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 low luminosities (M-V > -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 3 mag fainter than a typical GC (mu(g) > 20 mag arcsec (-2)). The median colors of diffuse star cluster systems (1.1 < g - z < 1: 6) are 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 supersolar metallicities, implying that diffuse star clusters are likely to be long-lived. Twelve galaxies in our sample contain a significant excess of diffuse star cluster candidates; nine are morphologically classified as lenticulars (S0s), and five visibly contain dust. We also find DSCs in the halo of the giant elliptical M49, near the companion galaxy VCC 1199. Most DSC systems appear spatially associated with galactic disks, but substantial DSC populations are not present in all lenticular galaxies, and environment is not a good predictor of their existence. Diffuse star clusters are similar to and include the locus of "faint fuzzies'' identified in other nearby galaxies. Unlike luminous GCs, whose sizes are constant with luminosity, 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 mag brighter than DSCs, ultracompact dwarfs have similar surface brightnesses. The closest Galactic analogs to the DSCs are the old open clusters, and if a diffuse star cluster population did exist in the disk of the Milky Way, it would be very difficult to find.