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


  • Aaron A. Dutton,

    Corresponding author
    1. Department of Physics and Astronomy, University of Victoria, Victoria, BC V8P 5C2, Canada
    2. Department of Physics, University of California, Santa Barbara, CA 93106, USA
    3. Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
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  • J. Trevor Mendel,

    1. Department of Physics and Astronomy, University of Victoria, Victoria, BC V8P 5C2, Canada
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  • Luc Simard

    1. Herzberg Institute of Astrophysics, National Research Council of Canada, 5071 West Saanich Road, Victoria, BC V9E 2E7, Canada
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CITA National Fellow.


We determine an absolute calibration of stellar mass-to-light ratios for the densest ≃3 per cent of early-type galaxies in the local Universe (redshift z≃ 0.08) from Sloan Digital Sky Survey (SDSS) Data Release 7. This sample of ∼4000 galaxies has, assuming a Chabrier initial mass function (IMF), effective stellar surface densities Σe > 2500 M pc−2, stellar population synthesis (SPS) stellar masses log10(MSPS/M) < 10.8 and aperture velocity dispersions of inline image (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 (Mdyn), obtained from the spherical Jeans equations, are only weakly sensitive to the choice of anisotropy (β) due to the relatively large aperture of the SDSS fibre for these galaxies: Rap≃ 1.5Re. Assuming isotropic orbits (β= 0), we find a median log10(Mdyn/MSPS) = 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 Mdyn/MSPS∝ (gr)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.