On the formation of massive galaxies: A simultaneous study of number density, size and intrinsic colour evolution in GOODS

TL;DR

This study investigates the evolution of massive early-type galaxies in GOODS, finding little change in their number density, size, and intrinsic colour from redshift 1.2 to 0, suggesting limited evolution over half the universe's age.

Contribution

It provides a simultaneous analysis of number density, size, and intrinsic colour evolution of massive galaxies, combining HST/ACS data with 2dFGRS results, and discusses implications for galaxy formation models.

Findings

Massive galaxies (>3E11 Msun) show no significant change in number density or size from z=1.2 to 0.

Massive galaxies have homogeneous intrinsic colours with red cores and small scatter.

Size evolution is consistent with models linking dissipation during mergers to size changes.

Abstract

The evolution of number density, size and intrinsic colour is determined for a volume-limited sample of visually classified early-type galaxies selected from the HST/ACS images of the GOODS North and South fields (version 2). The sample comprises 457 galaxies over 320 arcmin2 with stellar masses above 3E10 Msun in the redshift range 0.4<z<1.2. Our data allow a simultaneous study of number density, intrinsic colour distribution and size. We find that the most massive systems (>3E11 Msun) do not show any appreciable change in comoving number density or size in our data. Furthermore, when including the results from 2dFGRS, we find that the number density of massive early-type galaxies is consistent with no evolution between z=1.2 and 0, i.e. over an epoch spanning more than half of the current age of the Universe. Massive galaxies show very homogeneous **intrinsic** colour distributions, featuring red cores with small scatter. The distribution of half-light radii -- when compared to z=0 and z>1 samples -- is compatible with the predictions of semi-analytic models relating size evolution to the amount of dissipation during major mergers. However, in a more speculative fashion, the observations can also be interpreted as weak or even no evolution in comoving number density **and size** between 0.4<z<1.2, thus pushing major mergers of the most massive galaxies towards lower redshifts.