Public Data Access Overview / Data Specifications

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
• Contents
• Tracer Quantities
• Subboxes
• 2. Group Catalogs
• FoF Halos
• Subfind Subhalos
• Header and Offsets
• 3. Merger Trees
• SubLink
• LHaloTree
• 4. Supplementary Data Catalogs
• (a) Stellar Mocks: Multi-band Images and SEDs
• (b) Photometric Non-Parametric Stellar Morphologies
• (c) Stellar Circularities, Angular Momenta, Axis Ratios
• (d) Subhalo Matching To Dark
• (e) Multi-Redshift Non-Parametric Stellar Morphologies
• (f) Blackhole Mergers and Details
• (g) Stellar Assembly
• (h) Photometry, Morphology, Strong-lensing and Dynamics (PMSD)
• (i) Halo/galaxy angular momentum and baryon content
• (j) Mock Deep Field Lightcones (HST, JWST, WFIRST)
• (l) SKIRT Synthetic Images and Optical Morphologies
• (m) Star Formation Rates

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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:

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:

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.

The complete snapshot field listings, including dimensions, units and descriptions, are given for all the particles types in the following large table.

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)

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.

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.

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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):

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.

Header and Offsets

The following table describes the Header group of each file:

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.

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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.

Note that this merger tree comes in two varieties: "dark-matter based" and "baryonic based". The dark-matter 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 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.

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:

TreeX Groups:

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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:

(*) 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 $M_\star \gtrsim 10^{9.7} {\rm M}_\odot$ 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.

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 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 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 $J_z$ 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:

• Available for all baryonic simulations: TNG50-1, TNG50-2, TNG50-3, TNG50-4, TNG100-1, TNG100-2, TNG100-3, TNG300-1, TNG300-2, TNG300-3, Illustris-1 (all snapshots/redshifts).
• Restricted to subhalos with stellar mass $M^\star > 3.4 \times 10^8 M_\odot$ (TNG50/TNG100/Illustris) or $M^\star > 10^{10} M_\odot$ (TNG300), measured within the usual twice stellar half mass radius, and at least 100 stars.

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.

(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).

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 $10.22 M_\odot / \rm{yr}$, 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:

• Available for all TNG baryonic runs, as well as all original Illustris baryonic runs.
• Black hole mergers: TNG50-1, TNG50-2, TNG50-3, TNG50-4, TNG100-1, TNG100-2, TNG100-3, TNG300-1, TNG300-2, TNG300-3.
• Black hole details: TNG50-1, TNG50-2, TNG50-3, TNG50-4, TNG100-1, TNG100-2, TNG100-3, TNG300-1, TNG300-2, TNG300-3.
• IMPORTANT NOTE: For both the mergers and details files, there are a few small missing data gaps. In general these files are fragile and may contain inconsistencies. In TNG100-1, this includes redshift periods [0.96 - 0.98], [1.18 - 1.20], [1.27 - 1.40], [2.06 - 2.48]. For Illustris-1, data is missing in the redshift range from approximately z = 0.14 to z = 0.38, where 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. All other runs are unaffected. This information was lost to data corruption and cannot be recovered, and appropriate care should be taken in analysis of this data.

Blackhole mergers:

Blackhole details:

(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.

Simulation and snapshot coverage:

• Available for all baryonic simulations:
• TNG50-1, TNG50-2, TNG50-3, TNG50-4, TNG100-1, TNG100-2, TNG100-3, TNG300-1, TNG300-2, TNG300-3, Illustris-1, Illustris-2, Illustris-3 (all snapshots/redshifts).
• All subhalos with nonzero stellar mass.

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

The above descriptions make reference to two values computed for every star particle in the simulation, 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: 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.

(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. (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:

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,e1,e2} 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 (N_g is the number of groups in snapshot 135, while N_s is the number of subhalos):

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").

