Gauging the dark matter fraction in an L* S0 galaxy at z = 0.47 through gravitational lensing from deep Hubble space telescope/advanced camera for surveys imaging

Abstract

We analyze a new gravitational lens OAC-GL J1223-1239, serendipitously found in a deep I814-band image of the Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS). The lens is an L*, edge-on S0 galaxy at z l = 0.4656. The gravitational arc has a radius of 042 ≃ 1.74 h -1 kpc. We have determined the total mass and the dark matter (DM) fraction within the Einstein radius R E as a function of the lensed source redshift, which is presently unknown. For z s ∼ 1.3, which is in the middle of the redshift range plausible for the source according to some external constraints, we find the central velocity dispersion to be ∼180 km s-1. With this value, close to that obtained by means of the Faber-Jackson relation at the lens redshift, we compute a 30% DM fraction within R E (given the uncertainty in the source redshift, the allowed range for the DM fraction is 25%-35% in our lensing model). When compared with the galaxies in the local universe, the lensing galaxy OAC-GL J1223-1239 seems to fall in the transition regime between massive, DM-dominated galaxies and lower-mass, DM-deficient systems. © 2009. The American Astronomical Society. All rights reserved.