The SLUGGS survey: Combining stellar and globular cluster metallicities
  in the outer regions of early-type galaxies
  The outer halo regions of early-type galaxies carry key information about
their past accretion history. However, spectroscopically probing the stellar
component at such galactocentric radii is still challenging. Using Keck/DEIMOS,
we have been able to measure the metallicities of the stellar and globular
cluster components in 12 early-type galaxies out to more than $10~\rm{R_{e}}$.
We find similar metallicity gradients for the metal-poor and metal-rich
globular cluster subpopulations, suggesting a common formation process for the
two subpopulations. This is in conflict with most current theoretical
predictions, where the metal-poor globular clusters are thought to be purely
accreted and metal-rich globular clusters mostly formed in-situ. Moreover, we
find that the globular cluster metallicity gradients show a trend with galaxy
mass, being steeper in lower-mass galaxies than in higher-mass galaxies. This
is similar to what we find for the outermost galaxy stars and suggests a more
active accretion history, with a larger role played by major mergers, in the
most massive galaxies. This conclusion is qualitatively consistent with
expectations from two-phase galaxy assembly models. Finally, we find that the
small difference in metallicity between galaxy stars and metal-rich globular
clusters at $1~\rm{R_{e}}$ may correlate with galaxy mass. The origin of this
difference is not currently clear.
