Date Published:November 1, 2006
We present a detailed investigation into the recent star formationhistories of 5697 luminous red galaxies (LRGs) based on the Hδ(4101 Å), and [OII] (3727 Å) lines and the D4000 index. LRGsare luminous (L > 3L*) galaxies which have been selected to havephotometric properties consistent with an old, passively evolvingstellar population. For this study, we utilize LRGs from the recentlycompleted 2dF-SDSS LRG and QSO Survey (2SLAQ). Equivalent widths of theHδ and [OII] lines are measured and used to define three spectraltypes, those with only strong Hδ absorption (k+a), those withstrong [OII] in emission (em) and those with both (em+a). All other LRGsare considered to have passive star formation histories. The vastmajority of LRGs are found to be passive (~80 per cent); however,significant numbers of k+a (2.7 per cent), em+a (1.2 per cent) and emLRGs (8.6 per cent) are identified. An investigation into the redshiftdependence of the fractions is also performed. A sample of SDSS MAINgalaxies with colours and luminosities consistent with the 2SLAQ LRGs isselected to provide a low-redshift comparison. While the em and em+afractions are consistent with the low-redshift SDSS sample, the fractionof k+a LRGs is found to increase significantly with redshift. Thisresult is interpreted as an indication of an increasing amount of recentstar formation activity in LRGs with redshift. By considering theexpected lifetime of the k+a phase, the number of LRGs which willundergo a k+a phase can be estimated. A crude comparison of thisestimate with the predictions from semi-analytic models of galaxyformation shows that the predicted level of k+a and em+a activities isnot sufficient to reconcile the predicted mass growth for massive earlytypes in a hierarchical merging scenario.