This page contains a description of all fields in the snapshots, group catalogs, merger trees, and supplementary data sets.

Table of Contents


1. Snapshots

Organization

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, smaller-size 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 Illustris-3 94,196,375 32 2 22 GB 100 MB 3 TB
L75n455DM Illustris-3-Dark 94,196,375 8 2 3.2 GB 50 MB 0.4 TB
L75n910FP Illustris-2 753,571,000 256 4 176 GB 500 MB 24 TB
L75n910DM Illustris-2-Dark 753,571,000 32 4 26 GB 320 MB 3.5 TB
L75n1820FP Illustris-1 6,028,568,000 512 8 1.5 TB 3.6 GB 204 TB
L75n1820DM Illustris-1-Dark 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.

Contents

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):

  • PartType0 - GAS
  • PartType1 - DM
  • PartType2 - (unused)
  • PartType3 - TRACERS
  • PartType4 - STARS & WIND PARTICLES
  • PartType5 - BLACK HOLES

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 star-forming gas (see comment below for NeutralHydrogenAbundance).
GFM_AGNRadiation N $ erg / s / cm^{2} $ Bolometric intensity (physical units) at the position of this cell arising from the radiation fields of nearby AGN.
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 star-forming 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 cubic-spline 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 $ 1 / (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 DM-only runs).
SubfindDensity N $ (10^{10} M_\odot/h) / (ckpc/h)^3 $ The local total comoving mass density, estimated using the standard cubic-spline 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 cubic-spline 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.
BH_CumMassGrowth_QM N $ (10^{10} M_\odot/h) $ Cumulative mass accreted onto the BH in the quasar mode.
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 nearest-neighbor 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 $ (\gamma-1) \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 quasar-heated 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 cubic-spline 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 mass-over-time 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.

Tracer Quantities

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:

  • Gas -> Gas (finite volume fluxes, refinement, derefinement)
  • Gas -> Stars (star formation, both spawning new stars and converting cells into stars)
  • Stars -> Gas (stellar mass return)
  • Gas -> Wind (galactic scale stellar winds)
  • Wind -> Gas (recoupling stellar wind)
  • Gas -> BHs (black hole accretion)
  • BHs -> BHs (black hole mergers)
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.

Subboxes

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%

2. Group Catalogs

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 sub-files, 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:

  • "Group", "FoF Group'', and ``FoF Halo'' all refer to halos.
  • "Subgroup", ``Subhalo'', and ``Subfind Group'' all refer to subhalos.
  • The first (most massive) subgroup of each halo is the ``Primary Subgroup'' or ``Central Subgroup''.
  • All other following subgroups within the same halo are ``Secondary Subgroups'', or ``Satellite Subgroups''.

FoF Halos

The Group fields are derived with a standard friends-of-friends (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 Illustris-3 run)
GroupFirstSub int32 N - Index into the Subhalo table of the first/primary/most massive Subfind group within this FoF group. 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 - Mass-weighted 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 maximum bound particle. 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 - Mass-weighted 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 top-hat 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.

Subfind Subhalos

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 - Mass-weighted 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 - Mass-weighted 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 star-formation 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 maximum bound particle. 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 - Mass-weighted 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 summed-up 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 $ One-dimensional velocity dispersion of all the member particles/cells (the 3D dispersion divided by $\sqrt{3}$).
SubhaloVmax float32 N $ km/s $ Maximum value of the spherically-averaged rotation curve.
SubhaloVmaxRad float32 N $ kpc/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 wind-phase cells in this subhalo (with Type==4 and BirthTime<=0).

Header and Offsets

The following table describes the Header group of each file:

Field Type Description
SimulationName string e.g. 'Illustris-1' or 'Illustris-2-Dark'
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 Side-length 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 two-dimensional 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 one-dimensional 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 one-dimensional 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 one-dimensional 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 two-dimensional 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 two-dimensional 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).

3. Merger Trees

Merger trees have been created for the various Illustris simulations using SubLink (Rodriguez-Gomez+ 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 depth-first fashion. Numbering also indicates the on-disk storage ordering for the SubLink trees, which adopt the approach of Lemson+ (2006). For example, the main progenitor branch (from 5-7 in the example) and the full progenitor tree (from 5-13 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.

