The UV colours of high-redshift early-type galaxies: evidence for recent star formation and stellar mass assembly over the last 8 billion years

TL;DR

This study uses UV photometry and galaxy morphology data to show that high-redshift early-type galaxies have experienced recent star formation, contributing significantly to their stellar mass assembly over the last 8 billion years.

Contribution

It provides the first large-scale quantitative analysis of recent star formation in early-type galaxies at 0.5<z<1, revealing ongoing star formation across luminosities.

Findings

Only ~1.1% of early-types are passively evolving since z=2.

Early-types show 5-13% recent star formation mass fraction between z=0.5 and 1.

Luminous early-types may form 10-15% of their mass since z=1, less luminous ones up to 60%.

Abstract

We combine deep UBVRIzJK photometry from the Multiwavelength Survey by Yale-Chile (MUSYC) with redshifts from the COMBO-17 survey to perform a large-scale study of the rest-frame ultraviolet (UV) properties of 674 high-redshift (0.5<z<1) early-type galaxies, drawn from the Extended Chandra Deep Field South (E-CDFS). Galaxy morphologies are determined through visual inspection of Hubble Space Telescope (HST) images taken from the GEMS survey. We harness the sensitivity of the UV to young (<1 Gyr old) stars to quantify the recent star formation history of early-type galaxies across a range of luminosities (-23.5 < M(V) < -18). Comparisons to simple stellar populations forming at high redshift indicate that only ~1.1 percent of early-types in this sample are consistent with purely passive ageing since z=2. Parametrising the recent star formation (RSF) in terms of the mass fraction of stars less than a Gyr old, we find that the early-type population as a whole shows a typical RSF between 5 and 13% in the redshift range 0.5<z<1. Early-types on the UV red sequence show RSF values less than 5% while the reddest early-types are virtually quiescent with RSF values of ~1%. We find compelling evidence that early-types of all luminosities form stars over the lifetime of the Universe, although the bulk of their star formation is already complete at high redshift. This tail-end of star formation is measurable and not negligible, with luminous (-23<M(V)<-20.5) early-types potentially forming 10-15% of their mass since z=1, with their less luminous (M(V)>-20.5) counterparts potentially forming 30-60 percent of their mass in the same redshift range. (abridged)