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 SPIDER 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). 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 fit by a Kroupalike IMF, whereas massive galaxies require bottom-heavy IMFs, exceeding the Salpeter slope at σ > ∼ 200 km s −1 .
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