The X-Shooter Lens Survey - I. Dark matter domination and a Salpeter-type initial mass function in a massive early-type galaxy

We present the first results from the X-Shooter Lens Survey: 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 European Southern Observatory Very Large Telescope with strong gravitational lensing and multicolour information derived from Sloan Digital Sky Survey (SDSS) images. Our main results are as follows. (i) The luminosity-weighted stellar velocity dispersion is (less than or similar to Reff) = 352 +/- 10 +/- 16 km s(-1), extracted from a rectangular aperture of 1.8 x 1.6 arcsec(2) centred on the galaxy, more accurate and considerably lower than a previously published value of similar to 450 km s(-1). (ii) A single-component (stellar plus dark) mass model of the lens galaxy yields a logarithmic total-density slope of gamma'= 1.72(- 0.06)(+0.05) (68 per cent confidence level, CL; ) within a projected radius of similar to 2.16 arcsec. (iii) The projected stellar mass fraction, derived solely from the lensing and dynamical data, is f(*)(= 3 (with dN/dM alpha M-alpha) such as that recently suggested for massive early-type galaxies with alpha= 3 in the mass range 0.1-1 M-circle dot - are excluded at the >90 per cent CL and in some cases violate the total lensing-derived mass limit. We conclude that this very massive early-type galaxy is dark-matter-dominated inside one effective radius, consistent with the trend recently found from massive Sloan Lens ACS (SLACS) galaxies, with a total density slope shallower than isothermal and an IMF normalization consistent with Salpeter.