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

TL;DR

This study explores the connection between E+A galaxies and UV-excess early-type galaxies, revealing a transitional population called E+a galaxies that bridge the gap in galaxy evolution from starburst to quiescent states.

Contribution

It introduces the concept of E+a galaxies as a new class bridging E+As and red sequence ETGs, based on FP relations and UV-optical colors, highlighting their role in galaxy evolution.

Findings

Identification of UV-excess ETGs bridging E+As and quiescent galaxies.

E+a galaxies are either mild starburst remnants or evolved E+As.

E+a galaxies likely represent the final phase of transition to the red sequence.

Abstract

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)