The ACS Fornax Cluster Survey VII. Half-Light Radii of Globular Clusters
  in Early-Type Galaxies
  We measure the half-light radii of globular clusters (GCs) in 43 galaxies
from the ACS Fornax Cluster Survey (ACSFCS). We use these data to extend
previous work in which the environmental dependencies of the half-light radii
of GCs in early type galaxies in the ACS Virgo Cluster Survey (ACSVCS) were
studied, and a corrected mean half-light radius (corrected for the observed
environmental trends) was suggested as a reliable distance indicator. This work
both increases the sample size for the study of the environmental dependencies,
and adds leverage to the study of the corrected half-light radius as a possible
distance indicator (since Fornax lies at a larger distance than the Virgo
cluster). We study the environmental dependencies of the size of GCs using both
a Principal Component Analysis as well as 2D scaling relations. We largely
confirm the environmental dependencies shown in Jordan et al. (2005), but find
evidence that there is a residual correlation in the mean half-light radius of
GC systems with galaxy magnitude, and subtle differences in the other
correlations - so there may not be a universal correction for the half-light
radii of lower luminosity galaxy GC systems. The main factor determining the
size of a GC in an early type galaxy is the GC color. Red GCs have <r_h> =
2.8+/-0.3 pc, while blue GCs have <r_h> = 3.4+/-0.3 pc. We show that for bright
early-type galaxies (M_B < -19 mag), the uncorrected mean half-light radius of
the GC system is by itself an excellent distance indicator (with error ~11%),
having the potential to reach cosmologically interesting distances in the era
of high angular resolution adaptive optics on large optical telescopes.
