INTRINSIC COLORS AND AGES OF EXTREMELY RED ELLIPTICAL GALAXIES AT HIGH REDSHIFT

摘要

In order to know the formation epoch of the oldest elliptical galaxies as a function of mass and observed redshift,a statistical analysis of 333 extremely red objects (EROs) classified as old galaxies (OGs) at 0.8 o z 2.3is carried out. Once we get My and (B — V) at rest for each galaxy, we calculate the average variation of thisintrinsic color with redshift and derive the average age through a synthesis model (the code for the calculationof the age has been made publicly available). The average gradient of the (B — V) color at rest of EROs/OGsis 0.07-0.10 Gyr~(-1)for a fixed luminosity. The stars in these extremely red elliptical galaxies were formed when mass:the universeand stewasllar 2 Gyr old on average. We have not found a significant enough dependence on the observedredshift dtfix, = —0.46 ± 0.32), (fodtrmabon dtobserved 0.81 ± 0.98) Gyr. This fits a scenario in which' d logio the stellar formation of the objects that we denominate as EROS—OGs is more intense at higher redshifts, at whichthe stellar populations of the most massive galaxies form earlier than or at the same time as less massive galaxies.