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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 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 $Δm_{12} = 2.5$ 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 given host stellar mass decreases as $Δm_{12}$ 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 $Δm_{12}$ (like NGC 4437) have assembled their halo mass more recently than those in larger gap groups, and that their stellar-to-halo mass ratios (SHMRs) 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.