RXJ1648.7+6109: Witnessing the Formation of a Massive Group/Poor Cluster and its Brightest Galaxy

TL;DR

This study observes a young, forming massive galaxy group at intermediate redshift, revealing its early stage of assembly, the properties of its galaxy population, and how its X-ray and dynamical relations compare to low-redshift systems.

Contribution

It provides detailed observations of a forming galaxy group, highlighting its unique features and predicting the future evolution of its brightest galaxy, while analyzing its scaling relations at intermediate redshift.

Findings

RXJ1648 is in early formation stage with elongated X-ray emission.

The central galaxy string is likely to merge into a BCG by z=0.

Intermediate-redshift groups show similar L_X-T_X relations to low-redshift systems.

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(H_delta)) 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 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-sigma_v and sigma_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.