F Turnoff Distribution in the Galactic Halo Using Globular Clusters as
  Proxies
  F turnoff stars are important tools for studying Galactic halo substructure
because they are plentiful, luminous, and can be easily selected by their
photometric colors from large surveys such as the Sloan Digital Sky Survey
(SDSS). We describe the absolute magnitude distribution of color-selected F
turnoff stars, as measured from SDSS data, for eleven globular clusters in the
Milky Way halo. We find that the M_g distribution of turnoff stars is
intrinsically the same for all clusters studied, and is well fit by two half
Gaussian functions, centered at mu=4.18, with a bright-side sigma=0.36, and
with a faint-side sigma=0.76. However, the color errors and detection
efficiencies cause the observed sigma of the faint-side Gaussian to change with
magnitude due to contamination from redder main sequence stars (40% at 21st
magnitude). We present a function that will correct for this
magnitude-dependent change in selected stellar populations, when calculating
stellar density from color-selected turnoff stars. We also present a consistent
set of distances, ages and metallicities for eleven clusters in the SDSS Data
Release 7. We calculate a linear correction function to Padova isochrones so
that they are consistent with SDSS globular cluster data from previous papers.
We show that our cluster population falls along the Milky Way Age-Metallicity
Relationship (AMR), and further find that isochrones for stellar populations on
the AMR have very similar turnoffs; increasing metallicity and decreasing age
conspire to produce similar turnoff magnitudes and colors for all old clusters
that lie on the AMR.
