitorID pointer. For subhalos with no progenitors, MainLeafProgenitorID == SubhaloID. 
RootDescendantID  int64  (N)    The SubhaloID of the latest subhalo that can be reached by following the DescendantID link, i.e. the root of the tree to which this subhalo belongs. For subhalos with no descendants, RootDescendantID == SubhaloID. 
TreeID  int64  (N)    Unique identifier of the tree to which this subhalo belongs. 
SnapNum  int16  (N)    The snapshot in which this subhalo is found. 
FirstProgenitorID  int64  (N)    The SubhaloID of this subhalo's first progenitor. The first progenitor is the one with the "most massive history" behind it (following De Lucia & Blaizot 2007). For subhalos with no progenitors, FirstProgenitorID == 1. 
NextProgenitorID  int64  (N)    The SubhaloID of the subhalo with the next most massive history which shares the same descendant as this subhalo. If there are no more subhalos sharing the same descendant, NextProgenitorID == 1. 
DescendantID  int64  (N)    The SubhaloID of this subhalo's descendant. If this subhalo has no descendants, DescendantID == 1. 
FirstSubhaloInFOFGroupID  int64  (N)    The SubhaloID of the first subhalo (i.e., the one with the most massive history) from the same FOF group. 
NextSubhaloInFOFGroupID  int64  (N)    The SubhaloID of the next subhalo (ordered by their mass history) from the same FOF group. If there are no more subhalos in the same FOF group, NextSubhaloInFOFGroupID == 1. 
NumParticles  uint32  (N)    Number of particles in the current subhalo which were used in the merger tree to determine descendants (e.g. DMonly or stars + starforming gas). 
Mass  float32  (N)  $ 10^{10} M_\odot/h $  Mass of the current subhalo, including only the particles which were used in the merger tree to determine descendants (e.g. DMonly or stars + starforming gas). 
MassHistory  float32  (N)  $ 10^{10} M_\odot/h $  Sum of the Mass field of all progenitors along the main branch (De Lucia & Blaizot 2007). 
SubfindID  int32  (N)    Index of this subhalo in the Subfind group catalog. 
The following fields are also available, and are copied exactly from the group catalogs. The advantage is that they are ordered in the same order as the tree structure.
See the group catalog description for their units, dimensions, and descriptions.
Fields: Group_M_Crit200, Group_M_Mean200, Group_M_TopHat200, SubhaloBHMass, SubhaloBHMdot, SubhaloCM, SubhaloGasMetallicity, SubhaloGasMetallicityHalfRad, SubhaloGasMetallicityMaxRad, SubhaloGasMetallicitySfr, SubhaloGasMetallicitySfrWeighted, SubhaloGrNr, SubhaloHalfmassRad, SubhaloHalfmassRadType, SubhaloIDMostbound, SubhaloLen, SubhaloLenType, SubhaloMass, SubhaloMassInHalfRad, SubhaloMassInHalfRadType, SubhaloMassInMaxRad, SubhaloMassInMaxRadType, SubhaloMassInRad, SubhaloMassInRadType, SubhaloMassType, SubhaloParent, SubhaloPos, SubhaloSFR, SubhaloSFRinHalfRad, SubhaloSFRinMaxRad, SubhaloSFRinRad, SubhaloSpin, SubhaloStarMetallicity, SubhaloStarMetallicityHalfRad, SubhaloStarMetallicityMaxRad, SubhaloStellarPhotometrics, SubhaloStellarPhotometricsMassInRad, SubhaloStellarPhotometricsRad, SubhaloVel, SubhaloVelDisp, SubhaloVmax, SubhaloVmaxRad, SubhaloWindMass. Note: Group_M_Crit200, Group_M_Mean200, and Group_M_Tophat200 are FOF group quantities, so that all subhalos from the same FOF group will have the same value. 
The LHaloTree algorithm is virtually identical to that used for the Millennium simulation, constructing trees based on subhalos instead of main halos, described fully in the supplementary information of Springel+ (2005). In short, to determine the appropriate descendant, the unique IDs that label each particle are tracked between outputs. For a given halo, the algorithm finds all halos in the subsequent output that contain some of its particles. These are then counted in a weighted fashion, giving higher weight to particles that are more tightly bound in the halo under consideration, and the one with the highest count is selected as the descendant. In this way, preference is given to tracking the fate of the inner parts of a structure, which may survive for a long time upon infall into a bigger halo, even though much of the mass in the outer parts can be quickly stripped. To allow for the possibility that halos may temporarily disappear for one snapshot, the process is repeated for Snapshot n to Snapshot n+2. If either there is a descendant found in Snapshot $n + 2$ but none found in Snapshot n+1, or, if the descendant in Snapshot n+1 has several direct progenitors and the descendant in Snapshot n+2 has only one, then a link is made that skips the intervening snapshot.
The LHaloTree merger tree is one large data structure split across several HDF5 files named
trees_sf1_135.[chunkNum].hdf5
, where [chunkNum] goes from e.g. 0 to 511 for the Illustris1 run.
Within each file there are a number of groups named TreeX
, where X
is an integer
which simply increases from zero to the number of tree groups in that file chunk. Note that a given TreeX
group may contain subhalos spanning different FoF groups as well as snapshots, and to efficiently locate a
specific subhalo at a specific snapshot (e.g z=0) the offsets can be used. The pair
(SubhaloNumber,SnapNum)
provides the indexing into the Subfind group
catalog. The five other indices for each entry in a TreeX group (e.g. Descendant
) index into
that same group in the tree file.
The following tables describe the fields. First, the Header group:
Dataset  Dimensions  Units  Description 

Redshifts  {N_snap}    List of redshifts of the snapshots used to create this merger tree. 
TotNsubhalos  {N_snap}    Equal to the number of Subfind/Subhalo groups in the group catalog, for each snapshot used to create this merger tree. 
TreeNHalos  {N_halos}    'The size of {N} for each "TreeX" group in this file', e.g. the total number of halos (across time) in that group. 
FirstSnapshotNr  1    First snapshot number used to make these merger trees (should be 0). 
LastSnapshotNr  1    Last snapshot number used to make these merger trees (should be 135). 
SnapSkipFac  1    Snapshot stride when making these merger trees (should be 1). 
NtreesPerFile  1    'The size of {N_halos} for this file', can be used to calculate the offset to map a FoF group number to a "TreeX" group name (made to be roughly equal across chunks). 
NhalosPerFile  1    The total number of tree members (subhalos) 'in this file.' Equals the sum of all elements of TreeNHalos. 
ParticleMass  1  $ 10^{10} M_\odot / h $  The dark matter particle mass used to make these merger trees. 
TreeX Groups:
Dataset  Dimensions  Description 

