2dF–SDSS LRG and QSO (2SLAQ) Survey: Evolution of the Most Massive Galaxies

Abstract

The 2dF-SDSS LRG and QSO (2SLAQ) survey is a new survey of distant Luminous Red Galaxies (LRGs) and faint quasars selected from the Sloan Digital Sky Survey (SDSS) multi-color photometric data and spectroscopically observed using the 2dF instrument on the Anglo-Australian Telescope (AAT). In total, the 2SLAQ survey has measured over 11000 LRG redshifts, covering 180deg^2 of SDSS imaging data, from 87 allocated nights of AAT time. Over 90% of these galaxies are within the range 0.45<z<0.7 and have luminosities consistent with >=3L*. When combined with the lower redshift SDSS LRGs, the evolution in the luminosity function of these LRGs is fully consistent with that expected from a simple passive (luminosity) evolution model. This observation suggests that at least half of the LRGs seen at z~0.2 must already have more than half their stellar mass in place by z~0.6, i.e., our observations are inconsistent with a majority of LRGs experiencing a major merger in the last 6 Gyrs. However, some "frosting" (i.e., minor mergers) has taken place with ~5% of LRGs showing some evidence of recent and/or on-going star-formation, but it only contributes ~1% of their stellar mass.