Systematic variation of the stellar initial mass function with velocity dispersion in early-type galaxies

Abstract

An essential component of galaxy formation theory is the stellarinitial mass function (IMF) that describes the parent distribution ofstellar mass in star-forming regions. We present observational evidence in a sample of early-type galaxies (ETGs) of a tight correlation between central velocity dispersion and the strength of several absorption features sensitive to the presence of low-mass stars. Our samplecomprises ~40 000 ETGs from the Spheroids Panchromatic Investigation in Different Environmental Regions survey (z ∼ 0.1). The data-extracted from the Sloan Digital Sky Survey-are combined, rejecting both noisy data, and spectra with contamination from telluric lines, resulting in a set of 18 stacked spectra at high signal-to-noise ratio (S/N ≳ 400å-1). A combined analysis of IMF-sensitive line strengths and spectral fitting is performed with the latest state-of-the-art population synthesis models (an extended version of the MILES models). Asignificant trend is found between IMF slope and velocity dispersion, towards an excess of low-mass stars in the most massive galaxies. Although we emphasize that accurate values of the IMF slope will requirea detailed analysis of chemical composition (such as [α/Fe] or even individual element abundance ratios), the observed trends suggest thatlow-mass ETGs are better fitted by a Kroupa-like IMF, whereas massivegalaxies require bottom-heavy IMFs, exceeding the Salpeter slope at σ ≳ 200 km s-1. © 2012 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society.