SubhaloNumber  (N)  The ID of this subhalo, unique within the full simulation for this snapshot. Indexes the Subfind group catalog at SnapNum. 
Descendant  (N)  The index of the subhalo's descendant within the merger tree, if any (1 otherwise). Indexes this TreeX group. 
FirstProgenitor  (N)  The index of the subhalo's first progenitor within the merger tree, if any (1 otherwise). The first progenitor is defined as the most massive one (1 if none). Indexes this TreeX group. 
NextProgenitor  (N)  The index of the next subhalo from the same snapshot which shares the same descendant, if any (1 if this is the last). Indexes this TreeX group. 
FirstHaloInFOFGroup  (N)  The index of the main subhalo (i.e. the most massive one) from the same FOF group. Indexes this TreeX group. 
NextHaloInFOFGroup  (N)  The index of the next subhalo from the same FOF group (1 if this is the last). Indexes this TreeX group. 
FileNr  (N)  File number in which the subhalo is found. (Redundant, i.e. for a given [chunkNum] file, this array will be constant and equal to [chunkNum]) 
SnapNum  (N)  The snapshot in which this subhalo was found. 
The following fields are also available, and are copied exactly from the group catalogs. The advantage is that they are ordered in the same order as the tree structure.
See the group catalog description for their units, dimensions, and descriptions.
Fields: Group_M_Crit200, Group_M_Mean200, Group_M_TopHat200, SubhaloBHMass, SubhaloBHMdot, SubhaloGasMetallicity, SubhaloGasMetallicitySfr, SubhaloGrNr, SubhaloHalfmassRadType, SubhaloIDMostBound, SubhaloLen, SubhaloLenType, SubhaloMassInRadType, SubhaloMass, SubhaloMassType, SubhaloOffsetType, SubhaloPos, SubhaloSFR, SubhaloSFRinRad, SubhaloSpin, SubhaloStarMetallicity, SubhaloStellarPhotometrics, SubhaloVMax, SubhaloVel, SubhaloVelDisp. Note: Group_M_Crit200, Group_M_Mean200, and Group_M_Tophat200 are FOF group quantities, so that only the first subgroup from each FOF group will have a nonzero value. 
A catalog of synthetic stellar images and integrated spectra of galaxies in Illustris1 at thirteen redshifts, produced using the radiative transfer code SUNRISE. For complete details on this data product, see Torrey+ (2015) where it was first described and made available. Spatially resolved photometric maps are computed in 36 broadband filters, including GALEX, SDSS, IRAC, Johnson, 2MASS, ACS, and preliminary NIRCAM filters. This dataset is available for these snapshots:
Snapshot Number  35  38  41  45  49  54  60  64  68  75  85  103  135 

Redshift  9.0  8.0  7.0  6.0  5.0  4.0  3.0  2.5  2.0  1.5  1.0  0.5  0.0 
Minimum $M_\star [{\rm M}_\odot]$ ^{(*)}  $10^{9}$  $10^{9}$  $10^{9}$  $10^{9}$  $10^{9}$  $10^{10}$  $10^{10}$  $10^{10}$  $10^{10}$  $10^{10}$  $10^{10}$  $10^{10}$  $10^{10}$ ^{(**)} 
(*) At snapshots >50, for all galaxies with stellar masses $M_\star \gt 10^{10} {\rm M}_\odot$ ($\sim10^4$ star particles and above), both integrated SEDs and spatially resolved photometric maps are computed ("FITS files"). There are approximately 7000 galaxies above this limit at $z=0$. At snapshots <50, this minimum stellar mass is reduced to $M_\star > 10^{9} {\rm M}_\odot$ ($\sim10^3$ star particles and greater).
At redshifts above zero two separate sets of FITS files are available for (i) restframe and (ii) redshifted values. Note: both of these datasets, including broadband images and integrated SEDs, are computed by considering all particles/stars within the parent FoF halo restricted to the imaging region. Therefore, they will include contributions from e.g. background ICL or nearby companions, if they exist.
(**) At redshift zero only, a third set of FITS files are available, restricted to (iii) subhalo stars only. Therefore, all stars not gravitationally bound to the galaxy of interest are excluded, including any other nearby galaxies. These contain only integrated SEDs, and not broadband images. These are available for subhalos with greater than 500 star particles.
Note that this is the only data product which is in a format other than HDF5 (namely, FITS). However, the API provides extractions of individual bands and viewing angles in HDF5 format, as well as SEDs in text format, if requested (see API reference). Finally, we have developed the Python code SUNPY to add observational realism and make figures based on the raw stellar mock image FITS files.
A catalog of photometric nonparametric morphologies of Illustris1 galaxies at $z=0$. This is meant to replicate automated diagnostics of galaxy stellar structure commonly used observationally, and is calculated by first adding observational realism to the idealized 'stellar mocks' of (a), then measuring $(G_{\rm ini}, M_{20}, C, r_P, r_E)$ statistics in four bands, restframe u, g, i, and H, each from four directions. For full details on the calculation of each value, see the following table and Snyder+ (2015). This data is available for essentially all subhalos with $M_\star \gtrsim 10^{9.7} {\rm M}_\odot$ at $z=0$ in Illustris1. Treating each viewing direction as an independent object, values have been computed for a uniform set of 42531 sources per filter.
Group Name  Units  Description 

/Snapshot_135/SubfindID_cam{0,1,2,3}    The Subfind IDs these values correspond to (different for each camera view, but the same for all bands and fields). {10654,10618,10639,10620} entries. 
/Snapshot_135/{band_name}/Gini_cam{0,1,2,3}    The $G_{\rm ini}$ coefficient, which measures the relative distribution of the galaxy pixel flux values. 
/Snapshot_135/{band_name}/M20_cam{0,1,2,3}    $M_{20}$, the secondorder moment of the brightest 20% of the galaxy's flux. 
/Snapshot_135/{band_name}/C_cam{0,1,2,3}    The concentration parameter $C$. 
/Snapshot_135/{band_name}/RP_cam{0,1,2,3}  $kpc$  The elliptical Petrosian radius $r_P$. 
/Snapshot_135/{band_name}/RE_cam{0,1,2,3}  $kpc$  The elliptical halflight radius $r_E$. 
The four bands which replace {band_name} are: gSDSS, iSDSS, uSDSS, and hWFC3 (WFC3IR/F160W). The four camera views are indexed 0, 1, 2, and 3.
A catalog of circularities, angular momenta and axis ratios of the stellar component of galaxies. For complete definitions on the calculation of each value, see the following table and Genel+ (2015), where they were first presented. Citation to that paper is requested if you use these data catalogs.
The first four quantities are calculated after alignment with the angular momentum vector of the stars within
10 times the stellar halfmass radius, and measure the quantities inside that radius. The Circ*
fields are
based on the distribution of the circularity parameter $\epsilon$. First the system is rotated such that the
zaxis is aligned with the angular momentum vector as described above. Then, for every stellar particle with
specific angular momentum $J_z$ we calculate $ \epsilon = \frac{ J_z }{ J(E) } $ where $J(E)$ is the maximum
angular momentum of the stellar particles at positions between 50 before and 50 after the particle in question
in a list where the stellar particles are sorted by their binding energy ($=U_{\rm grav} + v^2$).
Simulation and snapshot coverage:
Group Name  Units  Description 

