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

Abstract

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 [O-ii]λλ3726, 3729, Hβ and [O-iii]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) Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS) imaging of systems such as these, where the lensed source is brighter than r∼ 23, are presented. These demonstrate the feasibility of accurately measuring 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 per cent, and the relatively modest observational resources necessary to confirm the gravitational lens nature of the candidates, demonstrates that compilation of a sample of ∼100 galaxy–galaxy lenses from the SDSS is readily achievable, opening up a rich new field in dark matter studies.