Journal Article

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

Ignacio Ferreras, Francesco La Barbera, Ignacio G. de la Rosa, Alexandre Vazdekis, Reinaldo R. de Carvalho, Jesús Falcón-Barroso and Elena Ricciardelli

in Monthly Notices of the Royal Astronomical Society: Letters

Published on behalf of The Royal Astronomical Society

Volume 429, issue 1, pages L15-L19
Published in print February 2013 |
Published online November 2012 | e-ISSN: 1745-3933 | DOI: http://dx.doi.org/10.1093/mnrasl/sls014

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An essential component of galaxy formation theory is the stellar initial mass function (IMF) that describes the parent distribution of stellar 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 sample comprises ∼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 ≳ 400 Å−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). A significant 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 require a detailed analysis of chemical composition (such as [α/Fe] or even individual element abundance ratios), the observed trends suggest that low-mass ETGs are better fitted by a Kroupa-like IMF, whereas massive galaxies require bottom-heavy IMFs, exceeding the Salpeter slope at σ ≳ 200 km s−1.

Keywords: galaxies: elliptical and lenticular, cD; galaxies: evolution; galaxies: formation; galaxies: fundamental parameters; galaxies: stellar content

Journal Article.  3649 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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