The Stellar Initial Mass Function in Early-Type Galaxies From Absorption
  Line Spectroscopy. I. Data and Empirical Trends
  The strength of gravity-sensitive absorption lines in the integrated light of
old stellar populations is one of the few direct probes of the stellar initial
mass function (IMF) outside of the Milky Way. Owing to the advent of fully
depleted CCDs with little or no fringing it has recently become possible to
obtain accurate measurements of these features. Here we present spectra
covering the wavelength ranges 0.35 - 0.55 micron and 0.72 - 1.03 micron for
the bulge of M31 and 34 early-type galaxies from the SAURON sample, obtained
with the Low Resolution Imaging Spectrometer on Keck. The signal-to-noise ratio
is >200 per Angstrom out to 1 micron, which is sufficient to measure
gravity-sensitive features for individual galaxies and to determine how they
depend on other properties of the galaxies. Combining the new data with
previously obtained spectra for globular clusters in M31 and the most massive
elliptical galaxies in the Virgo cluster we find that the dwarf-sensitive Na I
doublet and the FeH Wing-Ford band increase systematically with velocity
dispersion, while the giant-sensitive Ca II triplet decreases with dispersion.
These trends are consistent with a varying IMF, such that galaxies with deeper
potential wells have more dwarf-enriched mass functions. In a companion paper
(Conroy & van Dokkum 2012) we use a comprehensive stellar population synthesis
model to demonstrate that IMF effects can be separated from age and abundance
variations and quantify the IMF variation among early-type galaxies.