SubLink

The SubLink algorithm constructs merger trees at the subhalo level. A unique descendant is assigned to each subhalo in three steps (see Rodriguez-Gomez+ 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.

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 Illustris-1 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 "depth-first" 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 "depth-first" 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. DM-only or stars + star-forming 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. DM-only or stars + star-forming 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.

LHaloTree

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 Illustris-1 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.

4. Supplementary Data Catalogs

(a) Stellar Mocks: Multi-band Images and SEDs

A catalog of synthetic stellar images and integrated spectra of galaxies in Illustris-1 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) rest-frame 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.

(b) Photometric Non-Parametric Stellar Morphologies

A catalog of photometric non-parametric morphologies of Illustris-1 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, rest-frame 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 $M_\star \gt 10^{9.7} {\rm M}_\odot$ at $z=0$ in Illustris-1. 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 second-order 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 half-light radius $r_E$.

The four bands which replace {band_name} are: gSDSS, iSDSS, uSDSS, and hWFC3 (WFC3-IR/F160W). The four camera views are indexed 0, 1, 2, and 3.

(c) Stellar Circularities, Angular Momenta, Axis Ratios

A catalog for the circularities, angular momenta and axis ratios of the stellar component, for Illustris-1 galaxies (at all snapshots). Data is available for all subhalos with stellar mass (inside twice the stellar half mass radius) bigger than $10^9 {\rm M}_\odot $. For complete definitions on the calculation of each value, see the following table and Genel+ (2015), where they were presented and used. The first four quantities are calculated after alignment with the angular momentum vector of the stars within 10 times the stellar half-mass 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 z-axis is aligned with the angular momentum vector as described above. Then, for every stellar particle with specific angular momentum $J_z$ 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 ($=U_{\rm grav} + v^2$).

Group Name Units Description
/Snapshot_N/SubfindID - The Subfind IDs these values correspond to at this snapshot (27345 entries for snapshot 135).
/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, which are the eigenvalues of the mass tensor of the stellar mass inside the stellar $2R_{1/2}$. This means that 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 $R_j\equiv\sum\limits_i r_{j,i}^2$ is the distance of star $j$ from the centre of the galaxy.

Note: 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.

(d) Subhalo Matching To Dark

A catalog for cross-matching subhalos between the full physics (e.g. Illustris-1) and dark matter only (e.g. Illustris-1-Dark) 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 DM-only 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.

(e) Multi-Redshift Non-Parametric Stellar Morphologies

A catalog of photometric non-parametric morphologies of Illustris-1 galaxies across redshifts. This is an updated and extended version of the catalog described in "(B) Photometric Non-Parametric Stellar Morphologies", which was available only at $z=0$. This new catalog contains roughly CANDELS-depth measurements ("SB25") for two observed-frame filters: ACS-F606W (V-band) and WFC3-F160W (H-band).

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 second-order 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 half-light semi-major 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 signal-to-noise 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: ACS-F606W (V-band) and WFC3-F160W (H-band). The four camera views replacing {X} are indexed 0, 1, 2, and 3.

(f) Blackhole Mergers and Details

This catalog is comprised of two data sets: 'mergers' and 'details'. The mergers file contains a record of each BH-BH 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' BH--which 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 $M_\odot / yr$, gas densities in units of $(10^{10} M_\odot / h) / (ckpc / h)^3$, and gas sound speeds in units of $km / s$ (calculated as $c_s = \sqrt{\gamma (\gamma-1) {\rm BH\_U}}$). They were generated with the illustris_blackholes code. Citation recommmended to Kelley et al. (2016) and Blecha et al. (2016) as appropriate.

IMPORTANT NOTE: For both the mergers and details files, there is a missing data gap for the Illustris-1 simulation (only)! For Illustris-1, data is missing in the redshift range from approximately z = 0.14 to z = 0.38. For this range, roughly half of the BH data is missing, except at the higher-z end of the range, where nearly all of the mergers are missing around snapshots 110-111. This information was lost to data corruption and cannot be recovered. The lower resolution runs Illustris-2 and Illustris-3 are unaffected.

