Extensive light profile fitting of galaxy-scale strong lenses

Towards an automated lens detection method

Institut d’Astrophysique de Paris, UMR7095 CNRS & Université Pierre et Marie Curie, 98bis Bd Arago, 75014 Paris, France
e-mail: brault@iap.fr

Received: 4 November 2014
Accepted: 3 February 2015

Abstract

Aims. We investigate the merits of a massive forward-modeling of ground-based optical imaging as a diagnostic for the strong lensing nature of early-type galaxies, in the light of which blurred and faint Einstein rings can hide.

Methods. We simulated several thousand mock strong lenses under ground- and space-based conditions as arising from the deflection of an exponential disk by a foreground de Vaucouleurs light profile whose lensing potential is described by a singular isothermal ellipsoid. We then fitted for the lensed light distribution with sl_fit after subtracting the foreground light emission (ideal case) and also after fitting the deflector light with galfit. By setting thresholds in the output parameter space, we can determine the lensed or unlensed status of each system. We finally applied our strategy to a sample of 517 lens candidates in the CFHTLS data to test the consistency of our selection approach.

Results. The efficiency of the fast modeling method at recovering the main lens parameters such as Einstein radius, total magnification, or total lensed flux is quite similar under CFHT and HST conditions when the deflector is perfectly subtracted (only possible in simulations), fostering a sharp distinction between good and poor candidates. Conversely, a substantial fraction of the lensed light is absorbed into the deflector model for a more realistic subtraction, which biases the subsequent fitting of the rings and then disturbs the selection process. We quantify completeness and purity of the lens-finding method in both cases.

Conclusions. This suggests that the main limitation currently resides in the subtraction of the foreground light. Provided further enhancement of the latter, the direct forward-modeling of large numbers of galaxy-galaxy strong lenses thus appears tractable and might constitute a competitive lens finder in the next generation of wide-field imaging surveys.