/Snapshot_N/SubfindID    The Subfind IDs these values correspond to at this snapshot. 
/Snapshot_N/SpecificAngMom  $ km/s \times kpc $  The specific angular momentum of the stars. 
/Snapshot_N/CircAbove07Frac    The fractional mass of stars with $ \epsilon \gt 0.7 $. This is a common definition of the "disk" stars  those with significant (positive) rotational support. 
/Snapshot_N/CircAbove07MinusBelowNeg07Frac    The fractional mass of stars with $ \epsilon \gt 0.7 $ minus the fraction of stars with $ \epsilon \lt 0.7 $. This removes the contribution of the "bulge" to the "disk", assuming the bulge is symmetric around $ \epsilon=0 $. 
/Snapshot_N/CircTwiceBelow0Frac    The fractional mass of stars with $ \epsilon \lt 0 $, multiplied by two. This is another common way in the literature to define the "bulge". 
/Snapshot_N/MassTensorEigenVals  $ kpc $  Three numbers for each galaxy that correspond to the eigenvalues of the mass tensor of the stellar mass inside the stellar $2R_{1/2}$. In a coordinate system that is aligned with the eigenvectors (principal axes), the component $i$ equals $M_i\equiv\sqrt{\sum\limits_j m_jr_{j,i}^2}/\sqrt{\sum\limits_j m_j}$, where $j$ enumerates over stellar particles inside that radius, $r_{j,i}$ is the distance of stellar particle $j$ in the $i$ axis from the most bound particle of the galaxy, and $m_j$ is its mass, and $i\in(1,2,3)$. They are sorted such that $M_1 \lt M_2 \lt M_3$. Example use: $M_1/\sqrt{M_2M_3}$ can represent the flatness of the galaxy. 
/Snapshot_N/ReducedMassTensorEigenVals    Similar to the above, except less weight is given to further away particles. The orientation of the system is the same, but the quantity measured for each axis is instead $M_i\equiv\sqrt{\sum\limits_j m_jr_{j,i}^2/R_j^2}/\sqrt{\sum\limits_j m_j}$, where $R_j\equiv\sqrt{\sum\limits_i r_{j,i}^2}$ is the distance of star $j$ from the centre of the galaxy. 
Note: For original Illustris (only), the "SpecificAngMom" and "Circ*" fields are available for snapshots 13 through 135, while the "*MassTensor*" fields are available for snapshots 38 through 135.
Note: In addition to the values measured within $10 R_E$, the "SpecificAngMom" and "Circ*" fields
are also computed including all stars in the subhalo. These are available as the _allstars
datasets.
A catalog for crossmatching subhalos between the full physics (e.g. Illustris1) and dark matter only (e.g. Illustris1Dark) runs, at the same resolution.
Is a single file for each full physics run, with a separate dataset for each snapshot named Snap_NNN_SubhaloIndexDark
, where NNN
is the snapshot number.
This is an array of integer indices, with a length equal to the number of subhalos in the full physics run. Each value in the array gives the corresponding index
of the matched subhalo in the DM only run. This was calculated as, for each DMonly subhalo, the FP subhalo with the greatest number of matching dark matter
particles. If no suitable match was found, a value of 1
is present for that subhalo index.
A catalog of photometric nonparametric morphologies of Illustris1 galaxies across redshifts. This is an updated and extended version of the catalog described in "(B) Photometric NonParametric Stellar Morphologies", which was available only at $z=0$. This new catalog contains roughly CANDELSdepth measurements ("SB25") for two observedframe filters: ACSF606W (Vband) and WFC3F160W (Hband).
The goal is again to replicate automated diagnostics of galaxy stellar structure, which are commonly used observationally. For the 'stellar mock' images of catalog (A), observational realism is first added, then several quantities are measured. These are available for each subhalo at each of the twelve $z>0$ snapshots/redshifts for which the stellar mock images are available. Throughout, NaN values indicate that the galaxy was either too faint to have its morphology characterized, or that it had a negative flux owing to shot noise. For full details on the calculation of each value, see the following table, Snyder+ (2015), and Snyder+ (in prep).
Group Name  Units  Description 

SubfindID    The Subfind IDs these values correspond to (the same for all bands, fields, and cameras). 
CC    The concentration parameter $C$, as measured by Lotz et al. (2004). 
GINI    The $G_{\rm ini}$ coefficient, which measures the relative distribution of the galaxy pixel flux values. As in Lotz et al. (2004). 
M20    $M_{20}$, the secondorder moment of the brightest 20% of the galaxy's flux. As in Lotz et al. (2004). 
MAG  $mag$  The apparent AB magnitude within the object segment in image. 
MAG_ERR  $mag$  The 1 sigma uncertainty in the magnitude. 
PIX_ARCSEC  $arcsec$  The linear width of one pixel. 
RHALF  $px$  The halflight semimajor radius. 
RP  $px$  The elliptical Petrosian radius $r_P$. 
SEG_AREA  $px^2$  Area of the object segment (bad if this approaches 0 or $N^2$). 
SNPIX    Average signaltonoise ratio of a pixel in the source (bad if this is <<3). 
The prefix for all datasets is /nonparamorphs/Snapshot_{NNN}/{filter}/CAMERA{X}/
,
except for SubfindID
which is available once per snapshot.
The two bands which replace {filter}
are: ACSF606W (Vband) and WFC3F160W (Hband).
The four camera views replacing {X}
are indexed 0, 1, 2, and 3.
This catalog is comprised of two data sets: 'mergers' and 'details'. The mergers file contains a record of each BHBH merger in the simulation. The details file contain properties of each BH at significantly higher time resolution than the snapshots. In both cases the information is extracted from output files separate from the snapshots, and so represents additional data which is otherwise not available from the snapshots alone. Here we distinguish the two BHs which participate in the merger as 'in' and 'out' BH. The difference, which during the simulation is chosen randomly by the code, is which BH ID number persists along with the remnant after the merger. The 'out' BH survives after the merger, increased in mass by that of the 'in' BHwhich no longer exists after the merger event.
There exists one
blackhole_mergers.hdf5
and one blackhole_details.hdf5
per baryonic run.
All datasets corresponding to physical values are in code units, with
masses in units of $10^{10} M_\odot / h$,
mass accretion rates in units of $10.22 M_\odot / \rm{yr}$,
gas densities in units of $(10^{10} M_\odot / h) / (\rm{ckpc} / h)^3$,
and gas sound speeds in units of $\rm{km / s}$.
They were generated with the
illustris_blackholes code.
Citation recommmended to Kelley et al. (2016) and
Blecha et al. (2016) as appropriate.
Simulation and snapshot coverage:
Blackhole mergers:
Dataset  Shape/Datatype  Description 

/Header/unique_ids  (NumBHsTot) uint64  The ID numbers of all unique BH participating in mergers. 
/Header/num_mergers  (attribute)  The total number of mergers stored $(N)$. 
/tree/*    Information describing the BH merger tree. If one of the below events does not
exist, the value in the array is 1, NOTE: not zero. For example, if
next[123] = 345 , then merger 345 is the next merger that
the BH remnant from merger 123 is involved in. If, prev_in[345] == 123
and prev_out[345] = 1 , then the 123 merger remnant is the 'in' BH of
merger 345, and the other BH of that merger was never in a previous merger (and so has
a prev value of 1).