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: in-before, 1: out-before, 2: out-aft]. Frequently these details were not found, in which case the array values are zero.
/details/time (N,3) float Time (scale-factor) for each entry.
/details/mass (N,3) float Blackhole mass.
/details/mdot (N,3) float Blackhole mass accretion rate.
/details/rho (N,3) float Local gas density in the vicinity of the blackhole.
/details/cs (N,3) float Local gas sound-speed in the vicinity of the blackhole.
/time (N) float Time (scale-factor) 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 scale-factor) for each entry.
/mass (N) float Blackhole mass.
/mdot (N) float Blackhole mass accretion rate.
/rho (N) float Local gas density in the vicinity of the blackhole.
/cs (N) float Local gas sound-speed in the vicinity of the blackhole.

(g) Stellar Assembly

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 Rodriguez-Gomez et al. (2015), Rodriguez-Gomez et al. (2016a), and/or Rodriguez-Gomez et al. (2016b) as appropriate. Further information and usage examples are available in these same papers.

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):

  • StellarMassInSitu + StellarMassExSitu == StellarMassTotal
  • StellarMassBeforeInfall + StellarMassAfterInfall + StellarMassFormedOutsideGalaxies == StellarMassExSitu
  • StellarMassFromCompletedMergers + StellarMassFromOngoingMergers + StellarMassFromFlybys + StellarMassFormedOutsideGalaxies == StellarMassExSitu

Note the above descriptions make reference to two values computed for every star particle in the simulation. They are derived as follows (see references for more details):

  • AccretionOrigin: A value which can take the following integer values: 0, 1, and 2 for ex situ stellar particles that were accreted from completed mergers (i.e., when the subhalo in which the stellar particle formed has already merged with the current subhalo), ongoing mergers (i.e., when the subhalo in which the stellar particle formed has not yet merged with the current subhalo, but will do so at a later snapshot in the simulation), and flybys (i.e., when the subhalo in which the stellar particle formed has not merged with the current subhalo, and will not do so at any future snapshot in the simulation), respectively; and -1 if not applicable (i.e., if the particle was formed in situ or if it was formed outside of any subhalo). NOTE: towards the end of the simulation, it becomes impossible to distinguish a flyby from an ongoing merger. Therefore, cases (1) and (2) are usually considered as being part of the same category: "stripped from surviving galaxies".
  • MergerMassRatio: The stellar mass ratio of the merger in which a given ex-situ stellar particle was accreted (if applicable). The mass ratio is measured at the time when the secondary progenitor reaches its maximum stellar mass. NOTE: this quantity was calculated also in the case of flybys, without a merger actually happening.

A small caveat: thanks to the subhalo "switching" problem, some galaxies can have (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 in which this happens is negligible, but still noticeable in e.g. a scatter plot.

(h) Photometry, Morphology, Strong-lensing and Dynamics (PMSD)

