The late stellar assembly of massive cluster galaxies via major merging

Kim Vy H. Tran, John Moustakas, Anthony H. Gonzalez, Lei Bai, Dennis Zaritsky, Stefan J. Kautsch

Research output: Contribution to journalArticlepeer-review


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 x 1011 M&o.). BGG-1 and 2 have bright, gravitationally bound companions and BGG-3 has two nuclei separated by only 2.5 kpc; thus, merging atz < 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 (σBV p 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 mm, 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.

Original languageEnglish (US)
Pages (from-to)L17-L20
JournalAstrophysical Journal
Issue number1 PART 2
StatePublished - 2008


  • Cd
  • Clusters
  • Elliptical and lenticular
  • Evolution
  • Galaxies
  • General

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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