The dark matter halo concentration and stellar initial mass function of a CASSOWARY group

Abstract

We exploit the group environment of the CAmbridge Sloan Survey Of Wide ARcs in the skY (CASSOWARY) z=0.3 lens J2158+0257 to measure the group dynamical mass as a complement to the central dynamical and lensing mass constraints. Follow-up spectroscopy of candidate group members is performed using VLT/FORS2. From the resulting N=21 confirmed members we measure the group dynamical mass by calibrating an analytic tracer mass estimator with cosmological simulations. The luminosity weighted line-of-sight velocity dispersion and the Einstein radius of the lens are used as mass probes in the inner regions of the galaxy. Combining these three observational probes allows us to independently constrain the mass and concentration of the dark matter halo, in addition to the total stellar mass of the central galaxy. We find a dark matter halo in remarkably good agreement with simulations (log_10 M_200/M_sun = 14.2 +/- 0.2, c_200 = 4.4 (+1.6, -1.4)) and a stellar mass-to-light ratio which favors a Salpeter initial mass function ((M/L)* = 5.7 +/- 1.2). Our measurement of a normal halo concentration suggests that there is no discrepancy between simulations and observations at the group mass scale. This is in contrast to the cluster mass scale for which a number of studies have claimed over-concentrated halos. While the halo mass is robustly determined, and the halo concentration is not significantly affected by systematics, the resulting stellar mass-to-light ratio is sensitive to the choice of stellar parameters, such as density profile and velocity anisotropy.

9 pages, 8 figures. Accepted for publication by ApJ

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Cited by in corpus (7)

• The CASSOWARY spectroscopy survey: A new sample of gravitationally lensed galaxies in SDSS
• Galaxy structure from multiple tracers: II. M87 from parsec to megaparsec scales
• Stellar mass-gap as a probe of halo assembly history and concentration: youth hidden among old fossils
• Are group- and cluster-scale dark matter halos over-concentrated?
• CLASH: Extending galaxy strong lensing to small physical scales with distant sources highly-magnified by galaxy cluster members
• Constraining the multi-scale dark-matter distribution in CASSOWARY 31 with strong gravitational lensing and stellar dynamics
• Very high energy emission of Crab-like pulsars driven by the Cherenkov drift radiation