This catalog contains measurements of galaxy photometry, morphology, strong lensing, and dynamics. There exists one pmsd.hdf5 file for Illustris-1. 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. (2016), where additional details are available. 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 half-stellar-mass radius, output of SUBFIND (converted to arcsec)
hgmr arcsec The 3D half-gas-mass radius, output of SUBFIND (converted to arcsec)
Rein_{x,y,z} arcsec The Einstein radius in {x,y,z}-projection; set to 0.0 if its physical scale is smaller than 710 pc (the softening length)
Rc50_{x,y,z} arcsec The radius within which the projected cumulative dark matter fraction is 50% in {x,y,z}-projection; set to 0.0 if its physical scale is smaller than 710 pc (the softening length); or set to 1E10 if it is larger than min(4xhsmr, 30pkpc)
Rl50_{x,y,z} arcsec The radius at which the projected local dark matter fraction is 50% in {x,y,z}-projection; set to 0.0 if its physical scale is smaller than 710 pc (the softening length)
Reff_ser_{x,y,z} arcsec The effective radius calculated by fitting Sersic profile in rest-frame SDSS r-band in {x,y,z}-projection
Reff_dev_{x,y,z} arcsec The effective radius calculated by fitting de Vaucouleurs profile in rest-frame SDSS r-band in {x,y,z}-projection
Reff_exp_{x,y,z} arcsec The effective radius calculated by fitting exponential profile in rest-frame SDSS r-band in {x,y,z}-projection
Reff_of_{bands}mod_{x,y,z} arcsec The Sersic-fit effective radius in observer-frame {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 observer-frame {band} in {x,y,z}-projection
Reff_rf_{bands}mod_{x,y,z} arcsec The Sersic-fit effective radius in rest-frame {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 rest-frame {band} in {x,y,z}-projection
Sersic_m_{x,y,z} - The Sersic index of the best-fitted Sersic profile measured in rest-frame SDSS r-band in {x,y,z}-projection
IRe_ser_{x,y,z} mag/arcsec^2 The surface brightness at Reff_ser_{x,y,z} in rest-frame SDSS r-band in {x,y,z}-projection
IRe_dev_{x,y,z} mag/arcsec^2 The surface brightness at Reff_dev_{x,y,z} in rest-frame SDSS r-band in {x,y,z}-projection
IRe_exp_{x,y,z} mag/arcsec^2 The surface brightness at Reff_exp_{x,y,z} in rest-frame SDSS r-band in {x,y,z}-projection
Mag_of_{bands}mod_{x,y,z} mag The total (absolute AB) magnitude in observer-frame {band} derived from best-fit Sersic model in {x,y,z}-projection
Mag_of_{bands}_{x,y,z} mag The total (absolute AB) magnitude in observer-frame {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 rest-frame {band} derived from best-fit Sersic model in {x,y,z}-projection
Mag_rf_{bands}_{x,y,z} mag The total (absolute AB) magnitude in rest-frame {band} derived from direct measurement within a projected radius of 30 pkpc from galaxy centre in {x,y,z}-projection
Mag{B,V}_{ep5,ep1,ep2}_{x,y,z} mag The rest-frame 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,ep1,ep2}_{x,y,z} - The axial ratio of the projected (rest-frame Johnson-V 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,ep1,ep2}_{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,ep1,ep2}_{x,y,z} radian The orientation angle of the projected (rest-frame Johnson-V 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,ep1,ep2}_{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 x-coordinate (in the plane of projection) of the light centre in {x,y,z}-projection, measured in rest-frame Johnson-V band and with respect to the centre of subhalo
Galyc_{x,y,z} arcsec The y-coordinate (in the plane of projection) of the light centre in {x,y,z}-projection, measured in rest-frame Johnson-V band and with respect to the centre of subhalo
TypeDec_{x,y,z} - Galaxy Type (in {x,y,z}-projection): 1 for early-type; 0 for late-type; -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 0.0 if Rein_{x,y,z} < 710 pc (the softening length)
Mstar{B,V}_{ep5,ep1,ep2}_{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,ep1,ep2}_{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
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 -1.0 if Rein_x < 710 pc (the softening length)
