First case of strong gravitational lensing by a QSO: SDSS J0013+1523 at z = 0.120*

We present the first case of strong gravitational lensing by a QSO: SDSS J0013+1523 at z = 0.120. The discovery is the result of a systematic search for emission lines redshifted behind QSOs, among 22 298 spectra of the SDSS data release 7. Apart from the z = 0.120 spectral features of the foreground QSO, the spectrum of SDSS J0013+1523 also displays the [O ii] and Hβ emission lines and the [O iii] doublet, all at the same redshift, z = 0.640. Using sharp Keck adaptive optics K-band images obtained using laser guide stars, we unveil two objects within a radius of 2′′ from the QSO. Deep Keck optical spectroscopy clearly confirms one of these objects at z = 0.640 and shows traces of the [O iii] emission line of the second object, also at z = 0.640. Lens modeling suggests that they represent two images of the same z = 0.640 emission-line galaxy. Our Keck spectra also allow us to measure the redshift of an intervening galaxy at z = 0.394, located 3.2′′ away from the line of sight to the QSO. If the z = 0.120 QSO host galaxy is modeled as a singular isothermal sphere, its mass within the Einstein radius is ME(r < 1 h−1 kpc) = 2.16× 1010 h−1 M and its velocity dispersion is σSIS = 169 km s−1. This is about 1-σ away from the velocity dispersion estimated from the width of the QSO Hβ emission line, σ∗(MBH) = 124 ± 47 km s−1. Deep optical HST imaging will be necessary to constrain the total radial mass profile of the QSO host galaxy using the detailed shape of the lensed source. This first case of a QSO acting as a strong lens on a more distant object opens new directions in the study of QSO host galaxies.