The Age of Ellipticals and the Color-Magnitude Relation

TL;DR

This study uses new narrowband observations and spectroscopic data to analyze the color-magnitude relation in early-type galaxies, finding that assuming old stellar ages better explains the observed colors than spectroscopically derived young ages.

Contribution

It demonstrates that the color-magnitude relation is consistent with old stellar populations, challenging previous interpretations based on Hβ indices indicating young ages.

Findings

The CMR cannot be reproduced by spectroscopically determined young ages.

Assuming a mean age of 12 Gyr yields colors matching the CMR.

Current stellar population models may misinterpret Hβ indices.

Abstract

Using new narrowband color observations of early-type galaxies in clusters, we reconstruct the color-magnitude relation (CMR) with a higher degree of accuracy than previous work. We then use the spectroscopically determined ages and metallicities from three samples (Trager et al 2008, Thomas et al 2005, Gallazzi et al 2006), combined with multi-metallicity SED models, to compare predicted colors for galaxies with young ages (less than 8 Gyr) with the known CMR. We find that the CMR cannot by reproduced by the spectroscopically determined ages and metallicities in any of the samples despite the high internal accuracies to the spectroscopic indices. In contrast, using only the $<$Fe$>$ index to determine [Fe/H], and assuming a mean age of 12 Gyr for a galaxy's stellar population, we derive colors that exactly match not only the color zeropoint of the CMR but also its slope. We consider the source of young age estimates, the H$\beta$ index, and examine the conflict between red continuum colors and large H$\beta$ values in galaxy spectra. We conclude that our current understanding of stellar populations is insufficient to correctly interpret H$\beta$ values and that the sum of our galaxy observations supports an old and monolithic scenario of galaxy formation. This result has a devastating impact on every study that has used the H$\beta$ index to calculate galaxy age, as the use of the H$\beta$ versus MgFe diagram will result in incorrectly deduced young ages.