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

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 (Δm 12) between the brightest and second-brightest galaxies as an indicator. From 20 deg2 of the 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 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 Δm 12 = 2.5 mag) with other groups in the literature, we find a stratification of the satellite number by Δm 12 for a given host stellar mass. The satellite number for the given host stellar mass decreases as Δm 12 increases. The same trend is found in simulated galaxy groups in the TNG50 simulation of the IllustrisTNG project. We also find that the host galaxies in groups with a smaller Δm 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 increase as Δm 12 increases. These results show that the large scatter in the satellite number is consistent with a large range of Δm 12, indicating diverse group assembly histories.