RX J1648.7+6109: Witnessing the Formation of a Massive Group/Poor Cluster and Its Brightest Galaxy

Abstract

Using deep Chandra and optical spectroscopic observations, we investigate an intriguing young massive group, RX J1648.7+6109, at z = 0.376, and we combine these observations with previous measurements to fit the scaling relations of intermediate-redshift groups and poor clusters. RX J1648 appears to be in an early stage of formation; while it follows X-ray scaling relations, its X-ray emission is highly elongated, and it lacks a central, dominant BCG. Instead, RX J1648 contains a central string of seven bright galaxies, which have a smaller velocity dispersion, are on average brighter, and have less star formation [lower EW([O II]) and EW(Hδ)] than other group galaxies. The four to five brightest galaxies in this string should sink to the center and merge through dynamical friction by z = 0, forming a BCG consistent with a system of RX J1648's mass even if 5%-50% of the light is lost to an intracluster light component. The L_X - T_X relation for intermediate-redshift groups/poor clusters is very similar to the low-redshift cluster relation and consistent with the low-redshift group relation. In contrast, the L_X - σ_v and σ_v - T_X relations reveal that intermediate-redshift groups/poor clusters have significantly lower velocity dispersions for their X-ray properties compared to low-redshift systems; however, the intermediate-redshift relations are currently limited to a small range in luminosity.