Gauging the dark matter fraction in an L* S0 galaxy at Z=0.47 through gravitational lensing from Deep HST advanced Camera Surveys Imaging

We analyze a new gravitational lens OAC-GL J1223-1239, serendipitously found in a deep I-814-band image of the Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS). The lens is an L*, edge-on S0 galaxy at z(1) = 0.4656. The gravitational arc has a radius of 0 ''.42 similar or equal to 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) similar to 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 similar to 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.