THE STELLAR POPULATIONS OF EARLY-TYPE GALAXIES AS A FUNCTION OF THEIR ENVIRONMENT

We have used gravity-sensitive spectral indices in the blue spectral region to study the mean stellar surface gravity of early-type galaxies in a variety of galaxian environments. We confirm earlier work that the mean surface gravity in integrated light is high for isolated and Virgo Cluster E/S0 galaxies, but find evidence to suggest that the mean surface gravity for similar galaxies in rich clusters is significantly lower. This indicates that galaxies in dense environments may be older on average than their counter-parts in lower-density regions. Possible selection effects and systematic errors are discussed. We conclude that star formation in galaxies was truncated at an earlier epoch (on average) in dense environments, but note that the strength of our result would be weakened if there were large "age" gradients within these galaxies.