RXJ1648.7+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, RXJ1648.7+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. RXJ1648 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, RXJ1648 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([OII]) and EW(Hdelta)) than other group galaxies. The 4-5 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 RXJ1648's mass even if 5-50% of the light is lost to an intracluster light component (ICL). The LX-TX 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 LX-sigmav and sigmav-TX 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.