The Acs Virgo Cluster Survey Xi. the Nature of Diffuse Star Clusters in Early-type Galaxies

We use HST/ACS imaging of 100 early-type galaxies in the ACS Virgo Cluster Survey to investigate the nature of diffuse star clusters (DSCs). Compared to globular clusters (GCs), these star clusters have moderately low luminosities (MV > −8) and a broad distribution of sizes (3 < rh < 30 pc), but they are principally characterized by their low mean surface brightnesses which can be more than three magnitudes fainter than a typical GC (μg > 20 mag arcsec ). The median colors of diffuse star cluster systems are red, 1.1 < g − z < 1.6, which is redder than metal-rich GCs and often as red as the galaxy itself. Most DSC systems thus have mean ages older than 5 Gyr or else have super-solar metallicities implying that diffuse star clusters are likely to be long-lived, surviving for significant fraction of a Hubble time. We find that 12 galaxies in our sample contain a significant excess of diffuse star cluster candidates. Nine of them are morphologically classified as lenticulars (S0s), and five of them visibly contain dust. We also find a substantial population of DSCs in the halo of the giant elliptical M49, associated with the companion galaxy VCC 1199. Most DSC systems appear to be both aligned with the galaxy light and associated with galactic disks, but at the same time many lenticular galaxies do not host substantial DSC populations, and environment and clustercentric radius do not appear to be good predictors of their existence. Diffuse star clusters in our sample share similar characteristics to those identified in other nearby lenticular, spiral, and dwarf galaxies. Unlike luminous GCs, whose sizes are constant with luminiosity, DSCs are bounded at the bright end by an envelope of nearly constant surface brightness. We suggest that populations of diffuse star clusters preferentially form, survive, and coevolve with galactic disks. Their properties are broadly consistent with those of merged star cluster complexes, and we note that despite being 3–5 magnitudes brighter than DSCs, ultra-compact dwarfs have similar surface brightnesses. The closest Galactic analogs to the DSCs are the old open clusters. We suggest that if a diffuse star cluster population did exist in the disk of the Milky Way, it would be very difficult to find. Subject headings: galaxies: elliptical and lenticular, cD — galaxies: evolution — galaxies: star clusters, globular clusters: general — open clusters and associations: general