The Late Stellar Assembly of Massive Cluster Galaxies Via Major Merging

Details The Late Stellar Assembly of Massive Cluster Galaxies Via Major Merging The Late Stellar Assembly of Massive Cluster Galaxies Via Major Merging

2008-06-26

arxiv.org/abs/0806.4387v1

Astronomy and Astrophysics

Astrophysics

Accepted for publication in ApJ Letters

Scientific paper

10.1086/591422

We present multi-wavelength observations of the brightest galaxies in four X-ray luminous groups at z~0.37 that will merge to form a cluster comparable in mass to Coma. Ordered by increasing stellar mass, the four brightest group galaxies (BGGs) present a time sequence where BGG-1, 2, and 3 are in merging systems and BGG-4 is a massive remnant [M(stars)=6.7x10^(11) Msun]. BGG-1 and 2 have bright, gravitationally bound companions and BGG-3 has two nuclei separated by only 2.5 kpc, thus merging at z<0.5 increases the BGG mass by >40% (merging timescale<2 Gyr) and V-band luminosity by ~0.4 mag. The BGGs' rest-frame (B-V) colors correspond to stellar ages of >3 Gyr, and their tight scatter in (B-V) color [sigma(BV)=0.032] confirms they formed the bulk of their stars at z>0.9. Optical spectroscopy shows no signs of recent (<1.5 Gyr) or ongoing star formation. Only two BGGs are weakly detected at 24 microns, and X-ray and optical data indicate the emission in BGG-2 is due to an AGN. All four BGGs and their companions are early-type (bulge-dominated) galaxies, and they are embedded in diffuse stellar envelopes up to ~140 kpc across. The four BGG systems must evolve into the massive, red, early-type galaxies dominating local clusters. Our results show that: 1) massive galaxies in groups and clusters form via dissipationless merging; and 2) the group environment is critical for this process.

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