(j) Mock Deep Field Lightcones (HST, JWST, WFIRST)

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 multi-extension FITS files. The filenames all begin with "hlsp_misty_illustris_", and then a string as: {instrument}_{field}_{filter}_v1_lightcone.fits where:

• {instrument} = "hst-acs", "hst-wfc3", "jwst-miri", "jwst-nircam", or "wfirst-wfidrm15"
• {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, "mag30-fielda-11-10" 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 rest-frame g-band apparent magnitude of the source, such that g ≤ 30.0.

The FITS files contain the following HDUs:

• "IMAGE_NOPSF": Raw image without PSF or noise.
• "SimulationAssumptions": URL links to Illustris documentation and API.
• "MockDataAssumptions": Header contains code and parameter choices for creating mock images.
• "IMAGE_PSF" : [Optional] Same image, now convolved with model PSF from TinyTim or WebbPSF.
• "MODELPSF" : [Optional] The PSF model at the same pixel scale as HDU 3 "IMAGE_PSF".
• "Catalog" : Lightcone Catalog, containing intrinsic simulation info, including galaxy ID numbers, image positions, and other info.
• "CatalogDocumentation": Strings containing explainers of catalog columns.

At present, all associated data can be downloaded from the STScI MAST Data Product Page.

(l) SKIRT Synthetic Images and Optical Morphologies

This catalog contains synthetic images and the corresponding morphological measurements, including Gini-M20, CAS and MID statistics, as well as 2D Sersic fits. The full description of the synthetic images and measurements can be found in Rodriguez-Gomez+ (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 Pan-STARRS and SDSS, respectively.

Simulation and snapshot coverage:

• Available for: TNG50-1, TNG100-1, and Illustris-1.
• For each, redshifts: z=0, z=0.05.
• Restricted to subhalos with (total) stellar mass M^* > 10^9.5 solar masses.

There are two ways to download this data.

• The entire dataset (for one survey/snapshot combination) can be downloaded as a single tar file (above).
• The FITS and PNG images can be downloaded separately for each subhalo for which they are available through the API, similar to the original Illustris FITS files. For example, /api/TNG100-1/snapshots/99/subhalos/397866/skirt/broadband_sdss.fits downloads the broadband FITS file matched to the SDSS survey, for the central subhalo of FoF group #500 in TNG100-1 at redshift zero.

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 i-band, g-band, (and r-band), respectively (using Pan-STARRS 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 Rodriguez-Gomez+ (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 Pan-STARRS or SDSS. Note that the SDSS z-band data should be used with caution (they are reasonable for morphologies, but not for magnitudes and colors), since the long-wavelength 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 Pan-STARRS (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 Rodriguez-Gomez+ (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 Pan-STARRS and 0.276 ckpc/h for SDSS.

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2022 addition: KiDS Survey Mock Images. A new, third set of images were generated with the same procedure as in Rodriguez-Gomez et al. (2019), except that:

1. the field-of-view is fixed for all images (does not scale with galaxy size), and
2. neighboring galaxies (from the same FoF group) were included.

The image settings are consistent with the Kilo-Degree Survey (KiDS). In particular, each FITS file has 4 layers corresponding to OmegaCAM g,r,i,z filters.

Simulation and snapshot coverage:

• Available for: TNG50-1.
• Redshift: z=0.0337 (snapshot 96) only.
• Restricted to subhalos with (total) stellar mass M^* > 10^8.0 solar masses.

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 Guzmán-Ortega 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.

(m) Star Formation Rates

A catalog of time-averaged, 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 time-averaged 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:

• Available for all baryonic simulations: TNG50-1, TNG50-2, TNG50-3, TNG50-4, TNG100-1, TNG100-2, TNG100-3, TNG300-1, TNG300-2, TNG300-3, Illustris-1, Illustris-2, Illustris-3.
• All snapshots/redshifts.
• Restricted to subhalos with a reasonable number of stellar particles (>~ 100).

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