Exploring the formation of spheroidal galaxies out to z~1.5 in GOODS

TL;DR

This study investigates the formation and evolution of massive spheroidal galaxies up to redshift 1.5, revealing stable number densities, the presence of blue cores, and consistent structural relations over time.

Contribution

It provides new insights into the internal colour gradients and the constancy of the Kormendy relation in spheroidal galaxies across a significant redshift range.

Findings

Number density of massive galaxies remains stable up to z~1.5.

Approximately 25% of early-types are blue, with blue cores indicating inside-out formation.

The Kormendy relation slope does not evolve significantly from z=0 to z=1.4.

Abstract

(Abridged) The formation of massive spheroidal galaxies is studied on a visually classified sample of 910 galaxies extracted from the ACS/HST images of the GOODS North and South fields (0.4<z<.5). Three key observables are considered: comoving number density, internal colour distribution; and the Kormendy relation. The comoving number density of the most massive galaxies is found not to change significantly with redshift. One quarter of the whole sample of early-types are photometrically classified as blue galaxies. On a volume-limited subset out to z<0.7, the average stellar mass of the blue ellipticals is 5E9Msun compared to 4E10Msun for red ellipticals. On a volume-limited subsample of bright galaxies (Mv<-21) out to z=1.4 we find only 4% are blue early-types, in contrast with 26% for the full sample. The intrinsic colour distribution correlates overall bluer colours with **blue cores** (positive radial gradients of colour), suggesting an inside-out process of formation. The redshift evolution of the observed colour gradients is incompatible with a significant variaton in stellar age within each galaxy. The slope of the Kormendy relation in the subsample of massive galaxies does not change between z=0 and z=1.4.