Snapshots
Snapshots contain the most complete description of the simulation available. They contain a large number of properties for each resolution element of the simulation. Each snapshot provides a "picture" of the simulation at a given cosmic time.
THESAN provides 80 of such snapshots. The scale factor and redshift at which they are produced are listed under Snapshot times below (click to open).
Note: THESAN includes two DMonly runs that were run to z=0. For these, at \( z<5.5 \) we provide snapshots every \( \Delta z = 0.25 \). The simulations actually include significantly more snapshots, but they are not provided for storage reason, but are available upon request. We do, however, provide all group catalogs available and the merger trees are constructed at the highest time resolution possible.
Snapshot times ▾ 

Snapshot 
scale factor 
redshift 

0  0.0476  20.00 
1  0.0495  19.20 
2  0.0514  18.47 
3  0.0532  17.81 
4  0.0550  17.19 
5  0.0567  16.63 
6  0.0585  16.11 
7  0.0602  15.62 
8  0.0618  15.17 
9  0.0635  14.75 
10  0.0651  14.35 
11  0.0668  13.98 
12  0.0684  13.63 
13  0.0699  13.30 
14  0.0715  12.98 
15  0.0731  12.69 
16  0.0746  12.41 
17  0.0761  12.14 
18  0.0776  11.89 
19  0.0791  11.64 
20  0.0806  11.41 
21  0.0820  11.19 
22  0.0835  10.98 
23  0.0849  10.78 
24  0.0863  10.59 
25  0.0877  10.40 
26  0.0891  10.22 
Snapshot 
scale factor 
redshift 

27  0.0905  10.05 
28  0.0919  9.88 
29  0.0933  9.72 
30  0.0946  9.57 
31  0.0960  9.42 
32  0.0973  9.28 
33  0.0986  9.14 
34  0.0999  9.01 
35  0.1013  8.88 
36  0.1026  8.75 
37  0.1039  8.63 
38  0.1051  8.51 
39  0.1064  8.40 
40  0.1077  8.29 
41  0.1090  8.18 
42  0.1102  8.07 
43  0.1115  7.97 
44  0.1127  7.87 
45  0.1139  7.78 
46  0.1152  7.68 
47  0.1164  7.59 
48  0.1176  7.50 
49  0.1188  7.42 
50  0.1200  7.33 
51  0.1212  7.25 
52  0.1224  7.17 
53  0.1236  7.09 
Snapshot 
scale factor 
redshift 

54  0.1248  7.01 
55  0.1260  6.94 
56  0.1271  6.87 
57  0.1283  6.79 
58  0.1295  6.72 
59  0.1306  6.66 
60  0.1318  6.59 
61  0.1329  6.52 
62  0.1341  6.46 
63  0.1352  6.40 
64  0.1363  6.34 
65  0.1375  6.28 
66  0.1386  6.22 
67  0.1397  6.16 
68  0.1408  6.10 
69  0.1419  6.05 
70  0.1430  5.99 
71  0.1441  5.94 
72  0.1452  5.89 
73  0.1463  5.83 
74  0.1474  5.78 
75  0.1485  5.73 
76  0.1496  5.69 
77  0.1506  5.64 
78  0.1517  5.59 
79  0.1528  5.55 
80  0.1538  5.50 
header
group, whose attributes contain general information about the simulation setup and the current snapshot. The available attributes are listed here (click to open). Header attributes ▾ 

Attribute  Dimensions  Units  Description 

BoxSize  1  \( ckpc / h \)  Spatial extent of the periodic box (in comoving units). 
HubbleParam  1    Little \( h \), defined as \( h = H_0 / (100 km/s/Mpc) \) 
MassTable  6  \( 10^{10} M_\odot / h \)  Masses of particle types which have a constant mass (only DM). 
NumFilesPerSnapshot  1    Number of file chunks per snapshot. 
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 \(2^{32}\) 
Omega0  1    The cosmological density parameter for matter. 
OmegaBaryon  1    The cosmological density parameter for baryons. 
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. 
UnitLength_in_cm  1    Length unit used in the simulation, expressed in comoving cm. 
UnitLength_in_g  1    Mass unit used in the simulation, expressed in g. 
UnitLength_in_cm_per_s  1    Velocity unit used in the simulation, expressed in comoving cm / s. 
PartTypeN
(where N is the type number). Inside each one of these groups, each simulated property is saved in a separate dataset. For example, the positions of all dark matter (PartType2
) particles in a single file are saved in the dataset PartType2/Positions
. The list of available datasets for each particle type is shown in the tables below (click to open). NOTE: Most fields have identical meaning to those in the IllustrisTNG outputs. Those that differ have the tag (modified), while the new ones show the tag (new).
Part Type 0 (gas) ▾ 

