Redshifts and lens profile for the double quasar QJ 0158-4325

TL;DR

This paper determines the redshifts of the lensing galaxy and quasar in QJ 0158-4325, models the lens profile using HST and VLT data, and constrains the mass distribution with flux ratios, favoring a truncated-PIEMD model over SIE.

Contribution

It provides the first detailed lens model of QJ 0158-4325 incorporating flux ratio constraints, favoring the truncated-PIEMD profile.

Findings

Redshift of lensing galaxy: 0.317

Redshift of quasar: 1.294

Time delay between images: -14.5 days

Abstract

We report on the redshift of the lensing galaxy and of the quasar QJ 0158-4325 and on the lens model of the system. A deep VLT/FORS2 spectrum and HST/NICMOS-F160W images are deconvolved. From the images we derive the light profile of the lensing galaxy and an accurate relative astrometry for the system. In addition we measure the flux ratio between the quasar images in the MgII emission line to constrain the mass model. From the spectrum we measure the redshift of the lensing galaxy (z=0.317+/-0.001) and of the quasar (z=1.294+/-0.008). Using the flux ratio in the lens model allows to discard the SIE as a suitable approximation of the lens potential. On the contrary the truncated-PIEMD gives a good fit to the lens and leads to a time delay of t(A-B)=-14.5+/-0.1 days, with H0=73 km/s/Mpc. Using the flux ratio to constrain the mass model favors the truncated-PIEMD over the SIE, while ignoring this constraint leaves the choice open.