Evidence for a non-universal stellar initial mass function in low-redshift high-density early-type galaxies

We determine an absolute calibration of stellar mass-to-light ratios for the densest similar or equal to 3 per cent of early-type galaxies in the local Universe (redshift z similar or equal to 0.08) from Sloan Digital Sky Survey (SDSS) Data Release 7. This sample of similar to 4000 galaxies has, assuming a Chabrier initial mass function (IMF), effective stellar surface densities Sigma(e) > 2500 M-circle dot pc(-2), stellar population synthesis (SPS) stellar masses log(10)(M-SPS/M-circle dot) < 10.8 and aperture velocity dispersions of sigma(ap) = 168(-34)(+37) km s(-1) (68 per cent range). In contrast to typical early-type galaxies, we show that these dense early-type galaxies follow the virial Fundamental Plane, which suggests that mass follows light. With the additional assumption that any dark matter does not follow the light, the dynamical masses of dense galaxies provide a direct measurement of stellar masses. Our dynamical masses (M-dyn), obtained from the spherical Jeans equations, are only weakly sensitive to the choice of anisotropy (beta) due to the relatively large aperture of the SDSS fibre for these galaxies: R-ap similar or equal to 1.5R(e). Assuming isotropic orbits (beta = 0), we find a median log(10)(M-dyn/M-SPS) = 0.233 +/- 0.003, consistent with a Salpeter IMF, while more bottom-heavy IMFs and standard Milky Way IMFs are strongly disfavoured. Our results are consistent with, but do not require, a dependence of the IMF on dynamical mass or velocity dispersion. We find evidence for a colour dependence to the IMF such that redder galaxies have heavier IMFs with M-dyn/M-SPS proportional to (g - r)(1.13 +/- 0.09). This may reflect a more fundamental dependence of the IMF on the age or metallicity of a stellar population, or the density at which the stars formed.