THE INITIAL MASS FUNCTION OF EARLY-TYPE GALAXIES

Abstract

We determine an absolute calibration of the initial mass function (IMF) of\nearly-type galaxies, by studying a sample of 56 gravitational lenses identified\nby the SLACS Survey. Under the assumption of standard Navarro, Frenk & White\ndark matter halos, a combination of lensing, dynamical, and stellar population\nsynthesis models is used to disentangle the stellar and dark matter\ncontribution for each lens. We define an "IMF mismatch" parameter\n\\alpha=M*(L+D)/M*(SPS) as the ratio of stellar mass inferred by a joint lensing\nand dynamical models (M*(L+D)) to the current stellar mass inferred from\nstellar populations synthesis models (M*(SPS)). We find that a Salpeter IMF\nprovides stellar masses in agreement with those inferred by lensing and\ndynamical models (<\\log \\alpha>=0.00+-0.03+-0.02), while a Chabrier IMF\nunderestimates them (<\\log \\alpha>=0.25+-0.03+-0.02). A tentative trend is\nfound, in the sense that \\alpha appears to increase with galaxy velocity\ndispersion. Taken at face value, this result would imply a non universal IMF,\nperhaps dependent on metallicity, age, or abundance ratios of the stellar\npopulations. Alternatively, the observed trend may imply non-universal dark\nmatter halos with inner density slope increasing with velocity dispersion.\nWhile the degeneracy between the two interpretations cannot be broken without\nadditional information, the data imply that massive early-type galaxies cannot\nhave both a universal IMF and universal dark matter halos.\n