/tree/next  (N) int  The index number of the next merger this remnant takes part in. 
/tree/prev_in  (N) int  The index number of the previous merger this 'in' BH was part of. 
/tree/prev_out  (N) int  The index number of the previous merger this 'out' BH was part of. 
/details/*    Information from the 'details' catalog for the BHs in each merger. Details entries were searched by trying to match the 'in' BH just before merger, and the 'out' BH both just before, and just after merger. This corresponds to the three 'columns' for each entry 'row': [0: inbefore, 1: outbefore, 2: outaft]. Frequently these details were not found, in which case the array values are zero. 
/details/time  (N,3) float  Time (scalefactor) for each entry. 
/details/mass  (N,3) float  Blackhole mass. 
/details/mdot  (N,3) float  Blackhole mass accretion rate. NOTE: This is the expected Bondi accretion rate, before the Eddington limit or local pressure criterion are applied, and thus will not in general reflect the true mdot of the BH used during the simulation. Should be used with caution. 
/details/rho  (N,3) float  Local gas density in the vicinity of the blackhole. 
/details/cs  (N,3) float  Local gas soundspeed in the vicinity of the blackhole. 
/time  (N) float  Time (scalefactor) for each merger event. 
/id_in  (N) uint64  ID number of the 'in' BH. 
/id_out  (N) uint64  ID number of the 'out' BH. 
/mass_in  (N) float  Mass of the 'in' BH (immediately preceding merger). 
/mass_out  (N) float  Mass of the 'out' BH (immediately preceding merger). 
/snapshot  (N) int  Output snapshot during which, or immediately following, this merger event occured. 
Blackhole details:
Dataset  Shape/Datatype  Description 

/Header/num_entries  (attribute)  The total number of details entries stored $(N)$. 
/Header/num_blackholes  (attribute)  The total number of unique BHs with details entries $(M)$. 
/unique/id  (M) uint64  ID numbers of each unique BH. 
/unique/first_index  (M) int  The index number (into any of the size N arrays) of the first entry for each unique BH. 
/unique/num_entries  (M) int  The total number of entries (in any of the size N arrays) for each unique BH. 
/id  (N) uint64  ID number of the BH for each details entry. 
/time  (N) float  Time (cosmological scalefactor) for each entry. 
/mass  (N) float  Blackhole mass. 
/mdot  (N) float  Blackhole mass accretion rate. NOTE: This is the expected Bondi accretion rate, before the Eddington limit or local pressure criterion are applied, and thus will not in general reflect the true mdot of the BH used during the simulation. Should be used with caution. 
/rho  (N) float  Local gas density in the vicinity of the blackhole. 
/cs  (N) float  Local gas soundspeed in the vicinity of the blackhole. 
This large catalog contains information about the stellar assembly of all galaxies across all
snapshots, focusing on in situ vs. ex situ stellar mass growth, the contribution from star
formation before or after infall, the role of major and/or minor mergers and flyby encounters.
There exists one stellar_assembly.hdf5
file per baryonic run. All datasets are masses,
and are given in code units, that is, $10^{10} M_\odot / h$. There is one group per snapshot,
and within each group, all datasets have the same size, corresponding to exactly one entry per
Subfind subhalo. Citation recommmended to
RodriguezGomez et al. (2015),
RodriguezGomez et al. (2016a), and/or
RodriguezGomez et al. (2016b) as appropriate.
Further information and usage examples are available in these same papers.
Simulation and snapshot coverage:
Dataset  Description 

/Snapshot_N/StellarMassInSitu  The amount of stellar mass that was formed in situ. 
/Snapshot_N/StellarMassExSitu  The amount of stellar mass that was formed ex situ. 
/Snapshot_N/StellarMassTotal  The total stellar mass of the galaxy. 
/Snapshot_N/StellarMassAfterInfall  The amount of (ex situ) stellar mass that was formed after entering the halo where the galaxy is currently found. 
/Snapshot_N/StellarMassBeforeInfall  The amount of (ex situ) stellar mass that was formed before entering the halo where the galaxy is currently found. 
/Snapshot_N/StellarMassFromCompletedMergers  The amount of (ex situ) stellar mass that was accreted from completed mergers, as defined by the "AccretionOrigin" property. 
/Snapshot_N/StellarMassFromCompletedMergersMajor  Same as above, but only considering major mergers (stellar mass ratio > 1/4), as defined by the "MergerMassRatio" property of each star. 
/Snapshot_N/StellarMassFromCompletedMergersMajorMinor  The same, but considering major and minor mergers (stellar mass ratio > 1/10). 
/Snapshot_N/StellarMassFromOngoingMergers  The amount of (ex situ) stellar mass that was accreted from ongoing mergers, as defined by the "AccretionOrigin" property. NOTE: by definition, this quantity is zero at z=0 (since a flyby cannot be distinguished from an ongoing merger). In fact, it is recommended to combine "ongoing mergers" with "flybys" into a single category called "stripped from surviving galaxies" (see references). 
/Snapshot_N/StellarMassFromOngoingMergersMajor  Same as above, but only considering major mergers (stellar mass ratio > 1/4), as defined by the "MergerMassRatio" property of each star. 
/Snapshot_N/StellarMassFromOngoingMergersMajorMinor  The same, but considering major and minor mergers (stellar mass ratio > 1/10). 
/Snapshot_N/StellarMassFromFlybys  The amount of (ex situ) stellar mass that was accreted during flybys, as defined by the "AccretionOrigin" property. 
/Snapshot_N/StellarMassFromFlybysMajor  Same as above, but only considering major flybys (stellar mass ratio > 1/4), as defined by the "MergerMassRatio" property of each star. 
/Snapshot_N/StellarMassFromFlybysMajorMinor  The same, but considering major and minor flybys (stellar mass ratio > 1/10). 
/Snapshot_N/StellarMassFormedOutsideGalaxies  The amount of (ex situ) stellar mass that was formed outside of any galaxy (as determined by SUBFIND). 
The 'galaxy' quantities above should satisfy the following invariants (up to rounding errors):
The above descriptions make reference to two values computed for every star particle in the simulation, derived as follows (see references for more details):
A small caveat: the "subhalo switching" problem" can result in some galaxies having (spurious) ex situ fractions very close to 1 (say, if a satellite suddenly becomes a central, then most of its newly assigned mass will appear as ex situ). The number of galaxies this affects is negligible, but still noticeable in e.g. a scatter plot.
This catalog contains measurements of galaxy photometry, morphology, strong lensing, and
dynamics. There exists one pmsd.hdf5
file for Illustris1. There is one group per
snapshot analyzed, of which ten exist: 127, 120, 114, 108, 103, 99, 95, 92, 89, 85 (redshifts
$z=0.1$ to $z=1.0$ in spacings of $0.1$). Within each snapshot group, all datasets have the same
size, corresponding to exactly one entry per Subfind subhalo which has been analyzed. The
selection criterion for a subhalo to be analyzed was that the stellar mass within 30 physical
kpc must be larger than $7 \times 10^{9} M_\odot / h \simeq 10^{10} M_\odot$. This corresponds to
a minimum of roughly 5,000 to 10,000 or more stellar particles within the 30 pkpc aperture.
If using this supplementary catalog, citation is recommmended to
Xu et al. (2017),
where additional details are given in Appendix A.
Data at other redshifts and galaxy images are available upon request, please contact Dandan Xu.
The contents of each /Snapshot_N/
group are:
Dataset  Units  Description 

