The OLS–lens survey: The discovery of five new galaxy–galaxy strong lenses from the SDSS⋆

Bright galaxy–galaxy strong lenses are much more powerful than lensed quasars for measuring the mass profiles of galaxies, but until this year only a handful have been known. Here we present five new examples, identified via the optimal line–of–sight gravitational lens search strategy applied to luminous red galaxies in the Sloan Digital Sky Survey (SDSS). Our survey largely complements a similar survey by Bolton et al., who recently presented several new lenses. The lensed background galaxies are selected from the SDSS spectra via the presence of narrow emission line signatures, including the [OII] λλ3726,3729, Hβ and [OIII] λλ4960,5008 lines, superposed on the spectra of the bright, intervening, deflector galaxies. Our five confirmed new systems include deflector galaxies with redshifts z = 0.17 − 0.28 and lensed galaxies with redshifts z = 0.47 − 1.18. Simulations of moderately deep (few orbits) HST–ACS imaging of systems such as these, where the lensed source is brighter than r ∼ 23, are presented. These demonstrate the feasibility of measuring accurately the inner slope of the dark matter halo to within an uncertainty σ(γ) ∼ 0.1, the dark matter fraction within the Einstein radius, and the mass–to–light ratio of the stars alone, independently of dynamical measurements. The high success rate of our search so far, > 60%, and the relatively modest observational resources necessary to confirm the gravitational lens nature of the candidates, demonstrate that compilation of a sample of ∼100 galaxygalaxy lenses from the SDSS is readily achievable, opening up a rich new field in dark matter studies.