Group Therapy for Halos: Advancing Halo Mass Estimation for Galaxy Groups

Abstract

Accurate estimation of dark matter halo masses for galaxy groups is central to studies of galaxy evolution and for leveraging group catalogues as cosmological probes. We present a calibration and evaluation of two complementary halo mass estimators: a dynamical estimator based on the virial theorem, and an empirical relation between the sum of the stellar masses of the three most massive group galaxies and the halo mass (SHMR). Using state-of-the-art semi-analytic models (SHARK, SAGE, and GAEA) to generate mock light-cone catalogues, we quantify the accuracy, uncertainty, and model dependence of each method. The calibrated virial theorem achieves negligible systematic bias (mean $\Delta$ = -0.01 dex) and low scatter (mean $\sigma$ = 0.20 dex) with no sensitivity to baryonic physics. The calibrated SHMR yields the highest precision (mean $\Delta$ = 0.02 dex, mean $\sigma$ = 0.14 dex) but shows greater model dependence due to sensitivity to baryonic physics across the models. We demonstrate applications to observational catalogues, including the empirical halo mass function and mapping quenched fractions in the stellar mass-halo mass plane. We provide guidance: the virial theorem is recommended for GAMA-like surveys (i < 19.2) at z < 0.1 where minimal model dependence is required, while the SHMR is optimal for high-precision halo mass estimates across diverse catalogues with limits of z < 0.3. These calibrated estimators will aid upcoming wide-area spectroscopic surveys in probing the connection between galaxies and their host dark matter halos.