Globular Cluster Systems in Giant Ellipticals: the Mass/Metallicity
  Relation
  Hubble Space Telescope ACS/WFC data in (B,I) are used to investigate the
globular cluster populations around 6 gE galaxies ~40 Mpc distant. The total
comprises a sample of ~8000 high-probability globular clusters. PSF-convolved
King-model profiles are used to measure their individual total magnitudes,
colors, and effective radii. The classic bimodal form of the GC color-magnitude
distribution shows up unambiguously in all the galaxies, allowing an accurate
definition of the mean colors along each of the two sequences as a function of
magnitude (the mass/metallicity relation or MMR). The blue, metal-poor cluster
sequence shows a clearly defined but nonlinear MMR, changing smoothly from a
near-vertical sequence at low luminosity to an increasingly redward slope at
higher luminosity, while the red, metal-rich sequence is nearly vertical at all
luminosities. All the observed features of the present data agree with the
interpretation that the MMR is created primarily by GC self-enrichment, along
the lines of the quantitative model of Bailin and Harris (2009): The
"threshold" mass at which this effect should become noticeable is near 1
million Solar masses, which is closely consistent with the transition region
that is seen in the data. Correlation of the median half-light radii of the GCs
with other parameters shows that the metal-poor clusters are consistently 17%
larger than those of the metal-rich clusters, at all galactocentric distances
and luminosities. At the same time, cluster size scales with halo location as
r_h ~ R_gc^0.11, indicating that both metallicity and the external tidal
environment play roles in determining the scale size of a given cluster.
Lastly, both the red and blue GC components show metallicity gradients with
galactocentric distance, following Z ~ R_gc^-0.1.
