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The X-Shooter Lens Survey - I. Dark-Matter Domination and a Salpeter-type IMF in a Massive Early-type Galaxy

We present the first results from the X-shooter Lens Survey (XLENS): an analysis of the massive early-type galaxy SDSS J1148+1930 at redshift z=0.444. We combine its extended kinematic profile -derived from spectra obtained with X-shooter on the ESO VLT- with strong gravitational lensing and multi-color information derived from SDSS images. Our main results are (i) The luminosity-weighted stellar velocity dispersion is σ*(<Reff) = 351\pm10 km/s, extracted from a rectangular aperture of 1.8"x1.6", more accurate and considerably lower than a previously published value of \sim450km/s. (ii) A single-component (stellar plus dark) mass model of the lens galaxy yields a logarithmic total-density slope of γ' = 1.72+0.05-0.06. (iii) The projected stellar mass fraction, derived solely from lensing, is f*(<Reff) = 0.19+0.04-0.09 inside the Einstein radius for a Hernquist profile and no anisotropy. The dark-matter fraction inside the effective radius f_DM(<Reff) = 0.60+0.15\pm0.1. (iv) Based on the SDSS colors, we find f*(<Reff)=0.17 \pm 0.06 for a Salpeter IMF and f*(<Reff)=0.07 \pm 0.02 for a Chabrier IMF. The lensing and dynamics constraints on the stellar mass fraction agree well with those independently derived from the SDSS colors for a Salpeter IMF, preferred over Chabrier IMF at variance with standard results for lower mass galaxies. Dwarf-rich IMFs in the lower mass range of 0.1-0.7 solar mass, with α >= 3 (with dN/dM \propto M^(-α)) -such as that recently suggested for massive ETGs- are excluded at the > 90 % C.L. and in some cases violate the total lensing-derived mass limit. We conclude that this very massive ETG is dark-matter dominated inside one effective radius, consistent with the trend recently found from massive SLACS galaxies, with a IMF normalization consistent with Salpeter.