Nonlinear Color-Metallicity Relations of Globular Clusters. III. On the
  Discrepancy in Metallicity between Globular Cluster Systems and their Parent
  Elliptical Galaxies
  One of the conundrums in extragalactic astronomy is the discrepancy in
observed metallicity distribution functions (MDFs) between the two prime
stellar components of early-type galaxies-globular clusters (GCs) and halo
field stars. This is generally taken as evidence of highly decoupled
evolutionary histories between GC systems and their parent galaxies. Here we
show, however, that new developments in linking the observed GC colors to their
intrinsic metallicities suggest nonlinear color-to-metallicity conversions,
which translate observed color distributions into strongly-peaked, unimodal
MDFs with broad metal-poor tails. Remarkably, the inferred GC MDFs are similar
to the MDFs of resolved field stars in nearby elliptical galaxies and those
produced by chemical evolution models of galaxies. The GC MDF shape,
characterized by a sharp peak with a metal-poor tail, indicates a virtually
continuous chemical enrichment with a relatively short timescale. The
characteristic shape emerges across three orders of magnitude in the host
galaxy mass, suggesting a universal process of chemical enrichment among
various GC systems. Given that GCs are bluer than field stars within the same
galaxy, it is plausible that the chemical enrichment processes of GCs ceased
somewhat earlier than that of field stellar population, and if so, GCs
preferentially trace the major, vigorous mode of star formation events in
galactic formation. We further suggest a possible systematic age difference
among GC systems, in that the GC systems in more luminous galaxies are older.
This is consistent with the downsizing paradigm of galaxies and supports
additionally the similar nature shared by GCs and field stars. Our findings
suggest that GC systems and their parent galaxies have shared a more common
origin than previously thought, and hence greatly simplify theories of galaxy
formation.
