Early-type galaxies at intermediate redshift observed with HST WFC3: perspectives on recent star-formation

TL;DR

This study analyzes the stellar populations of 102 early-type galaxies at intermediate redshift using HST WFC3 data, revealing recent star formation activity and its implications for galaxy evolution.

Contribution

It provides new insights into the recent star formation history and size-mass evolution of early-type galaxies at intermediate redshifts through combined spectral energy distribution modeling.

Findings

~40% of ETGs show evidence of recent star formation.

Massive star-forming ETGs tend to have larger sizes.

Quiescent ETGs with companions occupy distinct size-mass regions.

Abstract

We present an analysis of the stellar populations of 102 visually-selected early-type galaxies (ETGs) with spectroscopic redshifts (0.3<z<1.5) from observations in the Early Release Science program with the Wide Field Camera 3 (WFC3) on {\it Hubble Space Telescope} (HST). We fit one- and two-component synthetic stellar models to the ETGs UV-optical-near-IR spectral energy distributions and find a large fraction (~40%) are likely to have experienced a minor (f$\lesssim$10% of stellar mass) burst of recent (t$\lesssim$1 Gyr) star-formation. The measured ages and mass fraction of the young stellar populations do not strongly trend with measurements of galaxy morphology. We note that massive (log(M[$M_{\odot}$])>10.5) recently star-forming ETGs appear to have larger sizes. Furthermore, high-mass, quiescent ETGs identified with likely companions populate a distinct region in the size-mass parameter space, in comparison with the distribution of massive ETGs with evidence of RSF. We conclude that both mechanisms of the quenching of star-formation in disk-like ETGs and (gas-rich, minor) merger activity contribute to the formation of young stars and the size-mass evolution of intermediate redshift ETGs. The number of ETGs for which we have both HST WFC3 panchromatic (especially UV) imaging and spectroscopically-confirmed redshifts is relatively small, therefore a conclusion on the relative roles of both of these mechanisms remains an open question.