An Extremely Massive Dry Galaxy Merger in a Moderate Redshift Cluster

TL;DR

This paper reports the discovery of the largest dry galaxy merger observed at moderate redshift, involving a central galaxy and surrounding companions, shedding light on BCG and intracluster light formation.

Contribution

It presents the identification and detailed analysis of an extremely massive dry galaxy merger at z=0.39, including imaging, spectral, and redshift data, revealing insights into galaxy evolution.

Findings

The merger involves a plume with luminosity comparable to a 4L* galaxy.

The merger timescale for the companion galaxies is approximately 110 Myr.

The cluster's properties are consistent with a moderate-mass cluster undergoing a major dry merger.

Abstract

We have identified perhaps the largest major galaxy merger ever seen. While analysing Spitzer IRAC images of CL0958+4702, an X-ray selected cluster at z=0.39, we discovered an unusual plume of stars extending $\gtrsim$110 kpc outward from the bright central galaxy (BCG). Three galaxies 1-1.5 mag fainter than the BCG lie within 17 kpc (projected) of the BCG and are probably participating in the merger. The plume is detected in all four IRAC channels and at optical wavelengths in images from the WIYN telescope; the surface brightness is remarkably high ($\mu_r\approx$24.8 mag arcsec$^{-2}$ at 50 kpc). The optical and infrared colors are consistent with those of other BCGs, suggesting that the plume is composed of old stars and negligible recent star formation (hence a "dry merger"). The luminosity in the plume is at least equivalent to a 4L^* galaxy. A diffuse halo extending 110 kpc from the BCG in one IRAC image suggests the total amount of diffuse light is L_r\sim 1.3x10^{11}h^{-2} L_sun. A Chandra observation shows an X-ray image and spectrum typical of moderate-mass clusters. We use MMT/Hectospec to measure 905 redshifts in a 1 deg^2 region around the cluster. The velocities of two of the BCG companions indicate a merger timescale for the companion galaxies of $\sim$110 Myr and $\sim$0.5-1 Gyr for the plume. We conclude that the BCG and intracluster light of CL0958 is formed by major mergers at moderate redshifts. After the major merger is complete, CL0958 will likely become a fossil cluster.