I. Ferreras, F. La Barbera, R. R. de Carvalho, I. G. de la Rosa, A. Vazdekis, J. Falcon-Barroso, E. Ricciardelli
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. In this letter we present observational evidence of a strong correlation between the slope of the IMF and central velocity dispersion for a comprehensive population of early-type galaxies (ETGs) in the nearby Universe (z<0.1). We study a large sample of ~40,000 ETGs from the SPIDER survey. The spectroscopic data -- extracted from the Sloan Digital Sky Survey -- are carefully combined, rejecting both noisy data, and spectra with contamination from telluric lines in the regions of interest, resulting in a set of 18 stacked spectra at very high signal-to-noise ratio (S/N>400 per A). Spectral line strengths sensitive to age, metallicity, and IMF slope (Gamma) are compared against the latest state-of-the-art population synthesis models (MIUSCAT). A strong correlation is found between Gamma and velocity dispersion: Gamma = 3.8log s200+1.37, where s200 is the velocity dispersion measured in units of 200 km/s. This result is applicable in ETGs at s200>0.75, at z<0.1. At the low mass end, ETGs are better fit by a bottom-light IMF, with a Kroupa-like function corresponding to galaxies with s200~0.75, whereas massive galaxies require bottom-heavy IMFs, even exceeding the Salpeter slope at s200>1.
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http://arxiv.org/abs/1206.1594
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