The ACS Fornax Cluster Survey. X. Color Gradients of Globular Cluster
  Systems in Early-Type Galaxies
  We use the largest homogeneous sample of globular clusters (GCs), drawn from
the ACSVCS and ACSFCS, to investigate the color gradients of GC systems in 76
early-type galaxies. We find that most GC systems possess an obvious negative
gradient in g-z color (bluer outwards). For GC systems displaying color
bimodality, both metal-rich and metal-poor GC subpopulations present shallower
but significant color gradients on average, and the mean gradients of these two
subpopulations are of roughly equal strength. The FOV of ACS mainly restricts
us to measuring the inner gradients of GC systems. These gradients, however,
can introduce an aperture bias when measuring the mean colors of GC
subpopulations from relatively narrow central pointings. Inferred corrections
to previous work imply a reduced significance for the relation between the mean
color of metal-poor GCs and their host galaxy luminosity. The GC color
gradients also show a dependence with host galaxy mass where the gradiens are
weakest at the ends of the mass spectrum--in massive galaxies and dwarf
galaxies--and strongest in galaxies of intermediate mass, around a stellar mass
of M_stellar~10^10M_sun. We also measure color gradients for field stars in the
host galaxies. We find that GC color gradients are systematically steeper than
field star color gradients, but the shape of the gradient-mass relation is the
same for both. If gradients are caused by rapid dissipational collapse and
weakened by merging, these color gradients support a picture where the inner GC
systems of most intermediate-mass and massive galaxies formed early and rapidly
with the most massive galaxies having experienced greater merging. The lack of
strong gradients in the GC systems of dwarfs, which probably have not
experienced many recent major mergers, suggests that low mass halos were
inefficient at retaining and mixing metals during the epoch of GC formation.
