The discovery of two new galaxy-galaxy lenses from the Sloan Digital Sky Survey

Abstract

The gravitational lens configuration where a background galaxy is closely aligned with a foreground galaxy can provide accurate measurement of the dark matter density profile in the foreground galaxy, free of dynamical assumptions. Currently, only three such galaxy-galaxy lenses are known where the lensed source has a confirmed redshift and is reasonably bright at optical wavelengths, and therefore suitable for observations with the Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS). Two of these were discovered by noting an anomalous emission line (from the source) in the spectrum of a massive early-type galaxy (the lens). To find further galaxy-galaxy lenses suitable for ACS imaging, we have looked for anomalous emission lines in the luminous red galaxy (LRG) subsample of the Sloan Digital Sky Survey (SDSS) data release 3 (DR3) spectroscopic data base. Our search methodology has similarities to that applied by Bolton et al. (which has had recent success), but extends the upper redshift limit for lensed sources to z ≃ 1.4. Here, we report follow-up imaging and spectroscopic observations of two candidates, confirmed as gravitational lenses by the detection of multiple images in the line of [OII]λλ3726, 3729. In the first system, J145957.1-005522.8, the lens at z = 0.58 consists of two LRGs. The anomalous emission line is confirmed as [OII] by the detection of the corresponding Hy line, providing a source redshift of z = 0.94. In the second system, J230946.3-003912.9, the lens is a single LRG at z = 0.29, and the source redshift is z = 1.00, confirmed by partially resolving the [OII] doublet.