2008/06/26 by Kim‐Vy Tran, Kim-Vy H. Tran, John Moustakas +5
Physics and Astronomy · #Astronomy and Astrophysical Research #Galaxies: Formation, Evolution, Phenomena #Stellar, planetary, and galactic studies #astro-ph
paper · pdf · doi:10.1086/591422
Accepted for publication in ApJ Letters
arxiv created 2008/06/26 · openalex publication_date 2008/07/22 · arxiv updated 2009/12/01 · openalex created_date 2020/11/23 · openalex updated_date 2026/07/31
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 × 10 11 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% ( t MGR < 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 (σ BV = 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.