Dataset  Dim.  Units  Description 

CenterOfMass  (N,3)  \( ckpc / h \)  Spatial position of the center of mass, which in general differs from the geometrical center of the Voronoi cell (the offset should be small). Comoving coordinates. 
Coordinates  (N,3)  \( ckpc / h \)  Spatial position within the periodic simulation domain of BoxSize. Comoving coordinate. 
Density  N  \( 10^{10} M_\odot h^2 / ckpc^3 \)  Comoving mass density of cell (calculated as mass/volume). 
ElectronAbundance  N    Fractional electron number density with respect to the total hydrogen number density, so \( n_e = \) ElectronAbundance \( \times n_H \), where \(n_H = X_H \) \( \rho / m_p \). Use with caution for starforming gas (see comment below for NeutralHydrogenAbundance ). 
GFM_DustMetallicity (new)  N    Dusttogas ratio of the cell. The dust mass contained in the cell is therefore \( M_d = M_{gas} \times \) GFM_DustMetallicity . 
GFM_Metallicity (modified)  N    The ratio \( M_Z / M_{total} \) where \( M_Z \) is the total mass all metal elements (above He). It is not in solar units. To convert to solar metallicity, divide by 0.0127 (the primordial solar metallicity). Note: \( M_Z \) includes (metals locked in) dust. 
GFM_Metals (modified)  (N,9)    Individual abundances of nine species: H, He, C, N, O, Ne, Mg, Si, Fe (in this order). Each is the dimensionless ratio of mass in that species to the total gas cell mass. Unlike IllustrisTNG, the untracked metals are not stored separately. Their mass can be inferred from the total metal mass (computed from the gas metallicity). Note: Abundances include the metals locked in dust. 
HI_Fraction (new)  N    HI fraction of the cells, as tracked by the nonequilibrium thermochemistry module coupled to the radiation transport solver. 
HeI_Fraction (new)  N    HeI fraction of the cells, as tracked by the nonequilibrium thermochemistry module coupled to the radiation transport solver. 
HeII_Fraction (new)  N    HeII fraction of the cells, as tracked by the nonequilibrium thermochemistry module coupled to the radiation transport solver. 
InternalEnergy  N  \( (km/s)^2 \)  Internal (thermal) energy per unit mass for this gas cell. This field holds the corrected values, and is generally recommended for all uses. 
MagneticField  (N,3)  \( (h/a^2) \sqrt{}\)UnitPressure 
The (comoving) magnetic field 3vector (x,y,z) of this gas cell. Multiply by \( h/a^2 \) to obtain physical code units, then by UnitPressure \( ^{1/2} = ( \)UnitMass /UnitLength \( )^{1/2} \)/UnitTime \(= (10^{10} M_\odot / kpc)^{1/2} \times (km/s) / kpc = 2.60 \times 10^{6} \) to obtain CGS units (Gauss). 
MagneticFieldDivergence  N  \( (h^3/a^2) \) \( (10^{10} M_\odot)^{1/2} \) \( (km/s) \)\( (ckpc)^{5/2} \)  The divergence of the magnetic field in this cell. Units are (area)*MagneticFieldUnits/(volume). 
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. 
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. 
PhotonDensity (new)  (N,3)  # \( / (ckpc / h)^3 / 10^{63} \)  Density of photons in the cells in the three spectral bands tracked. 
Potential  N  \( (km/s) / a \)  Gravitational potential energy. 
StarFormationRate  N  \( M_\odot / yr \)  Instantaneous star formation rate of the gas cells. 
SubfindHsml  N  \( ckpc / h \)  The comoving radius of the sphere centered on this cell enclosing the \(64 \pm 1\) nearest dark matter particles. 
Velocities  (N,3)  \( km \sqrt{a} / s \)  Spatial velocity. Multiply this value by \(\sqrt{a}\) to obtain peculiar velocity. 
Part Type 1 (dark matter) ▾ 

