Universal IMF vs dark halo response in early-type galaxies: breaking the degeneracy with the fundamental plane

Details Universal IMF vs dark halo response in early-type galaxies: breaking the degeneracy with the fundamental plane Universal IMF vs dark halo response in early-type galaxies: breaking the degeneracy with the fundamental plane

2012-04-12

arxiv.org/abs/1204.2825v1

Astronomy and Astrophysics

Astrophysics

Cosmology and Extragalactic Astrophysics

14 pages, 12 figures, submitted to MNRAS

Scientific paper

We use the relations between aperture stellar velocity dispersion (\sigma_ap), stellar mass (M_sps), and galaxy size (R_e) for a sample of 150,000 early-type galaxies from SDSS/DR7 to place constraints on the stellar initial mass function (IMF) and dark halo response to galaxy formation. We build LCDM based mass models that reproduce, by construction, the relations between size, light concentration and stellar mass. Reproducing the median \sigma_ap vs M_sps relation is not possible in models that have {\it both} a universal IMF and a universal dark halo response. Significant departures from a universal IMF and/or dark halo response are required. We show that this degeneracy can be broken using the strength of the correlation between residuals of the velocity-mass (\Delta log \sigma_ap) and size-mass (\Delta log R_e) relations. The slope of this correlation, d_vr = \Delta log\sigma_ap/\Delta log R_e, varies systematically with galaxy mass from -0.45 at M_sps \sim 10^{10} M_sun, to -0.15 at M_sps \sim 10^{11.6}M_sun. The virial fundamental plane (FP) has d_vr=-1/2, and thus we find the tilt of the FP is mass dependent. Reproducing this tilt requires {\it both} a non-universal IMF and a non-universal halo response. Our best model has mass-follows-light at low masses (M_sps < 10^{11.2}M_sun) and unmodified NFW haloes at M_sps \sim 10^{11.5}M_sun. The stellar masses imply a mass dependent IMF which is "lighter" than Salpeter at low masses and "heavier" than Salpeter at high masses.

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