The SLUGGS survey: Exploring the metallicity gradients of nearby
  early-type galaxies to large radii
  Stellar metallicity gradients in the outer regions of galaxies are a critical
tool for disentangling the contributions of in-situ and ex-situ formed stars.
In the two-phase galaxy formation scenario, the initial gas collapse creates
steep metallicity gradients, while the accretion of stars formed in satellites
tends to flatten these gradients in the outskirts, particularly for massive
galaxies. This work presents the first compilation of extended metallicity
profiles over a wide range of galaxy mass. We use the DEIMOS spectrograph on
the Keck telescope in multi-slit mode to obtain radial stellar metallicity
profiles for 22 nearby early-type galaxies. From the calcium triplet lines in
the near-infrared we measure the metallicity of the starlight up to 3 effective
radii. We find a relation between the outer metallicity gradient and galaxy
mass, in the sense that lower mass systems show steeper metallicity gradients
than more massive galaxies. This result is consistent with a picture in which
the ratio of ex-situ to in-situ formed stars is lower in less massive galaxies
as a consequence of the smaller contribution by accretion. In addition, we
infer a correlation between the strength of the calcium triplet feature in the
near-infrared and the stellar initial mass function slope that is consistent
with recent models in the literature.
