The Fundamental Planes of E+A galaxies and GALEX UV-excess early-type galaxies: revealing their intimate connection

Abstract

Strong Balmer absorption lines and the lack of Hα and [O ii] emission lines signify that E+A galaxies 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 early-type galaxies. 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 Galaxy Evolution Explorer (GALEX) that an unexpectedly large fraction of early-type galaxies exhibit ultraviolet (UV) excess (i.e. blue UV – optical colours) as a sign of recent star formation (RSF), we investigate the possible connection of the UV-excess galaxies to E+As. In particular, we examine the Fundamental Plane (FP) scaling relations of the currently largest sample of ∼1000 E+As selected from the Sloan Digital Sky Survey (SDSS) and ∼20 000 morphologically selected SDSS early-type galaxies with GALEX UV data. The FP parameters, combined with stellar population indicators, reveal a certain group of UV-excess early types that bridge between E+As and quiescent red galaxies. The newly identified galaxies are the post-starburst systems characterized by UV-excess but no Hα emission. This is essentially a conceptual generalization 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 (named after ‘E+A’), which stand 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 per cent 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 (i.e. old ‘E+As’) represents the most recent arrival to the red sequence in the final phase of the E+A to red early-type transition.