Dark Matter Halos in Galaxies and Globular Cluster Populations. II:
  Metallicity and Morphology
  An increasing body of data reveals a one-to-one linear correlation between
galaxy halo mass and the total mass in its globular cluster (GC) population,
M_{GCS} ~ M_h^{1.03 \pm 0.03}, valid over 5 orders of magnitude. We explore the
nature of this correlation for galaxies of different morphological types, and
for the subpopulations of metal-poor (blue) and metal-rich (red) GCs. For the
subpopulations of different metallicity we find M_{GCS}(blue) ~ M_h^{0.96 \pm
0.03} and M_{GCS}(red) ~ M_h^{1.21 \pm 0.03} with similar scatter. The
numerical values of these exponents can be derived from the detailed behavior
of the red and blue GC fractions with galaxy mass and provide a self-consistent
set of relations. In addition, all morphological types (E, S0, S/Irr) follow
the same relation, but with a second-order trend for spiral galaxies to have a
slightly higher fraction of metal-rich GCs for a given mass. These results
suggest that the amount of gas available for GC formation at high redshift was
in nearly direct proportion to the dark-matter halo potential, in strong
contrast to the markedly nonlinear behavior of total stellar mass versus halo
mass. Of the few available theoretical treatments that directly discuss the
formation of GCs in a hierarchical merging framework,the model of Kravtsov &
Gnedin (2005) best matches these observations. They find that the blue,
metal-poor GCs formed in small halos at $z > 3$ and did so in nearly direct
proportion to halo mass. Similar models addressing the formation rate of the
red, metal-richer GCs in the same detail and continuing to lower redshift are
still needed for a comprehensive picture.