Dataset  Dim.  Units  Description 

Coordinates  (N,3)  \( ckpc / h \)  Spatial position within the periodic simulation domain of BoxSize. Comoving coordinate. 
ParticleIDs  N    The unique ID (uint64) of this star particle. Constant for the duration of the simulation. 
Potential  N  \( (km/s) / a \)  Gravitational potential energy. 
SubfindHsml  N  \( ckpc / h \)  The comoving radius of the sphere centered on this cell enclosing the \(64 \pm 1\) nearest dark matter particles. 
Velocities  (N,3)  \( km \sqrt{a} / s \)  Spatial velocity. Multiply this value by \(\sqrt{a}\) to obtain peculiar velocity. 
Part Type 4 (stars) ▾ 

Dataset  Dim.  Units  Description 

BirthDensity (new)  N  \( 10^{10} M_\odot h^2 / ckpc^3 \)  Comoving mass density where this star particle initially formed. 
BirthPos  (N,3)  \( ckpc / h \)  Spatial position within the periodic box where this star particle initially formed. Comoving coordinate. 
(N,3)  \( (km/s) / a \)  Spatial velocity of the parent starforming gas cell at the time of formation. The peculiar velocity is obtained by multiplying this value by \(\sqrt{a}\).  
Coordinates  (N,3)  \( ckpc / h \)  Spatial position within the periodic simulation domain of BoxSize. Comoving coordinate. 
GFM_DustMetallicity (new)  N    Dusttogas ratio of the stellar particle. This value is inherited directly from the parent gas cell and never changed. 
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 (modified)  N    See entry under PartType0. Inherited from the gas cell spawning/converted into this star, at the time of birth. 
GFM_Metals (modified)  (N,9)    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 \( ( \leq 0 ) \) 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. 
ParticleIDs  N    The unique ID (uint64) of this star particle or wind phase cell. Constant for the duration of the simulation. 
Potential  N  \( (km/s) / a \)  Gravitational potential energy. 
SubfindHsml  N  \( ckpc / h \)  The comoving radius of the sphere centered on this cell enclosing the \(64 \pm 1\) nearest dark matter particles. 
Velocities  (N,3)  \( km \sqrt{a} / s \)  Spatial velocity. Multiply this value by \(\sqrt{a}\) to obtain peculiar velocity. 
Part Type 5 (black holes) ▾ 

