The Stellar Initial Mass Function in Early-Type Galaxies From Absorption
  Line Spectroscopy. II. Results
  The spectral absorption lines in early-type galaxies contain a wealth of
information regarding the detailed abundance pattern, star formation history,
and stellar initial mass function (IMF) of the underlying stellar population.
Using our new population synthesis model that accounts for the effect of
variable abundance ratios of 11 elements, we analyze very high quality
absorption line spectra of 38 early-type galaxies and the nuclear bulge of M31.
These data extend to 1um and they therefore include the IMF-sensitive spectral
features NaI, CaII, and FeH at 0.82um, 0.86um and 0.99um, respectively. The
models fit the data well, with typical rms residuals ~1%. Strong constraints on
the IMF and therefore the stellar mass-to-light ratio, (M/L)_stars, are derived
for individual galaxies. We find that the IMF becomes increasingly bottom-heavy
with increasing velocity dispersion and [Mg/Fe]. At the lowest dispersions and
[Mg/Fe] values the derived IMF is consistent with the Milky Way IMF, while at
the highest dispersions and [Mg/Fe] values the derived IMF contains more
low-mass stars (is more bottom-heavy) than even a Salpeter IMF. Our best-fit
(M/L)_stars values do not exceed dynamically-based M/L values. We also apply
our models to stacked spectra of four metal-rich globular clusters in M31 and
find an (M/L)_stars that implies fewer low-mass stars than a Milky Way IMF,
again agreeing with dynamical constraints. We discuss other possible
explanations for the observed trends and conclude that variation in the IMF is
the simplest and most plausible.
