COSMOS 5921+0638: Characterization and analysis of a new strong gravitationally lensed AGN
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[abridged] We present VLT/FORS1 spectroscopic follow-up observations and HST/WFPC2 imaging of the system COSMOS 5921+0638, which shows quadruply lensed images and a perfect Einstein ring. With the available observations of COSMOS 5921+0638 we study the nature of the system as well as its photometric, spectroscopic and physical properties. We have done an environmental analysis and detailed analytical and grid-based mass modeling. We have measured the redshift of the lensing galaxy in COSMOS 5921+0638 (z_l=0.551+/-0.001) and of 9 additional galaxies in the field (5 of them at z~0.35). The redshift of the source was inferred by identifying a candidate Ly_alpha line at z_s=3.14+/-0.05. The mass modeling reveals the requirement of a small external shear (gamma=0.038), which is slightly larger than the lensing contribution expected due to galaxy groups in the line-of-sight (kappa_groups~0.01 and gamma_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 mu~150. The measured mass-to-light ratio of the lensing galaxy within the Einstein ring is M/L_B~8.5+/-1.6. Our analysis shows that the ring and point-like structures in COSMOS 5921+0638 are made from a lensed high redshift galaxy hosting a Low Luminosity AGN (LLAGN). Flux ratio anomalies observed in the lensed AGN images are likely due to microlensing by stars in the lensing galaxy and/or a combination of static phenomena. Due to its short time-delays and likely possibility of microlensing, COSMOS 5921+0638 is a promising laboratory for future studies of LLAGNs.
