Confirmation of Enhanced Dwarf-sensitive Absorption Features in the
  Spectra of Massive Elliptical Galaxies: Further Evidence for a Non-universal
  Initial Mass Function
  We recently found that massive cluster elliptical galaxies have strong Na I
8183,8195 and FeH 9916 Wing-Ford band absorption, indicating the presence of a
very large population of stars with masses <~ 0.3 M_sun. Here we test this
result by comparing the elliptical galaxy spectra to those of luminous globular
clusters associated with M31. These globular clusters have similar
metallicities, abundance ratios and ages as massive elliptical galaxies but
their low dynamical mass-to-light ratios rule out steep stellar initial mass
functions (IMFs). From high quality Keck spectra we find that the
dwarf-sensitive absorption lines in globular clusters are significantly weaker
than in elliptical galaxies, and consistent with normal IMFs. The differences
in the Na I and Wing-Ford indices are 0.027 +- 0.007 mag and 0.017 +- 0.006 mag
respectively. We directly compare the two classes of objects by subtracting the
averaged globular cluster spectrum from the averaged elliptical galaxy
spectrum. The difference spectrum is well fit by the difference between a
stellar population synthesis model with a bottom-heavy IMF and one with a
bottom-light IMF. We speculate that the slope of the IMF may vary with velocity
dispersion, although it is not yet clear what physical mechanism would be
responsible for such a relation.
