Journal Article

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

C. Spiniello, L. V. E. Koopmans, S. C. Trager, O. Czoske and T. Treu

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 417, issue 4, pages 3000-3009
Published in print November 2011 | ISSN: 0035-8711
Published online November 2011 | e-ISSN: 1365-2966 | DOI:
The X-Shooter Lens Survey – I. Dark matter domination and a Salpeter-type initial mass function in a massive early-type galaxy

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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 〈σ*〉(≲Reff) = 352 ± 10 ± 16 km s−1, extracted from a rectangular aperture of 1.8 × 1.6 arcsec2 centred on the galaxy, more accurate and considerably lower than a previously published value of ∼450 km s−1. (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 (68 per cent confidence level, CL; ) within a projected radius of ∼2.16 arcsec. (iii) The projected stellar mass fraction, derived solely from the lensing and dynamical data, is f*(<RE) = 0.19+0.04− 0.09 (68 per cent CL) inside the Einstein radius for a Hernquist stellar profile and no anisotropy. The dark matter fraction inside the effective radius fDM(<Reff) = 0.60+0.15− 0.06± 0.1 (68 per cent CL), where the latter error is systematic. (iv) Based on the SDSS colours, we find f*, Salp(<RE) = 0.17 ± 0.06 for a Salpeter initial mass function (IMF) and f*, Chab(<RE) = 0.07 ± 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 colours for a Salpeter IMF, which is preferred over a Chabrier IMF at variance with standard results for lower mass galaxies. Dwarf-rich IMFs in the lower mass range of 0.1–0.7 M, with α≥ 3 (with dN/dMM−α) – such as that recently suggested for massive early-type galaxies with α= 3 in the mass range 0.1–1 M– 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.

Keywords: gravitational lensing: strong; galaxies: elliptical and lenticular, cD; galaxies: formation; galaxies: kinematics and dynamics; galaxies: structure; dark matter

Journal Article.  6848 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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