The effect of baryons on the positions and velocities of satellite galaxies in the MTNG simulation

TL;DR

This work quantifies how baryons alter the positions and velocities of satellite galaxies in MTNG by directly comparing the hydrodynamic run to its dark-matter-only counterpart and by robustly matching satellites via their merger histories. Using a velocity- and radius-dependent framework, the authors show satellites in the hydro run are shifted inward by about $3$–$4\%$ and move more slowly in the inner regions, yielding $\sim10\%$ differences in clustering at $\sim0.1\,h^{-1}{\rm Mpc}$, though overall velocity dispersions are broadly similar. They develop a practical baryonification scheme (MTNG-DMO$_{mod}$) to mimic baryonic effects in a DMO simulation, reproducing MTNG clustering with good accuracy, and validate these corrections against a suite of zoom-in runs that explore baryonic physics variations. Crucially, the study finds selection effects (e.g., choosing by stellar mass, luminosity, or SFR) induce much larger clustering changes than baryons, underscoring that sample definition is the dominant source of uncertainty when constructing mocks, though baryons must still be accounted for in precision cosmology.

Abstract

Mock galaxy catalogues are often constructed from dark-matter-only simulations based on the galaxy-halo connection. Although modern mocks can reproduce galaxy clustering to some extent, the absence of baryons affects the spatial and kinematic distributions of galaxies in ways that remain insufficiently quantified. We compare the positions and velocities of satellite galaxies in the MTNG hydrodynamic simulation with those in its dark-matter-only counterpart, assessing how baryonic effects influence galaxy clustering and contrasting them with the impact of galaxy selection, i.e. the dependence of clustering on sample definition. Using merger trees from both runs, we track satellite subhaloes until they become centrals, allowing us to match systems even when their z=0 positions differ. We then compute positional and velocity offsets as functions of halo mass and distance from the halo centre, and use these to construct a subhalo catalogue from the dark-matter-only simulation that reproduces the galaxy distribution in the hydrodynamic run. Satellites in the hydrodynamic simulation lie 3-4% closer to halo centres than in the dark-matter-only case, with an offset that is nearly constant with halo mass and increases toward smaller radii. Satellite velocities are also systematically higher in the dark-matter-only run. At scales of 0.1 Mpc/h, these spatial and kinematic differences produce 10-20% variations in clustering amplitude -- corresponding to 1-3$σ$ assuming DESI-like errors -- though the impact decreases at larger scales. These baryonic effects are relevant for cosmological and lensing analyses and should be accounted for when building high-fidelity mocks. However, they remain smaller than the differences introduced by galaxy selection, which thus represents the dominant source of uncertainty when constructing mocks based on observable quantities.

Figures (12)

• Figure 1: Cumulative distribution of $V_{\rm peak}$ for subhaloes in the MTNG-DMO simulation. We define $V_{\rm peak}$ as the maximum circular velocity ($V_{\rm max}$) reached by a subhalo during its lifetime. The upper axis shows the corresponding stellar mass of these haloes after matching them to their counterparts in the hydrodynamic MTNG simulation.
• Figure 2: (Top) Median (solid lines) and 16th–84th percentile region (shaded area) of the mean radial distance between satellite galaxies and the halo centre as a function of halo mass. Results from the MTNG-DMO simulation are shown in black, and those from the hydrodynamic MTNG run in green. Subhaloes were selected according to their $V_{\rm peak}$ for a number density of ${\rm n_{den}=0.0316}~ h^{3}{\rm Mpc}^{-3}$ in MTNG-DMO, and the corresponding matched galaxies in MTNG were identified following the procedure described in Section \ref{['sec:matched']}. Distances are shown as a function of the MTNG-DMO halo masses. The black dashed line represents the value of one virial radius $r_{200c}$. (Bottom) Median (solid lines) and 16th–84th percentile region (shaded area) of the ratio of mean distances between matched haloes in the MTNG and MTNG-DMO simulations. The green solid line and shaded area correspond to the galaxy sample with number density $0.0316~ h^{3}{\rm Mpc}^{-3}$ (same as in the top panel), while the dashed and dotted lines represent galaxy samples with number densities of $0.01$ and $0.00316~ h^{3}{\rm Mpc}^{-3}$. Only the subhaloes successfully matched between the two simulations are shown here.
• Figure 3: Illustration of three satellite galaxies infalling into a halo of $\sim10^{13.5}, h^{-1}{\rm M_{ \odot}}$. Black circles show the satellite positions in the MTNG-DMO simulation, while green circles indicate their matched counterparts in the hydrodynamic MTNG run. The symbol sizes scale with the mass of the satellites. The dashed grey line represents one virial radius.
• Figure 4: Density of satellite galaxies as a function of distance from the halo centre for the $\rm n_{den}=0.0316~ h^{3}{\rm Mpc}^{-3}$ sample. The left, middle, and right panels show density profiles for halo masses of $10^{13.5}$, $10^{14}$, and $10^{14.5}~ h^{-1}{\rm M_{ \odot}}$, respectively. Black lines represent results from the MTNG-DMO simulation, while green lines correspond to the hydrodynamic MTNG simulation. The vertical dashed lines represent the value of one virial radius $r_{200c}$.
• Figure 5: (Top) Median (solid lines) and 16th–84th percentile region (shaded area) of the satellite velocity dispersion as a function of halo mass for a number density of $\rm n_{den}=0.0316~ h^{3}{\rm Mpc}^{-3}$. Results from the MTNG-DMO and MTNG simulations are shown in black and green, respectively. (Bottom) Median (solid lines) and 16th–84th percentile region (shaded area) of the ratio of velocity dispersions between matched haloes in the MTNG and MTNG-DMO simulations, for the three number densities used in this work (as labelled).