COSMOS 5921+0638: characterization and analysis of a new strong gravitationally lensed AGN

Abstract

Context: Strong lens candidates have been newly identified within the COSMOS field. We present VLT/FORS1 spectroscopic follow-up observations and HST/WFPC2 imaging of the system COSMOS 5921+0638, which exhibits quadruply lensed images and a perfect Einstein ring.
Aims: We investigate the nature of COSMOS 5921+0638 by studying its photometric, spectroscopic and physical properties.
Methods: By analyzing our VLT/FORS1 spectroscopy and Subaru/CFHT/HST imaging of COSMOS 5921+0638, we completed both an environmental analysis and detailed analytical and grid-based mass modeling to determine it properties.
Results: We measured the redshifts of the lensing galaxy in COSMOS 5921+0638 (z_l = 0.551 ± 0.001) and 9 additional galaxies in the field (5 of them at z ~ 0.35). The redshift of the lensed source was inferred by identifying a candidate Lyα line at z_s = 3.14 ± 0.05. The mass modeling reveals the requirement of a small external shear (γ = 0.038), which is slightly larger than the lensing contribution expected by galaxy groups along the line-of-sight obtained from the zCOSMOS optical group catalog (κ_groups ~ 0.01 and γ_groups ~ 0.005). The estimated time-delays between the different images are of the order of hours to half a week and the total magnification of the background source is μ ≈ 150. The measured mass-to-light ratio of the lensing galaxy within the Einstein ring is M/L_B ≈ 8.5 ± 1.6. Anomalies are observed between the measured and expected flux ratios of the images of the background AGN.
Conclusions: Our analysis indicates that the ring and point-like structures in COSMOS 5921+0638 consist of a lensed high redshift galaxy hosting a low luminosity AGN (LLAGN). The observed flux ratio anomalies are probably due to microlensing by stars in the lensing galaxy and/or a combination of static phenomena. Multi-epoch, multi-band space-based observations would allow us to differentiate between the possible causes of these anomalies, since static and/or dynamic variations could be identified. Because of its short time-delays and the possibility of microlensing, COSMOS 5921+0638 is a promising laboratory for future studies of LLAGNs.

Based on observations made with ESO Telescopes at Paranal Observatory programme IDs: 077.A-0473(A) and 175.A-0839(B,D) 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; also based on data collected at: the Subaru Telescope, which is operated by the National Astronomical Observatory of Japan; the European Southern Observatory under Large Program 175.A-0839, Chile; Kitt Peak National Observatory, Cerro Tololo Inter-American Observatory, and the National Optical Astronomy Observatory, which are operated by the Association of Universities for Research in Astronomy, Inc. (AURA) under cooperative agreement with the National Science Foundation; and the Canada-France-Hawaii Telescope operated by the National Research Council of Canada, the Centre National de la Recherche Scientifique de France and the University of Hawaii.