SubfindID    SUBFIND subhalo index (into the group catalogs at this snapshot) 
FileNo    Auxiliary file number (currently unused). 
Msub  Msun/h  SUBFIND subhalo mass 
Mstar_tot  Msun/h  The stellar mass of the entire subhalo 
Mstar_30  Msun/h  The stellar mass within a 3D radius of 30 pkpc from subhalo centre 
Mdm_30  Msun/h  The dark matter mass within a 3D radius of 30 pkpc from subhalo centre 
Mgas_30  Msun/h  The gas mass within a 3D radius of 30 pkpc from subhalo centre 
Nstar_30    The number of stellar particles within a 3D radius of 30 pkpc from subhalo centre 
hsmr  arcsec  The 3D halfstellarmass radius, output of SUBFIND (converted to arcsec) 
hgmr  arcsec  The 3D halfgasmass radius, output of SUBFIND (converted to arcsec) 
R_promin  arcsec  The minimum of the radial range, within which radidal distributions of relevant quantities were measured (and used for interpolation); set to be the angular size (at the snapshot redshift) corresponding to a physical scale of 0.7 kpc 
R_promax  arcsec  The maximum of the radial range, within which radidal distributions of relevant quantities were measured (and used for interpolation); set to be the angular size (at the snapshot redshift) corresponding to min(5*hsmr, 30 pkpc) 
Rein_{x,y,z}  arcsec  The Einstein radius in {x,y,z}projection; set to be 0.0 if Rein_{x,y,z} < R_promin 
Rc50_{x,y,z}  arcsec  The radius within which the projected cumulative dark matter fraction is 50% in {x,y,z}projection; set to be 0.0 if Rc50_{x,y,z} < R_promin; or set to be 1E10 if Rc50_{x,y,z} > R_promax 
Rl50_{x,y,z}  arcsec  The radius at which the projected local dark matter fraction is 50% in {x,y,z}projection; set to be 0.0 if Rl50_{x,y,z} < R_promin; or set to be 1E10 if Rl50_{x,y,z} > R_promax 
Reff_ser_{x,y,z}  arcsec  The effective radius calculated by fitting Sersic profile in restframe SDSS rband in {x,y,z}projection 
Reff_dev_{x,y,z}  arcsec  The effective radius calculated by fitting de Vaucouleurs profile in restframe SDSS rband in {x,y,z}projection 
Reff_exp_{x,y,z}  arcsec  The effective radius calculated by fitting exponential profile in restframe SDSS rband in {x,y,z}projection 
Reff_of_{bands}mod_{x,y,z}  arcsec  The Sersicfit effective radius in observerframe {band} in {x,y,z}projection 
Reff_of_{bands}_{x,y,z}  arcsec  The radius which encloses half of the total luminosity measured within a projected radius of 30 pkpc from the galaxy center, in observerframe {band} in {x,y,z}projection 
Reff_rf_{bands}mod_{x,y,z}  arcsec  The Sersicfit effective radius in restframe {band} in {x,y,z}projection 
Reff_rf_{bands}_{x,y,z}  arcsec  The radius which encloses half of the total luminosity measured within a projected radius of 30 pkpc from the galaxy center, in restframe {band} in {x,y,z}projection 
Sersic_m_{x,y,z}    The Sersic index of the bestfitted Sersic profile measured in restframe SDSS rband in {x,y,z}projection 
IRe_ser_{x,y,z}  mag/arcsec^2  The surface brightness at Reff_ser_{x,y,z} in restframe SDSS rband in {x,y,z}projection 
IRe_dev_{x,y,z}  mag/arcsec^2  The surface brightness at Reff_dev_{x,y,z} in restframe SDSS rband in {x,y,z}projection 
IRe_exp_{x,y,z}  mag/arcsec^2  The surface brightness at Reff_exp_{x,y,z} in restframe SDSS rband in {x,y,z}projection 
Mag_of_{bands}mod_{x,y,z}  mag  The total (absolute AB) magnitude in observerframe {band} derived from bestfit Sersic model in {x,y,z}projection 
Mag_of_{bands}_{x,y,z}  mag  The total (absolute AB) magnitude in observerframe {band} derived from direct measurement within a projected radius of 30 pkpc from galaxy centre in {x,y,z}projection 
Mag_rf_{bands}mod_{x,y,z}  mag  The total (absolute AB) magnitude in restframe {band} derived from bestfit Sersic model in {x,y,z}projection 
Mag_rf_{bands}_{x,y,z}  mag  The total (absolute AB) magnitude in restframe {band} derived from direct measurement within a projected radius of 30 pkpc from galaxy centre in {x,y,z}projection 
Mag{B,V}_{ep5,e1,e2}_{x,y,z}  mag  The restframe Johnson{B,V} (absolute AB) magnitude measured within a projected radius of {0.5,1.0,2.0}*Reff_rf_john_{b,v}mod_{x,y,z} from galaxy centre in {x,y,z}projection 
Galb2a_{ep5,e1,e2}_{x,y,z}    The axial ratio of the projected (restframe JohnsonV band) light distribution measured within a radius of {0.5,1.0,2.0}*Reff_rf_john_vmod_{x,y,z} from galaxy centre in {x,y,z}projection 
Subb2a_{ep5,e1,e2}_{x,y,z}    The axial ratio of the projected total matter distribution measured within a radius of {0.5,1.0,2.0}*Reff_rf_john_vmod_{x,y,z} from galaxy centre in {x,y,z}projection 
GalRA_{ep5,e1,e2}_{x,y,z}  radian  The orientation angle of the projected (restframe JohnsonV band) light distribution measured within a radius of {0.5,1.0,2.0}*Reff_rf_john_vmod_{x,y,z} from galaxy centre in {x,y,z}projection 
SubRA_{ep5,e1,e2}_{x,y,z}  radian  The orientation angle of the projected total matter distribution measured within a radius of {0.5,1.0,2.0}*Reff_rf_john_vmod_{x,y,z} from galaxy centre in {x,y,z}projection 
Galxc_{x,y,z}  arcsec  The xcoordinate (in the plane of projection) of the light centre in {x,y,z}projection, measured in restframe JohnsonV band and with respect to the centre of subhalo 
Galyc_{x,y,z}  arcsec  The ycoordinate (in the plane of projection) of the light centre in {x,y,z}projection, measured in restframe JohnsonV band and with respect to the centre of subhalo 
TypeDec_{x,y,z}    Galaxy Type (in {x,y,z}projection): 1 for earlytype; 0 for latetype; 1 if lack of resolution for surface brightness fitting 
Mrein_{x,y,z}  Msun/h  The mass projected within a radius of Rein_{x,y,z} from galaxy centre in {x,y,z}projection (Msun/h); set to be 0.0 if Rein_{x,y,z} < R_promin 
Mstar{B,V}_{ep5,e1,e2}_{x,y,z}  Msun/h  The stellar mass projected within a radius of {0.5,1.0,2.0}*Reff_rf_john_{b,v}mod_{x,y,z} from galaxy centre in {x,y,z}projection 
