Confirmation of Enhanced Dwarf-sensitive Absorption Features in the Spectra of Massive Elliptical Galaxies: Further Evidence for a Non-universal Initial Mass Function

TL;DR

This study confirms that massive elliptical galaxies have a bottom-heavy initial mass function (IMF) with more low-mass stars compared to globular clusters, supporting the idea that the IMF varies with galaxy properties.

Contribution

It provides further evidence for a non-universal, bottom-heavy IMF in massive elliptical galaxies by comparing their spectra to globular clusters with similar properties.

Findings

Elliptical galaxies show stronger dwarf-sensitive absorption lines than globular clusters.

The spectral differences are consistent with a bottom-heavy IMF in ellipticals.

The IMF slope may correlate with galaxy velocity dispersion.

Abstract

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.