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.Comment: Published in ApJ Letter

Keck Spectroscopy of distant GOODS Spheroidal Galaxies: Downsizing in a Hierarchical Universe

We analyze the evolution of the Fundamental Plane for 141 field spheroidal galaxies in the redshift range 0.2<z<1.2, selected morphologically to a magnitude limit F850LP=22.43 in the northern field of the Great Observatories Origin Survey. For massive galaxies we find that the bulk of the star formation was completed prior to z=2. However, for the lower mass galaxies, the luminosity-weighted ages are significantly younger. The differential change in mass-to-light ratio correlates closely with rest-frame color, consistent with recent star formation and associated growth. Our data are consistent with mass rather than environment governing the overall growth, contrary to the expectations of hierarchical assembly. We discuss how feedback, conduction, and galaxy interactions may explain the downsizing trends seen within our large sample.Comment: ApJ Letters, in press. 4 figure