The ACS Fornax Cluster Survey. XII. Diffuse Star Clusters in Early-type
  Galaxies
  Diffuse star clusters (DSCs) are old and dynamically hot stellar systems that
have lower surface brightness and more extended morphology than globular
clusters (GCs). Using the images from HST/ACS Fornax Cluster Survey, we find
that 12 out of 43 early-type galaxies (ETGs) in the Fornax cluster host
significant numbers of DSCs. Together with literature data from the HST/ACS
Virgo Cluster Survey, where 18 out of 100 ETGs were found to host DSCs, we
systematically study the relationship of DSCs with GCs, and their host galaxy
environment. Two DSC hosts are post-merger galaxies, with most of the other
hosts either having low mass or showing clear disk components. We find that
while the number ratio of DSCs to GCs is nearly constant in massive galaxies,
the DSC-to-GC ratio becomes systematically higher in lower mass hosts. This
suggests that DSCs may be more efficient at forming (or surviving) in low
density environments. DSC hosts are not special either in their position in the
cluster, or in the galactic color-magnitude diagram. Why some disk and low-mass
galaxies host DSCs while others do not is still a puzzle, however. The mean
ages of DSC hosts and non-hosts are similar at similar masses, implying that
formation efficiency, rather than survival, is the reason behind different DSC
number fractions in early-type galaxies.
