Stellar age versus mass of early-type galaxies in the Virgo Cluster

TL;DR

This study uses population synthesis models including binary evolution to explain ultraviolet colors of Virgo cluster early-type galaxies, showing no need for a dichotomy between dwarf and giant galaxies.

Contribution

It demonstrates that binary stellar evolution models can naturally explain UV color trends without assuming a fundamental difference between dwarf and giant galaxies.

Findings

UV colors are explained by binary evolution models.

No dichotomy between dwarf and giant galaxy populations is needed.

UV color trends are linked to star formation history and binary evolution.

Abstract

The flux excess of elliptical galaxies in the far-ultraviolet can be reproduced by population synthesis models when accounting for the population of old hot helium-burning subdwarf stars. This has been achieved by Han and coworkers through a quantitative model of binary stellar evolution. Here, we compare the resulting evolutionary population synthesis model to the GALEX far-near ultraviolet colors (FUV-NUV) of Virgo cluster early-type galaxies that were published by Boselli and coworkers. FUV-NUV is reddest at about the dividing luminosity of dwarf and giant galaxies, and becomes increasingly blue for both brighter and fainter luminosities. This behavior can be easily explained by the binary model with a continuous sequence of longer duration and later truncation of star formation at lower galaxy masses. Thus, in contrast to previous conclusions, the GALEX data do not require a dichotomy between the stellar population properties of dwarfs and giants. Their apparently opposite behavior in FUV-NUV occurs naturally when the formation of hot subdwarfs through binary evolution is taken into account.