Dataset  Dim.  Units  Description 

BH_CumEgyInjection_QM  N  \( 10^{10} M_\odot / h \)\( (ckpc/h)^2 \)\( /(0.978 Gyr/h)^2 \)  Cumulative amount of thermal AGN feedback energy injected into surrounding gas in the high accretionstate (quasar) mode, total over the entire lifetime of this blackhole. Field summed during BHBH merger. 
BH_CumEgyInjection_RM  N  \( 10^{10} M_\odot / h \)\( (ckpc/h)^2 \)\( /(0.978 Gyr/h)^2 \)  Cumulative amount of kinetic AGN feedback energy injected into surrounding gas in the low accretionstate (wind) mode, total over the entire lifetime of this blackhole. Field summed during BHBH merger. 
BH_CumMassGrowth_QM  N  \(10^{10} M_\odot / h\)  Cumulative amount of kinetic AGN feedback energy injected into surrounding gas in the low accretionstate (wind) mode, total over the entire lifetime of this blackhole. Field summed during BHBH merger. 
BH_CumMassGrowth_RM  N  \(10^{10} M_\odot / h\)  Cumulative mass accreted onto the BH in the high accretionstate (quasar) mode, total over the entire lifetime of this blackhole. Field summed during BHBH merger. 
BH_Density  N  \( 10^{10} M_\odot h^2 / ckpc^3 \)  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 128, 256 (for the THESAN2 and THESAN1 series, respectively) \(\pm 4\) nearestneighbor gas cells around the BH. Used for the BH feedback (excluding radiation). 
BH_MPB_CumEgyHigh (new)  N  \( 10^{10} M_\odot / h \)\( (ckpc/h)^2 \)\( /(0.978 Gyr/h)^2 \)  Identical to BH_CumEgyInjection_QM but only computed along the main progenitor branch, defined as the branch containing the most massive progenitor of each BH merger. 
BH_MPB_CumEgyLow (new)  N  \( 10^{10} M_\odot / h \)\( (ckpc/h)^2 \)\( /(0.978 Gyr/h)^2 \)  Identical to BH_CumEgyInjection_RM but only computed along the main progenitor branch, defined as the branch containing the most massive progenitor of each BH merger. 
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_Mdot  N  \( 10^{10} M_\odot / h \)\( /(0.978 Gyr/h) \)  The mass accretion rate onto the black hole, instantaneous. 
BH_MdotBondi  N  \( 10^{10} M_\odot / h \)\( /(0.978 Gyr/h) \)  Current estimate of the Bondi accretion rate for this BH. Calculated as \( \dot{M}_{Bondi} = ( \alpha 4 \pi G^2 M^2_{BH} \rho ) / (c^2_s + v^2_{BH})^{3/2} \) with \( \alpha=1 \) and \( v_{BH}=0 \) for THESAN. 
BH_MdotEddington  N  \( 10^{10} M_\odot / h \)\( /(0.978 Gyr/h) \)  Current estimate of the Eddington accretion rate for this BH. Calculated as \( \dot{M}_{edd} = ( 4\pi G M_{BH} m_p)/(\epsilon_r \sigma_T c) \) where \(\epsilon_r=0.2\) is the radiative efficieny parameter. 
BH_PhotonHsml (new)  N  \( ckpc/h \)  The comoving radius of the sphere enclosing the \( 64 \pm 4 \) nearestneighbor gas cells around the BH. Used for the injection of photons from the BH. 
BH_Pressure  N  \( 10^{10} M_\odot / h \)\( (ckpc/h)^2 \)\( /(0.978 Gyr/h)^2 \)  Physical gas pressure (in comoving units) near the BH, defined as \( (\gamma1) \rho u \), where \( \rho \) is the local comoving gas density (BH_Density , as above) \(u\) is BH_U (defined below). If this pressure is lower than the reference gas pressure, \(P_{ref}\), the BH accretion rate is lowered by \( (P_{ext}/P_{ref})^2 \). 
BH_Progs  N    Total number of BHs that have merged into this BH. 
BH_U  N  \( (km/s)^2 \)  Thermal energy per unit mass in quasarheated bubbles near the BH. Used to define the BH_Pressure . Not to be confused with the "radio mode" bubbles injected via the unified feedback model. 
Coordinates  (N,3)  \( ckpc/h \)  Spatial position within the periodic simulation domain of BoxSize. Comoving coordinate. 
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 ). 
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. 
SubfindHsml  N  \( ckpc/h \)  The comoving radius of the sphere centered on this blackhole particle enclosing the \(64 \pm 1 \) nearest dark matter particles. 
Velocities  (N,3)  \( km \sqrt{a} /s \)  Spatial velocity. Multiply this value by \(\sqrt{a}\) to obtain peculiar velocity. 
Dataset attributes
Attribute  Description 

a_scaling  Scaling of the dataset with expansion factor \( a \). 
h_scaling  Scaling of the dataset with little h \( (h = H_0 / 100 km/s/Mpc ) \). 
length_scaling 
Scaling of the dataset with the unit length of the simulations (available in the snapshot header as UnitLength_in_cm ).

mass_scaling 
Scaling of the dataset with the unit mass of the simulations (available in the snapshot header as UnitMass_in_g ).

to_cgs  Conversion factor from internal units to cgs units (Note that this does not convert from comoving to physical units!). 
velocity_scaling 
Scaling of the dataset with the unit velocity of the simulations (available in the snapshot header as UnitVelocity_in_cm_per_s ).

Particle order
PartType
.
Note: There is no guarantee that particles belonging to the same halo are stored in the same file. There is also no gurantee that particles of different types belonging to the same FoF group are stored in the same chunk file.