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

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 (Δ m 12 ) between the brightest and second-brightest galaxies as an indicator. From 20 deg 2 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 Δ m 12 = 2.5 mag) with other groups in the literature, we find a stratification of the satellite number by Δ m 12 for a given host stellar mass. The satellite number for the given host stellar mass decreases as Δ m 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 Δ 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 increase as Δ m 12 increases. These results show that the large scatter in the satellite number is consistent with a large range of Δ m 12 , indicating diverse group assembly histories.