Globular Clusters in Dense Clusters of Galaxies

Abstract

Deep imaging data from the Keck II Telescope are employed to study the globular cluster (GC) populations in the cores of six rich Abell clusters. The sample includes A754, A1644, A2124, A2147, A2151, and A2152 and covers the redshift range z = 0.035–0.066. These clusters also span a range in morphology from spiral-rich, irregular systems to centrally concentrated cD clusters rich in early-type galaxies. Globular cluster specific frequencies SN and luminosity function dispersions σLF are measured for a total of nine galaxies in six central fields. The measured values of SN for the six brightest cluster galaxies (BCGs) are all higher than typical values for giant elliptical galaxies, in accord with the SN-density correlations found by Blakeslee, Tonry, & Metzger. The three non-BCGs analyzed also have elevated values of SN, confirming that central location is a primary factor. Two different models are used to estimate ηGC, the number of GCs per unit mass, for these central cluster fields. The values for ηGC are consistent with those found in the earlier sample, again indicating that the number of GCs scales with mass and that the SN variations are due to a deficit of halo light, i.e., SN reflects mass-to-light ratio. The similarity of the GC color distributions of BCGs and more ordinary ellipticals further implies that no special mechanism is needed for explaining the properties of these GC populations. The discussion builds on an earlier suggestion that the GCs (both metal-rich and metal-poor) around the central cluster galaxies were assembled at early times, and that star formation halted prematurely in the central galaxies at the epoch of cluster collapse. It is argued that this saturation model is consistent with recent simulations of BCG/cluster formation. The subsequent addition of luminous material to the BCG through cluster dynamical evolution can cause SN to decrease while conserving ηGC, but both theory and observations indicate that the timescale for this is long. However, there may be some evidence of it in the present sample and elsewhere. Finally, the GC luminosity function measurements are used to constrain the relative distances of the three clusters that make up the Hercules supercluster.