A Rich Satellite Population of the NGC 4437 Group and Implications of a Magnitude Gap for Galaxy Group Assembly History

TL;DR

This study investigates how the number of satellite galaxies around Milky Way-like galaxies relates to the galaxy group assembly history, using observations of NGC 4437 and simulations, revealing a correlation with the magnitude gap.

Contribution

It introduces the use of the $r$-band magnitude gap as an indicator of galaxy group assembly history and confirms its correlation with satellite number and halo mass assembly in observations and simulations.

Findings

Satellite number decreases with increasing magnitude gap.

Groups with smaller magnitude gaps assembled their halos more recently.

Stellar-to-halo mass ratios increase with larger magnitude gaps.

Abstract

Both observations and cosmological simulations have recently shown that there is a large scatter in the number of satellites of Milky Way (MW)-like galaxies. In this study, we investigate the relation between the satellite number and galaxy group assembly history, using the $r-$band magnitude gap ($\Delta m_{12}$) between the first and the second brightest galaxy as an indicator. From 20 deg$^2$ of Hyper Suprime-Cam Subaru Strategic Program Wide layer, we identify 17 dwarf satellite candidates around NGC 4437, a spiral galaxy with about one-fourth of the MW stellar mass. We estimate their distances using the surface brightness fluctuation (SBF) method. Then we confirm five candidates as members of the NGC 4437 group, resulting in a total of seven group members. Combining the NGC 4437 group (with $\Delta m_{12} = 2.5$ mag) with other groups in the literature, we find a stratification of the satellite number by $\Delta m_{12}$ for a given host stellar mass. The satellite number for given host stellar mass decreases as $\Delta m_{12}$ increases. The same trend is found in simulated galaxy groups in IllustrisTNG50 simulations. We also find that the host galaxies in groups with a smaller $\Delta m_{12}$ (like NGC 4437) have assembled their halo mass more recently than those in larger gap groups, and that their stellar-to-halo mass ratios (SHMRs) increase as $\Delta m_{12}$ increases. These results show that the large scatter in the satellite number is consistent with a large range of $\Delta m_{12}$, indicating diverse group assembly histories.