A sharp look at the gravitationally lensed quasar SDSS J0806+2006 with Laser Guide Star Adaptive Optics

TL;DR

This paper demonstrates the use of laser guide star adaptive optics on the VLT to obtain high-resolution near-infrared images of a gravitationally lensed quasar, enabling precise astrometry and lens modeling comparable to HST, with implications for future observations.

Contribution

First VLT near-IR adaptive optics observations of a gravitationally lensed quasar, providing accurate astrometry and lens modeling without natural guide stars.

Findings

Astrometry with 0.05 arcsec resolution comparable to HST.

Lens galaxy is nearly circular, no strong external shear needed.

Predicted time delay of approximately 50 days.

Abstract

We present the first VLT near-IR observations of a gravitationally lensed quasar, using adaptive optics and laser guide star. These observations can be considered as a test bench for future systematic observations of lensed quasars with adaptive optics, even when bright natural guide stars are not available in the nearby field. With only 14 minutes of observing time, we derived very accurate astrometry of the quasar images and of the lensing galaxy, with 0.05 \arcsec spatial resolution, comparable to the Hubble Space Telescope (HST). In combination with deep VLT optical spectra of the quasar images, we use our adaptive optics images to constrain simple models for the mass distribution of the lensing galaxy. The latter is almost circular and does not need any strong external shear to fit the data. The time delay predicted for SDSS0806+2006, assuming a singular isothermal ellipsoid model and the concordance cosmology, is Delta t \simeq 50 days. Our optical spectra indicate a flux ratio between the quasar images of A/B=1.3 in the continuum and A/B=2.2 in both the MgII and in the CIII] broad emission lines. This suggests that microlensing affects the continuum emission. However, the constant ratio between the two emission lines indicates that the broad emission line region is not microlensed. Finally, we see no evidence of reddening by dust in the lensing galaxy.