Astrophysical Journal Letters
Galaxies: clusters: general, Galaxies: elliptical and lenticular, cD, Galaxies: evolution
We present multiwavelength 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* = 6.7 × 1011 M☉). 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% (tMGR < 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 (σBV = 0.032) confirms that 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 μm, and X-ray and optical data indicate that 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.
Tran, Kim-Vy H.; Moustakas, John; Gonzalez, Anthony H.; Bai, Lei; Zaritsky, Dennis; and Kautsch, Stefan, "The Late Stellar Assembly of Massive Cluster Galaxies via Major Merging" (2008). Chemistry and Physics Faculty Articles. 111.