Intrinsic colors and ages of extremely red elliptical galaxies at high redshift

TL;DR

This study analyzes the intrinsic colors and ages of extremely red elliptical galaxies at high redshift, revealing their stars formed when the universe was about 2 billion years old, with little dependence on redshift or mass.

Contribution

It provides a statistical analysis of 333 old galaxy candidates at 0.8<z<2.3, deriving average ages and color gradients using a publicly available synthesis model.

Findings

Stars in these galaxies formed when the universe was ~2 Gyr old.

No significant dependence of formation age on redshift or stellar mass.

Massive galaxies' stellar populations formed earlier or simultaneously with less massive ones.

Abstract

In order to know the formation epoch of the oldest elliptical galaxies as a function of mass and observed redshift, a statistical analysis for 333 extremely red objects (EROs) classified as old galaxies (OGs) at 0.8<z<2.3 is carried out. Once we get M_V and (B-V) at rest for each galaxy, we calculate the average variation of this intrinsic color with redshift and derive the average age through a synthesis model (the code for the calculation of the age has been made publicly available). The average gradient of the (B-V) color at rest of EROs/OGs is 0.07-0.10 Gyr^{-1} for a fixed luminosity. The stars in these extremely red elliptical galaxies were formed when the Universe was ~2 Gyr old on average. We have not found a significant enough dependence on the observed redshift and stellar mass: dt_{formation}/dt_{observed}=-0.46+/-0.32, dt_{formation}/(d log_10 M_*)=-0.81+/-0.98 Gyr. This fits a scenario in which the stellar formation of the objects that we denominate as EROs-OGs is more intense at higher redshifts, at which the stellar populations of the most massive galaxies form earlier than or at the same time as less massive galaxies.