Fdm2_{ep5,ep1,ep2}_{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,ep1,ep2}_{x,y,z} - The 3D cumulative dark matter fraction within a 3D radius of {0.5,1.0,2.0}*Reff_rf_john_vmod_{x,y,z}
Fgas3_{ep5,ep1,ep2}_{x,y,z} - The 3D cumulative gas fraction within a 3D radius of {0.5,1.0,2.0}*Reff_rf_john_vmod_{x,y,z}
Fcgs3_{ep5,ep1,ep2}_{x,y,z} - The 3D 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 stellar-mass-weighted stellar line-of-sight 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 (rest-frame SDSS-r band) luminosity-weighted stellar line-of-sight mean velocity measured within a projected radius of 1.5 arcsec from galaxy centre in {x,y,z}-projection
Vmean_{ep5,ep1,ep2}_{x,y,z} km/s The (rest-frame SDSS-r band) luminosity-weighted stellar line-of-sight mean velocity measured within a projected radius of {0.5,1.0,2.0}*Reff_rf_sdss_rmod_{x,y,z} from galaxy centre in {x,y,z}-projection
Vsigma_mav_{x,y,z} km/s The stellar-mass-weighted stellar line-of-sight 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 (rest-frame SDSS-r band) luminosity-weighted stellar line-of-sight velocity dispersion measured within a projected radius of 1.5 arcsec from galaxy centre in {x,y,z}-projection
Vsigma_{ep5,ep1,ep2}_{x,y,z} km/s The (rest-frame SDSS-r band) luminosity-weighted stellar line-of-sight velocity dispersion measured within a projected radius of {0.5,1.0,2.0}*Reff_rf_sdss_rmod_{x,y,z} from galaxy centre in {x,y,z}-projection
Beta_mav_{x,y,z} - The stellar-mass-weighted 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 (rest-frame SDSS-r band) luminosity-weighted stellar orbital anisotropy parameter ({x,y,z} direction) measured within a 3D radius of 1.5 arcsec from galaxy centre
Beta_{ep5,ep1,ep2}_{x,y,z} - The (rest-frame SDSS-r band) luminosity-weighted stellar orbital anisotropy parameter measured within a 3D radius of {0.5,1.0,2.0}*Reff_rf_sdss_rmod_{x,y,z} from galaxy centre
slp3totMWinRein_{x,y,z} - The mass-weighted total density slope calculated using Eq. (1) of Dutton & Treu 2014, evaluated at a radius of Rein_{x,y,z} from galaxy centre
slpMWtot_{ep5,ep1,ep2}_{x,y,z} - The mass-weighted 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
slp3totAVGRein2RApt_{x,y,z} - The average total density slope calculated using Eq. (15) of the paper, between Rein_{x,y,z} and the aperture radius of 1.5 arcsec
slp3tot_{ep5,ep1,ep2}_{x,y,z} - The average total density slope calculated using Eq. (15) of the paper, in the radial range {0.2-0.5, 0.5-1.0, 0.5-2.0} times Reff_rf_john_vmod_{x,y,z}
slp3totPLfRein2RApt_{x,y,z} - The fitted power-law slope of the total density distribution between Rein_{x,y,z} and the aperture radius of 1.5 arcsec
slp3totPLf_{ep5,ep1,ep2}_{x,y,z} - The fitted power-law slope of the total density distribution in the radial range {0.2-0.5, 0.5-1.0, 0.5-2.0} 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 single-aperture stellar kinematics data of Vsigma_lav_{x,y,z}, assuming the stellar orbital anisotropy is given by Beta_lav_{x,y,z}; set to 1E10 for late-type galaxies
slpJEbeta0_{x,y,z} - The total density slope derived by combining strong lensing measurement of Mrein_{x,y,z} and single-aperture stellar kinematics data of Vsigma_lav_{x,y,z}, assuming isotropic stellar orbital distribution; set to 1E10 for late-type galaxies
slp3dm_{ep5,ep1,ep2}_{x,y,z} - The average dark matter density slope calculated using Eq. (15) of the paper, in the radial range {0.2-0.5, 0.5-1.0, 0.5-2.0} times Reff_rf_john_vmod_{x,y,z}
slp3dmPLf_{ep5,ep1,ep2}_{x,y,z} - The fitted power-law slope of the dark matter density distribution in the radial range {0.2-0.5, 0.5-1.0, 0.5-2.0} times Reff_rf_john_vmod_{x,y,z}
slp3st_{ep5,ep1,ep2}_{x,y,z} - The average stellar density slope calculated using Eq. (15) of the paper, in the radial range {0.2-0.5, 0.5-1.0, 0.5-2.0} times Reff_rf_john_vmod_{x,y,z}
slp3stPLf_{ep5,ep1,ep2}_{x,y,z} - The fitted power-law slope of the stellar density distribution in the radial range {0.2-0.5, 0.5-1.0, 0.5-2.0} 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 B-band, john_v = Johnson V-band, hst_b = HST B-F435w, hst_v = HST V-F606w hst_i = HST I-F814w, and sdss_* are the usual five SDSS bands.

