Evidence for the Evolution of Young Early-Type Galaxies in the GOODS/CDF-S Field

TL;DR

This study develops a photometric method to identify young early-type galaxies and finds evidence that their starburst activity and fraction increase with redshift, supporting early assembly in massive galaxies.

Contribution

The paper introduces a new photometric technique for selecting young early-type galaxy candidates and analyzes their evolution across redshifts and masses.

Findings

Higher starburst mass fractions at greater redshifts.

Larger young elliptical fractions among lower-luminosity galaxies.

Total early-type galaxy fraction increases with redshift.

Abstract

We have developed an efficient photometric technique for identifying young early-type galaxy candidates using a combination of photometric redshifts, spectral-type classification, and optical/near-infrared colors. Applying our technique to the GOODS HST/ACS and VLT/ISAAC data we have selected a complete and homogeneous sample of young elliptical candidates among early-type field galaxies. The distribution of structural parameters for these candidates shows that their selection, which is based on early spectral types, is fully consistent with early morphological types. We investigate the evolution of their luminosities and colors as a function of redshift and galaxy mass and find evidence for an increasing starburst mass fraction in these young early-type galaxy candidates at higher redshifts, which we interpret in terms of massive field galaxies experiencing more massive/intense starbursts at higher redshifts. Moreover, we find indications for a systematically larger young elliptical fraction among sub-L*/2 early-type galaxies compared to their brighter counterparts. The total fraction among the field early-type galaxies increases with redshift, irrespective of galaxy luminosity. Our results are most consistent with galaxy formation scenarios in which stars in massive early-type field galaxies are assembled earlier than in their low-mass counterparts.