Mtot{B,V}_{ep5,e1,e2}_{x,y,z}  Msun/h  The total mass projected within a radius of {0.5,1.0,2.0}*Reff_rf_john_{b,v}mod_{x,y,z} from galaxy centre in {x,y,z}projection 
Fdm2in5kpc_{x,y,z}    The cumulative dark matter fraction within a projected radius of 5 kpc from galaxy centre in {x,y,z}projection; set to be 1.0 if the angular scale of 5 pkpc is larger than R_promax 
Fdm2inRein_{x,y,z}    The cumulative dark matter fraction within a projected radius of Rein_{x,y,z} from galaxy centre in {x,y,z}projection; set to be 1.0 if Rein_x < R_promin 
Fdm2_{ep5,e1,e2}_{x,y,z}    The cumulative dark matter fraction within a projected radius of {0.5,1.0,2.0}*Reff_rf_john_vmod_{x,y,z} from galaxy centre in {x,y,z}projection 
Fdm3_{ep5,e1,e2}_{x,y,z}    The cumulative dark matter fraction within a 3D radius of {0.5,1.0,2.0}*Reff_rf_john_vmod_{x,y,z} 
Fgas3_{ep5,e1,e2}_{x,y,z}    The cumulative gas fraction within a 3D radius of {0.5,1.0,2.0}*Reff_rf_john_vmod_{x,y,z} 
Fcgs3_{ep5,e1,e2}_{x,y,z}    The cumulative cold gas (HI) fraction within a 3D radius of {0.5,1.0,2.0}*Reff_rf_john_vmod_{x,y,z} 
Vmean_mav_{x,y,z}  km/s  The stellarmassweighted stellar lineofsight mean velocity measured within a projected radius of 1.5 arcsec from galaxy centre in {x,y,z}projection 
Vmean_lav_{x,y,z}  km/s  The (restframe SDSSr band) luminosityweighted stellar lineofsight mean velocity measured within a projected radius of 1.5 arcsec from galaxy centre in {x,y,z}projection 
Vmean_{ep5,e1,e2}_{x,y,z}  km/s  The (restframe SDSSr band) luminosityweighted stellar lineofsight mean velocity measured within a projected radius of {0.5,1.0,2.0}*Reff_rf_john_vmod_{x,y,z} from galaxy centre in {x,y,z}projection 
Vsigma_mav_{x,y,z}  km/s  The stellarmassweighted stellar lineofsight velocity dispersion measured within a projected radius of 1.5 arcsec from galaxy centre in {x,y,z}projection 
Vsigma_lav_{x,y,z}  km/s  The (restframe SDSSr band) luminosityweighted stellar lineofsight velocity dispersion measured within a projected radius of 1.5 arcsec from galaxy centre in {x,y,z}projection 
Vsigma_{ep5,e1,e2}_{x,y,z}  km/s  The (restframe SDSSr band) luminosityweighted stellar lineofsight velocity dispersion measured within a projected radius of {0.5,1.0,2.0}*Reff_rf_john_vmod_{x,y,z} from galaxy centre in {x,y,z}projection 
Beta_mav_{x,y,z}    The stellarmassweighted stellar orbital anisotropy parameter ({x,y,z} direction) measured within a 3D radius of 1.5 arcsec from galaxy centre 
Beta_lav_{x,y,z}    The (restframe SDSSr band) luminosityweighted stellar orbital anisotropy parameter ({x,y,z} direction) measured within a 3D radius of 1.5 arcsec from galaxy centre 
Beta_{ep5,e1,e2}_{x,y,z}    The (restframe SDSSr band) luminosityweighted stellar orbital anisotropy parameter measured within a 3D radius of {0.5,1.0,2.0}*Reff_rf_john_vmod_{x,y,z} from galaxy centre 
slpMWtot_{ep5,e1,e2}_{x,y,z}    The massweighted total density slope calculated using Eq. (1) of Dutton & Treu 2014, evaluated at a radius of {0.5,1.0,2.0}*Reff_rf_john_vmod_{x,y,z} from galaxy centre 
slp3tot_{ep5,e1,e2}_{x,y,z}    The average total density slope calculated using Eq. (15) of the paper, in the radial range {0.20.5, 0.51.0, 0.52.0} times Reff_rf_john_vmod_{x,y,z} 
slp3totPLf_{ep5,e1,e2}_{x,y,z}    The fitted powerlaw slope of the total density distribution in the radial range {0.20.5, 0.51.0, 0.52.0} times Reff_rf_john_vmod_{x,y,z} 
slpJESER_{x,y,z}    The total density slope derived by combining strong lensing measurement of Mrein_{x,y,z} and singleaperture stellar kinematics data of Vsigma_lav_{x,y,z}, assuming the stellar orbital anisotropy is given by Beta_lav_{x,y,z}; set to be 1E10 if Rein_{x,y,z} < R_promin 
slpJEbeta0_{x,y,z}    The total density slope derived by combining strong lensing measurement of Mrein_{x,y,z} and singleaperture stellar kinematics data of Vsigma_lav_{x,y,z}, assuming isotropic stellar orbital distribution; set to be 1E10 if Rein_{x,y,z} < R_promin 
slp3dm_{ep5,e1,e2}_{x,y,z}    The average dark matter density slope calculated using Eq. (15) of the paper, in the radial range {0.20.5, 0.51.0, 0.52.0} times Reff_rf_john_vmod_{x,y,z} 
slp3dmPLf_{ep5,e1,e2}_{x,y,z}    The fitted powerlaw slope of the dark matter density distribution in the radial range {0.20.5, 0.51.0, 0.52.0} times Reff_rf_john_vmod_{x,y,z} 
slp3st_{ep5,e1,e2}_{x,y,z}    The average stellar density slope calculated using Eq. (15) of the paper, in the radial range {0.20.5, 0.51.0, 0.52.0} times Reff_rf_john_vmod_{x,y,z} 
slp3stPLf_{ep5,e1,e2}_{x,y,z}    The fitted powerlaw slope of the stellar density distribution in the radial range {0.20.5, 0.51.0, 0.52.0} times Reff_rf_john_vmod_{x,y,z} 
Note: {bands}
= {sdss_u,sdss_g,sdss_r,sdss_i,sdss_z,hst_b,hst_v,hst_i,john_b,john_v}.
Here, john_b = Johnson Bband, john_v = Johnson Vband, hst_b = HST BF435w, hst_v = HST VF606w hst_i = HST IF814w, and sdss_* are the usual five SDSS bands.
Note: {ep5,e1,e2}
correspond to either factors of {0.5,1.0,2.0}, or to radial
ranges of {0.20.5, 0.51.0, 0.52.0}, multiplying the appropriate Reff in the description.
All the above permutations are complete and exist as separate datasets, except please note that the following datasets are not available:
This catalog contains measurements of angular momenta and baryon content, measured separately
for both halos (FoF groups) and galaxies (Subfind subhalos). For Illustris1 and Illustris1Dark
there exists one angular_momentum.hdf5
file. All halos and galaxies are included, at
redshift zero only (snapshot 135).
If using this supplementary catalog, citation is recommmended to Zjupa & Springel (2016), where additional details are available. Data at other redshifts are available upon request, please contact Jolanta Zjupa. The contents of the data file are (N_{g} is the number of groups in snapshot 135, while N_{s} is the number of subhalos):
Dataset  Shape  Units  Description 

