The evolution of the scatter of the cosmic average color-magnitude relation: Demonstrating consistency with the ongoing formation of elliptical galaxies

TL;DR

This study measures the evolution of the scatter in the color-magnitude relation of early-type galaxies from redshift 1 to now, supporting models of ongoing galaxy formation through star formation quenching.

Contribution

It provides the first measurements of CMR scatter evolution and demonstrates consistency with models of continuous addition of galaxies to the red sequence via star formation truncation.

Findings

CMR scatter is ~0.1mag at 0.05<z<0.75

Scatter is higher at z=1

Model predictions match observed CMR scatter evolution

Abstract

We present first measurements of the evolution of the scatter of the cosmic average early-type galaxy color-magnitude relation (CMR) from z=1 to the present day, finding that it is consistent with models in which galaxies are constantly being added to the red sequence through truncation of star formation in blue cloud galaxies. We used a sample of over 700 red sequence, structurally-selected early-type galaxies (defined to have Sersic index >2.5) with redshifts 0<z<1 taken from the Extended Chandra Deep Field South (173 galaxies) and the Sloan Digital Sky Survey (550 galaxies), constructing rest-frame U-V colors accurate to <0.04mag. We find that the scatter of the CMR of cosmic average early-type galaxies is ~0.1mag in rest-frame U-V color at 0.05<z<0.75, and somewhat higher at z=1. We compared these observations with a model in which new red sequence galaxies are being constantly added at the rate required to match the observed number density evolution, and found that this model predicts the correct CMR scatter and its evolution. Furthermore, this model predicts approximately the correct number density of `blue spheroids' - structurally early-type galaxies with blue colors - albeit with considerable model dependence. Thus, we conclude that both the evolution of the number density and colors of the early-type galaxy population paint a consistent picture in which the early-type galaxy population grows significantly between z=1 and the present day through the quenching of star formation in blue cloud galaxies.