Starburst Cycle in Distant Clusters

Abstract

A major puzzle in observational cosmology is the physical origin of a significant excess population of blue galaxies in the cores of distant rich galaxy clusters. This ‘Butcher-Oemler’effect is now known to be a widespread starburst-related phenomenon. We test whether various spectral and photometrically-defined galaxy classes might represent different stages within a single cycle of star-formation. We compare the numbers of galaxies in various categories for three z = 0.31 clusters, AC103, AC114, and AC118, with evolutionary models generated according to the Bruzual & Charlot (1993) isochrone spectral synthesis code, assuming that some fraction of the model cluster population is viewed either before or during a secondary burst of star formation. We find good agreement between the model predictions and the number density of spectroscopically-confirmed members in the Hδ versus B — R plane for a cluster population in which 30 per cent of the member galaxies have undergone secondary bursts of star formation within the last ~ 2 Gyr prior to observation. As an additional check, we analyse a larger K _n-limited sample from newly-acquired infrared images and find good agreement between the models and the data in the U — I versus I — K _n plane for the same active cluster fraction. We conclude that the unusual galaxy population in distant clusters can be explained by a. single cycle in which about 30 per cent of t he cluster population experienced a secondary burst of star-formation within the last ~ 2 Gyr.