Strong Balmer absorption lines and the lack of Ha and [OII] emission lines signify that E+As are post-starburst systems. Recent studies suggest that E+As may undergo the transition from the `blue cloud to the `red sequence and eventually migrate to red sequence ETGs. An observational validation of this scenario is to identify the intervening galaxy population between E+As and the red-sequence. Motivated by recent findings with GALEX that a large fraction of ETGs exhibit UV-excess as a sign of RSF, we investigate the possible connection of the UV-excess galaxies to E+As. In particular, we examine the FP scaling relations of the largest sample of ~1,000 E+As selected from the SDSS and ~20,000 morphologically-selected SDSS ETGs with GALEX UV data. The FP parameters, combined with stellar population indicators, reveal a certain group of UV-excess ETGs that bridges between E+As and quiescent red galaxies. The newly identified galaxies are the post-starburst systems characterized by UV-excess but no Ha emission. This is a conceptual generalisation of E+A, in that the Balmer absorption line in the E+A definition is replaced with UV-optical colours that are far more sensitive to RSF than the Balmer lines. We refer to these UV-excess galaxies as E+a galaxies, which stands for elliptical (E) galaxies with a minority of A-type (a) young stars. The species are either (1) galaxies that experienced starbursts weaker than those observed in E+As (1~10% of E+As, mild E+As) or (2) the products of passively evolved E+As after quenching star formation quite a while ago (~1 Gyr, old E+As). We suggest that the latter type of E+a galaxies represents the most recent arrival to the red sequence in the final phase of the E+A to red early-type transition. (Abridged)
تحميل البحث