  A detailed imaging analysis of the globular cluster (GC) system of the
Sombrero galaxy (NGC 4594) has been accomplished using a six-image mosaic from
the Hubble Space Telescope Advanced Camera for Surveys. The quality of the data
is such that contamination by foreground stars and background galaxies is
negligible for all but the faintest 5% of the GC luminosity function (GCLF).
This enables the study of an effectively pure sample of 659 GCs until ~2 mags
fainter than the turnover magnitude, which occurs at M_V=-7.60+/-0.06 for an
assumed m-M=29.77. Two GC metallicity subpopulations are easily
distinguishable, with the metal-poor subpopulation exhibiting a smaller
intrinsic dispersion in color compared to the metal-rich subpopulation.
  Three new discoveries include: (1) A metal-poor GC color-magnitude trend. (2)
Confirmation that the metal-rich GCs are ~17% smaller than the metal-poor ones
for small projected galactocentric radii (less than ~2 arcmin). However, the
median half-light radii of the two subpopulations become identical at ~3 arcmin
from the center. This is most easily explained if the size difference is the
result of projection effects. (3) The brightest (M_V < -9.0) members of the GC
system show a size-magnitude upturn where the average GC size increases with
increasing luminosity. Evidence is presented that supports an intrinsic origin
for this feature rather than a being result from accreted dwarf elliptical
nuclei. In addition, the metal-rich GCs show a shallower positive
size-magnitude trend, similar to what is found in previous studies of young
star clusters.
