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

¹ Astronomisches Rechen-Institut, Zentrum für Astronomie der Universität Heidelberg, Mönchhofstr. 12–14, 69120 Heidelberg, Germany e-mail: tanguita@ari.uni-heidelberg.de
² Laboratoire d'Astrophysique, École Polytechnique Fédérale de Lausanne (EPFL), Observatoire de Sauverny, 1290 Versoix, Switzerland
³ Laboratoire d'Astrophysique de Marseille, UMR 6610, CNRS-Université de Provence, 38 rue Frédéric Joliot-Curie, 13 388 Marseille Cedex 13, France
⁴ Institute of Astronomy, Swiss Federal Institute of Technology (ETH Honggerberg), 8093 Zurich, Switzerland
⁵ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
⁶ Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen, Denmark

Accepted: 23 July 2009

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 ( ~ 0.01 and  ~ 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  ≈ 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.