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We use HST/ACS imaging of 100 early-type galaxies in the ACS Virgo ClusterSurvey to investigate the nature of diffuse star clusters (DSCs). Compared toglobular clusters (GCs), these star clusters have moderately low luminosities(M_V > -8) and a broad distribution of sizes (3 < r_h < 30 pc), but they areprincipally characterized by their low mean surface brightnesses which can bemore than three magnitudes fainter than a typical GC (mu_g > 20 mag arcsec^-2).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. MostDSC systems thus have mean ages older than 5 Gyr or else have super-solarmetallicities. We find that 12 galaxies in our sample contain a significantexcess of diffuse star cluster candidates -- nine are lenticulars (S0s), andfive visibly contain dust. We also find a substantial population of DSCs in thehalo of the giant elliptical M49, associated with the companion galaxy VCC1199. Most DSC systems appear to be both aligned with the galaxy light andassociated with galactic disks, but at the same time many lenticulars do nothost substantial DSC populations. Diffuse star clusters in our sample sharesimilar characteristics to those identified in other nearby lenticular, spiral,and dwarf galaxies, and we suggest that DSCs preferentially form, survive, andcoevolve with galactic disks. Their properties are broadly consistent withthose of merged star cluster complexes, and we note that despite being 3 - 5magnitudes brighter than DSCs, ultra-compact dwarfs have similar surfacebrightnesses. The closest Galactic analogs to the DSCs are the old openclusters. We suggest that if a diffuse star cluster population did exist in thedisk of the Milky Way, it would be very difficult to find. (Abridged)

The ACS Virgo Cluster Survey. XI. The Nature of Diffuse Star Clusters in Early‐Type Galaxies