Recent star formation in local, morphologically disturbed spheroidal galaxies on the optical red sequence

TL;DR

This study combines UV and optical data to reveal that many morphologically disturbed, red sequence spheroidal galaxies have recent, merger-induced star formation, which impacts their UV colours and stellar populations.

Contribution

It provides new evidence linking tidal features to recent star formation in red sequence galaxies and offers a method to estimate the age of the last star formation event.

Findings

Over 70% of bulge-dominated galaxies show tidal features.

Bluer UV colours correlate with stronger tidal distortions.

Merger-induced star formation contributes less than 10% of stellar mass.

Abstract

We combine GALEX (ultra-violet; UV) and SDSS (optical) photometry to study the recent star formation histories of ~100 field galaxies on the optical red sequence, a large fraction of which exhibit widespread signs of disturbed morphologies in deep optical imaging that are consistent with recent merging events. More than 70% of bulge-dominated galaxies in this sample show tidal features at a surface brightness limit of 28 mag arcsec^-2. We find that, while they inhabit the optical red sequence, they show a wide spread in their UV colours (~4 mags), akin to what has been discovered recently in the general early-type population. A strong correlation is found between UV colour and the strength of the tidal distortions, such that the bluest galaxies are more distorted. This strongly suggests that the blue UV colours seen in many nearby early-types are driven by (low-level) merger-induced star formation within the last 3 Gyrs, contributing less than 10% of the stellar mass. If the ongoing mergers in this sample, which have a median mass ratio of 1:4, are representative of the nearby red merger population, then less than 25% of the new stellar mass in the remnants is typically added through merger-induced star formation. While the dust extinction in the inter-stellar medium (ISM) in these galaxies is small [E(B-V)<0.1], the local dust content of the star-forming regions is, on average, a factor of 3 higher. Finally, we use our theoretical machinery to provide a recipe for calculating the age of the most recent star formation event (t2) in nearby (z<0.1) red early-type galaxies: Log (t2) [Gyrs] ~ 0.6.[(NUV-u)-(g-z)-1.73], where NUV, u, g and z are the observed photometric magnitudes of the galaxies in the GALEX/SDSS filtersets.