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

Abstract

Aims: We report on the redshift of the lensing galaxy and of the quasar QJ 0158-4325 and on the lens model of the system.
Methods: 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 Mg ii emission line to constrain the mass model.
Results: 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 us 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 Δ t_A-B = -14.5 ±0.1 days, with H₀ = 73 km s^-1 Mpc^-1.
Conclusions: Using the flux ratio to constrain the mass model favors the truncated-PIEMD over the SIE, while ignoring this constraint leaves the choice open.

Based on observations made with ESO Telescopes at Paranal Observatory programme ID: 67.A-0502(A, B) and with NASA/ESA Hubble Space Telescope, obtained from the data archive at the Space Telescope Institute. STScI is operated by the association of Universities for Research in Astronomy, Inc. under the NASA contract NAS 5-26555.