The ACS Fornax Cluster Survey. III. Globular Cluster Specific
  Frequencies of Early-Type Galaxies
  The globular cluster (GC) specific frequency ($S_N$), defined as the number
of GCs per unit galactic luminosity, represents the efficiency of GC formation
(and survival) compared to field stars. Despite the naive expectation that star
cluster formation should scale directly with star formation, this efficiency
varies widely across galaxies. To explore this variation we measure the z-band
GC specific frequency ($S_{N,z}$) for 43 early-type galaxies (ETGs) from the
Hubble Space Telescope (HST)/Advanced Camera for Surveys (ACS) Fornax Cluster
Survey. Combined with the homogenous measurements of $S_{N,z}$ in 100 ETGs from
the HST/ACS Virgo Cluster Survey from Peng et al. (2008), we investigate the
dependence of $S_{N,z}$ on mass and environment over a range of galaxy
properties. We find that $S_{N,z}$ behaves similarly in the two galaxy
clusters, despite the clusters' order-of-magnitude difference in mass density.
The $S_{N,z}$ is low in intermediate-mass ETGs ($-20<M_z<-23$), and increases
with galaxy luminosity. It is elevated at low masses, on average, but with a
large scatter driven by galaxies in dense environments. The densest
environments with the strongest tidal forces appear to strip the GC systems of
low-mass galaxies. However, in low-mass galaxies that are not in strong tidal
fields, denser environments correlate with enhanced GC formation efficiencies.
Normalizing by inferred halo masses, the GC mass fraction,
$\eta=(3.36\pm0.2)\times10^{-5}$, is constant for ETGs with stellar masses
$\mathcal{M}_\star \lesssim 3\times10^{10}M_\odot$, in agreement with previous
studies. The lack of correlation between the fraction of GCs and the nuclear
light implies only a weak link between the infall of GCs and the formation of
nuclei.