Note: {ep5,ep1,ep2} correspond to either factors of {0.5,1.0,2.0}, or to radial ranges of {0.2-0.5, 0.5-1.0, 0.5-2.0}, 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:

  • {Reff,Mag}_rf_hst_* (Reff and magnitudes for rest-frame HST bands)
  • {Reff,Mag}_of_john_* (Reff and magnitudes for observer frame Johnson bands)
  • {Reff,Mag}_rf_sdss_{u,z}* (Reff and magnitudes for rest-frame SDSS-u and SDSS-z bands)

(i) Halo/galaxy angular momentum and baryon content

This catalog contains measurements of angular momenta and baryon content, measured separately for both halos (FoF groups) and galaxies (Subfind subhalos). For Illustris-1 and Illustris-1-Dark 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 (Ng is the number of groups in snapshot 135, while Ns is the number of subhalos):

Dataset Shape Units Description
GroupEkin Ng $10^{10} M_\odot/h \ (km/s)^2$ kinetic energy of FOF-haloes (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} Ng $10^{10} M_\odot/h \ (km/s)^2$ kinetic energy of SO-haloes (due to the particle and cell velocities; the total physical kinetic energy is obtained adding the thermal energy of the gas, if present.)
GroupEpot Ng $10^{10} M_\odot/h \ (km/s)^2$ potential energy of FOF-haloes
Group_Epot_{SO} Ng $10^{10} M_\odot/h \ (km/s)^2$ potential energy of SO-haloes
GroupEthr Ng $10^{10} M_\odot/h \ (km/s)^2$ thermal energy of the gas component of FOF-haloes
Group_Ethr_{SO} Ng $10^{10} M_\odot/h \ (km/s)^2$ thermal energy of the gas component of SO-haloes
Group_J Ng,3 $10^{10} M_\odot/h \ kpc \ km/s$ total angular momentum of FOF-haloes
Group_J_{SO} Ng,3 $10^{10} M_\odot/h \ kpc \ km/s$ total angular momentum of SO-haloes
Group_Jdm Ng,3 $10^{10} M_\odot/h \ kpc \ km/s$ angular momentum of the dark matter component of FOF-haloes
Group_Jdm_{SO} Ng,3 $10^{10} M_\odot/h \ kpc \ km/s$ angular momentum of the dark matter component of SO-haloes
Group_Jgas Ng,3 $10^{10} M_\odot/h \ kpc \ km/s$ angular momentum of the gas component of FOF-haloes
Group_Jgas_{SO} Ng,3 $10^{10} M_\odot/h \ kpc \ km/s$ angular momentum of the gas component of SO-haloes
Group_Jstars Ng,3 $10^{10} M_\odot/h \ kpc \ km/s$ angular momentum of the stellar component of FOF-haloes
Group_Jstars_{SO} Ng,3 $10^{10} M_\odot/h \ kpc \ km/s$ angular momentum of the stellar component of SO-haloes
Group_CMFrac Ng - total counter-rotating mass fraction of FOF-haloes
Group_CMFrac_{SO} Ng - total counter-rotating mass fraction of SO-haloes
Group_CMFracType Ng,6 - counter-rotating mass fractions per type: dark matter, gas, stars
Group_CMFracType_{SO} Ng,6 - counter-rotating mass fractions per matter type in SO-haloes
Group_LenType_{SO} Ng,6 - number of particles/cells of each matter type in SO-haloes
Group_MassType_{SO} Ng,6 $10^{10} M_\odot / h$ mass per matter type in SO-haloes
SubhaloEkin Ns $10^{10} M_\odot/h \ (km/s)^2$ 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 Ns $10^{10} M_\odot/h \ (km/s)^2$ potential energy of subhaloes
SubhaloEthr Ns $10^{10} M_\odot/h \ (km/s)^2$ thermal energy of the gas component of subhaloes
Subhalo_J{rad} Ns,3 $10^{10} M_\odot/h \ kpc \ km/s$ total angular momentum of subhaloes
Subhalo_Jdm{rad} Ns,3 $10^{10} M_\odot/h \ kpc \ km/s$ angular momentum of the dark matter component of subhaloes
Subhalo_Jgas{rad} Ns,3 $10^{10} M_\odot/h \ kpc \ km/s$ angular momentum of the gas component of subhaloes
Subhalo_Jstars{rad} Ns,3 $10^{10} M_\odot/h \ kpc \ km/s$ angular momentum of the stellar component of subhaloes
Subhalo_CMFrac{rad} Ns - total counter-rotating mass fraction of subhaloes
Subhalo_CMFracType{rad} Ns,6 - counter-rotating 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 friends-of-friends (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 top-hat 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").