GroupEkin  N_{g}  $10^{10} M_\odot/h \ (km/s)^2$  kinetic energy of FOFhaloes (due to the particle and cell velocities; the total physical kinetic energy is obtained adding the thermal energy of the gas, if present.) 
Group_Ekin_{SO}  N_{g}  $10^{10} M_\odot/h \ (km/s)^2$  kinetic energy of SOhaloes (due to the particle and cell velocities; the total physical kinetic energy is obtained adding the thermal energy of the gas, if present.) 
GroupEpot  N_{g}  $10^{10} M_\odot/h \ (km/s)^2$  potential energy of FOFhaloes 
Group_Epot_{SO}  N_{g}  $10^{10} M_\odot/h \ (km/s)^2$  potential energy of SOhaloes 
GroupEthr  N_{g}  $10^{10} M_\odot/h \ (km/s)^2$  thermal energy of the gas component of FOFhaloes 
Group_Ethr_{SO}  N_{g}  $10^{10} M_\odot/h \ (km/s)^2$  thermal energy of the gas component of SOhaloes 
Group_J  N_{g},3  $10^{10} M_\odot/h^2 \ kpc \ km/s$  total angular momentum of FOFhaloes 
Group_J_{SO}  N_{g},3  $10^{10} M_\odot/h^2 \ kpc \ km/s$  total angular momentum of SOhaloes 
Group_Jdm  N_{g},3  $10^{10} M_\odot/h^2 \ kpc \ km/s$  angular momentum of the dark matter component of FOFhaloes 
Group_Jdm_{SO}  N_{g},3  $10^{10} M_\odot/h^2 \ kpc \ km/s$  angular momentum of the dark matter component of SOhaloes 
Group_Jgas  N_{g},3  $10^{10} M_\odot/h^2 \ kpc \ km/s$  angular momentum of the gas component of FOFhaloes 
Group_Jgas_{SO}  N_{g},3  $10^{10} M_\odot/h^2 \ kpc \ km/s$  angular momentum of the gas component of SOhaloes 
Group_Jstars  N_{g},3  $10^{10} M_\odot/h^2 \ kpc \ km/s$  angular momentum of the stellar component of FOFhaloes 
Group_Jstars_{SO}  N_{g},3  $10^{10} M_\odot/h^2 \ kpc \ km/s$  angular momentum of the stellar component of SOhaloes 
Group_CMFrac  N_{g}    total counterrotating mass fraction of FOFhaloes 
Group_CMFrac_{SO}  N_{g}    total counterrotating mass fraction of SOhaloes 
Group_CMFracType  N_{g},6    counterrotating mass fractions per type: dark matter, gas, stars 
Group_CMFracType_{SO}  N_{g},6    counterrotating mass fractions per matter type in SOhaloes 
Group_LenType_{SO}  N_{g},6    number of particles/cells of each matter type in SOhaloes 
Group_MassType_{SO}  N_{g},6  $10^{10} M_\odot / h$  mass per matter type in SOhaloes 
SubhaloEkin  N_{s}  $10^{10} M_\odot/h \ (km/s)^2$  kinetic energy of subhaloes (due to the particle and cell velocities; the total physical kinetic energy is obtained adding the thermal energy of the gas, if present.) 
SubhaloEpot  N_{s}  $10^{10} M_\odot/h \ (km/s)^2$  potential energy of subhaloes 
SubhaloEthr  N_{s}  $10^{10} M_\odot/h \ (km/s)^2$  thermal energy of the gas component of subhaloes 
Subhalo_J{rad}  N_{s},3  $10^{10} M_\odot/h^2 \ kpc \ km/s$  total angular momentum of subhaloes 
Subhalo_Jdm{rad}  N_{s},3  $10^{10} M_\odot/h^2 \ kpc \ km/s$  angular momentum of the dark matter component of subhaloes 
Subhalo_Jgas{rad}  N_{s},3  $10^{10} M_\odot/h^2 \ kpc \ km/s$  angular momentum of the gas component of subhaloes 
Subhalo_Jstars{rad}  N_{s},3  $10^{10} M_\odot/h^2 \ kpc \ km/s$  angular momentum of the stellar component of subhaloes 
Subhalo_CMFrac{rad}  N_{s}    total counterrotating mass fraction of subhaloes 
Subhalo_CMFracType{rad}  N_{s},6    counterrotating mass fractions per type: dark matter, gas, stars 
Note: multiple datasets where {SO}
= {Crit200,Crit500,Mean200,TopHat200}.
Note: multiple datasets where {rad}
= {,InHalfRad,InRad}.
Properties for each halo are measured using the friendsoffriends (FOF) group as well as several spherical overdensity (SO) criteria. The latter can be either with respect to 200 times the critical density (Crit200), 500 times the critical density (Crit500), 200 times the mean density (Mean200), or with the redshift dependent overdensity expected for the generalised tophat collapse model in a LCDM cosmology (TopHat200). The corresponding properties can be accessed by replacing {SO} with the terms in brackets. Properties for each galaxy are measured using all particles/cells in the subhalo ({rad} is blank, e.g. "Subhalo_CMFrac"). Furthermore, we include two more definitions of a galaxy closer to various observational approachs, by including only subhalo particles/cells that are in the stellar half mass radius ({rad} = "InHalfRad") or twice the stellar half mass radius ({rad} = "InRad").
This dataset consists of synthetic deep survey images. By projecting a line of sight through a periodic volume, we constructed realistic mock surveys which preserve the predicted geometry of the simulations. We can now create these mock galaxy surveys with detail down to the size scales and distances revealed by HST, and in the future, those JWST and WFIRST will reveal.
Here, we present three "mock ultra deep fields", each 2.8 arcminutes across, in common wide filters used by HST, as well as in filters expected to be used widely by observers with JWST and WFIRST. For HST and JWST simulations, we provide images with and without convolution by model PSFs. For each image, we also provide the simulation catalog from which we generated each image, enabling users to locate sources, link them to intrinsic simulation quantities, and conduct analyses across observation and theory space.
If using this supplementary catalog, citation is recommmended to Snyder et al. (2017), where additional details are available.
The deep fields are provided as multiextension FITS files. The filenames all begin with
"hlsp_misty_illustris_", and then a string as: {instrument}_{field}_{filter}_v1_lightcone.fits
where:
{instrument}
= "hstacs", "hstwfc3", "jwstmiri", "jwstnircam", or "wfirstwfidrm15"{field}
= The geometry/selection, described below.{filter}
= The filter, e.g., "f435w".The geometry/selection of these three fields is derived from the three "Thin" lightcones defined in Snyder et al. (2017). For example, "mag30fielda1110" corresponds to Field A from that paper. The string "mag30" refers to the selection of Illustris subhalos included in the mock image. For these files, the selection is based on the restframe gband apparent magnitude of the source, such that g ≤ 30.0.
