The life-cycle of star formation in distant clusters

Abstract

We analyse the detailed distribution of star-forming and post-starburst members in three distant (z = 0.31) galaxy clusters in terms of evolutionary sequences that incorporate secondary bursts of star formation on pre-existing stellar populations. Using the number density of spectroscopically confirmed members of the EW(Hδ) versus B - R plane from existing data, and for a larger K'-limited sample on the U - I versus I - K' plane from newly acquired infrared images, we demonstrate that the proportion of cluster members undergoing secondary bursts of star formation during the last ∼ 2 Gyr prior to the epoch of observation is probably as high as 30 per cent of the member galaxies. A key observation leading to this conclusion is the high proportion of Hδ-strong galaxies in all three clusters. The evolutionary modelling, whilst necessarily approximate, returns the correct proportions of galaxies in various stages of the star formation cycle in terms of both spectral and colour properties. Hubble Space Telescope (HST) images for the three clusters indicate that a high proportion of the active members show signs of interaction, whereas the Hδ-strong galaxies appear mainly to be regular spheroidals. We examine results from recent merger simulations in the context of the populations in these clusters and confirm that the merging of individual galaxies, triggered perhaps by the hierarchical assembly of rich clusters at this epoch, is consistent with the star formation cycle identified in our data. The implications of such a high fraction of active objects in cluster cores are briefly discussed.