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

Abstract

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 M E (r < 1 h −1 kpc) = 2.16 × 10 10 h −1 M and its velocity dispersion is σ SIS = 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 * (M BH ) = 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.

Three quasi-stellar objects acting as strong gravitational lenses

“…7. Note that SDSS J0013+1523 was already presented in Courbin et al (2010) but the present HST images do not clearly support the lensing nature of this object.…”

“…Courbin et al (2010) detected two point-like sources symmetrical about the foreground QSO, from near-IR adaptive optics images. The fainter of these sources is not visible in the optical HST images, either because it is heavily obscured by dust or because it was an artifact in the Keck adaptive optics images.…”

“…For each of the three objects, as well as for SDSS J0013+1523, previously published in Courbin et al (2010), we took six dithered exposures in the F814W filter and six exposures in the F475W filter. These observations correspond to one full orbit per filter and per object.…”

“…Our goal is to weigh QSO host galaxies for the first time with strong gravitational lensing and to compare their dynamical and lensing masses. We presented the search method in Courbin et al (2010) which included the first case of a QSO acting as a strong lens, based on Keck adaptive optics (AO) imaging observations. In the present paper, we show three new strong "QSO lenses" for which we have Keck optical imaging and spectroscopy as well as deep HST images in two bands with the WFC3 camera in the visible channel.…”

Quasi-stellar objects acting as potential strong gravitational lenses in the SDSS-III BOSS survey

“…In Section 4 we discuss the completeness of our algorithm search for quasar lenses. We then review the number of QSO-ELG candidates obtained compared to Courbin et al (2010).…”