The FITS files contain the following HDUs:
At present, all associated data can be downloaded from the STScI MAST Data Product Page.
This catalog contains synthetic images and the corresponding morphological measurements, including GiniM20, CAS and MID statistics, as well as 2D Sersic fits. The full description of the synthetic images and measurements can be found in RodriguezGomez+ (2019), to which citation is requested if you use this data. Full details are not repeated, for which the user is directed to the paper.
Synthetic images and measurements are available separately for: "pogs" and "sdss", designed to match PanSTARRS and SDSS, respectively.
Simulation and snapshot coverage:
z=0
, z=0.05
.There are two ways to download this data.
In the single tar collection, the files "morphs_i.hdf5" and "morphs_g.hdf5" (and "morphs_r.hdf5" for SDSS only) contain
statmorph morphological measurements in the observed iband, gband, (and rband),
respectively (using PanSTARRS or SDSS filter curves, depending on the dataset), and "subfind_ids.txt" contains the
associated subhalo IDs. For a description of the measurements,
see Section 4 of RodriguezGomez+ (2019). In general, only morphological
measurements with flag == 0
are reliable. If interested in parameters derived from the Sérsic fits, then
flag_sersic == 0
should be imposed as well. In addition, only measurements with S/N > 2.5 (included in
the catalog as sn_per_pixel
) should be trusted. See Section 5 of the paper, for more details.
The idealized synthetic images themselves are available in FITS format. The dimensions of each image are NxNx4
,
where N
is the number of pixels in each dimension and the 4 layers correspond to the observed g,r,i,z broadband
filters of PanSTARRS or SDSS. Note that the SDSS zband data should be used with caution (they are reasonable for morphologies,
but not for magnitudes and colors), since the longwavelength tail of the SDSS z filter curve extends beyond the
wavelength range of the SKIRT runs (clipped at 0.95 microns in the rest frame).
These synthetic broadband images are "idealized", meaning that some realism may need to be added before analyzing them. Most importantly, they should be convolved with an adequate PSF (for most purposes, a 2D Gaussian with the same FWHM as the observations should suffice) and background shot noise should be added. In general, Gaussian noise with sigma_sky ~ 1/20 (1/10) for PanSTARRS (SDSS) represents a good compromise between observational realism and detection strength. Note that the image units are electrons/s/pixel. For more details, see Section 3.4 of the paper.
In addition to the FITS files, PNG images are also available. These are analogous to Fig. 4 of RodriguezGomez+ (2019), such that different panels show different morphological diagnostics. These figures can be used to quickly inspect the morphology of a given galaxy.
Notes: The z=0 images and measurements were created by assuming that the galaxies are actually located at z=0.0485 (this determines the pixel scale, etc.). Galaxy sizes (e.g. "rhalf", "sersic_rhalf", "rpetro", "rmax") are given in pixels. To convert to simulation units (ckpc/h), note that the scale of each pixel at z=0.0485 corresponds to 0.174 ckpc/h for PanSTARRS and 0.276 ckpc/h for SDSS.
2022 addition: KiDS Survey Mock Images. A new, third set of images were generated with the same procedure as in RodriguezGomez et al. (2019), except that:
The image settings are consistent with the KiloDegree Survey (KiDS). In particular, each FITS file has 4 layers corresponding to OmegaCAM g,r,i,z filters.
Simulation and snapshot coverage:
z=0.0337
(snapshot 96) only.There are three projections for each galaxy (xy, yz, zx). The pixel scale: 0.2 arcsec/pixel, corresponding to 138.9 pc (physical) per pixel at z = 0.0337. The dimensions of each image are 240x240 pixels (33.34 kpc per side). Note that the most massive galaxies may extend beyond the FoV.
The image units are ADU/s, just like in the real survey, and the zeropoint is zero, i.e. mag = 2.5 * log10(data)
. The images are "idealized", i.e. they have not been convolved with a PSF and they do not include background or shot noise. See Section 2.4 from GuzmánOrtega et al. (2022) for information on how to apply realism. Complete details are available in that paper, to which citation is requested if you use this dataset.
A catalog of timeaveraged, rather than instantaneous, star formation rates of galaxies. To do so the SFRs are derived the stellar particles actually produced across different time spans, using their initial mass at birth. This is in contrast to all values in the group/subhalo catalogs, which are instantaneous values (measured from the gas cells). The timeaveraged methodology, across a given period of Myr or Gyr, better reflects the values obtained via various observational tracers. For complete definitions on the calculation of each value, see the following table, Donnari+ (2019), and Pillepich+ (2019) where they were first presented. Citation to these two papers is requested if you use these data catalogs.
Simulation and snapshot coverage:
Group Name  Units  Description 

/Snapshot_N/SubfindID    The Subfind IDs these values correspond to at this snapshot. 
/Snapshot_N/SFR_MsunPerYrs_in_r5pkpc_{X}Myrs  Msun/yr  The galaxy SFR including stars within a 3D aperture of 5 physical kpc, averaged
across the last {X} Myr. This roughly mimics the SDSS fiber aperture. 
/Snapshot_N/SFR_MsunPerYrs_in_InRad_{X}Myrs  Msun/yr  The galaxy SFR including stars within a 3D aperture of two times the stellar
half mass radius, averaged across the last {X} Myr.

/Snapshot_N/SFR_MsunPerYrs_in_r30pkpc_{X}Myrs  Msun/yr  The galaxy SFR including stars within a 3D aperture of 30 physical kpc,
averaged across the last {X} Myr.

/Snapshot_N/SFR_MsunPerYrs_in_all_{X}Myrs  Msun/yr  The galaxy SFR including all gravitationally bound stars of the subhalo,
averaged across the last {X} Myr.

For all measurements, time intervals {X}
exist for X = 10, 50, 100, 200, 1000
(Myr).