The ACS Fornax Cluster Survey. XII. Diffuse Star Clusters in Early-type Galaxies

TL;DR

This study investigates diffuse star clusters in early-type galaxies within the Fornax and Virgo clusters, revealing their prevalence, relationship with globular clusters, and dependence on galaxy mass and environment.

Contribution

It provides a systematic analysis of DSCs in early-type galaxies, highlighting their higher ratios in low-mass hosts and exploring their connection with galaxy properties and formation efficiency.

Findings

DSCs are present in 12 of 43 Fornax ETGs and 18 of 100 Virgo ETGs.

DSC-to-GC ratio increases in lower mass galaxies.

DSC hosts are not distinguished by their cluster position or color-magnitude.

Abstract

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.