Calibrating Galaxy Infall Times in Groups and Clusters with IllustrisTNG Simulations

Abstract

The time since a galaxy first became a satellite is central to understanding how environment drives galaxy evolution, yet it cannot be measured directly. Using the TNG300 and TNG-Cluster simulations, we track satellites from $z=1$ to $z=0$ and derive a simple, redshift-dependent prescription for ${T}_{\rm{inf}}$ based on position in projected phase space and stellar mass, via symbolic regression. The resulting calibration provides continuous, observation-ready estimates of infall time across projected phase space. In projected phase space, ${T}_{\rm{inf}}$ is often well described by two components, and we provide analytic expressions for the corresponding characteristic timescales. This framework can be applied directly to spectroscopic samples to infer environmental histories in galaxy groups and clusters.

Figures (12)

• Figure 1: Distributions of halo masses (top panel) and stellar masses (bottom panel) in our sample. The red histogram in the bottom panel includes galaxies in groups and clusters with $M_{\rm h} > 10^{13}M_\odot$, while the blue histogram contains the full sample of galaxies, which includes galaxies in lower-mass halos that are not part of our selection
• Figure 2: Projected phase-space (PPS) diagrams colour-coded by infall time, $T_{\rm inf}$, for all galaxies in the sample. In the top panel, $T_{\rm inf}$ is defined as the time when a galaxy first crosses $3R_{200}$; in the bottom panel, it is defined at $R_{200}$. The corresponding radii are marked by blue circles.
• Figure 3: Fraction of interloper galaxies in projected phase space (PPS) at $z=0$. Interlopers are defined as galaxies located between $3R_{200}$ and $10R_{200}$ in 3D space but projected within $3R_{200}$. The contamination fraction peaks at large projected radii ($r \gtrsim 2$) and high relative velocities, above and to the right of the black dashed and dotted lines. The dashed line marks $r=2$, and the dotted line marks $r+v=4$, above which there are too few galaxies in our sample for robust statistical analysis, as explained in Sec. \ref{['sec:distribs']}.
• Figure 4: Comparison between the true infall time measured directly from the simulation and the value predicted by Eq. (\ref{['eq:Tinf']}) at $z=0$. In the top panel, each dot corresponds to the mean value of a grid cell (grid dimensions of 20x20x6 in $(r, v, M_\ast)$). The 2D histogram in the bottom panel shows the distribution for individual galaxies. In both cases, the coefficients $A$, $B$, and $C$ are those fitted using the full (interloper-free) galaxy sample. The black lines indicate the one-to-one relation.
• Figure 5: RMSE in the prediction of infall time as a function of position in PPS for the $z=0$ sample.