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 followup 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.

Survey of Gravitationally lensed Objects in HSC Imaging (SuGOHI)

“…There are three known lenses in the HSC S16A footprint found again by Chitah: HSCJ092455+021923 (Inada et al 2012), HSCJ095921+020638 (Anguita et al 2009), and HSCJ115252+004733 (More et al 2017) 3 . We include two new lens candidates, HSCJ091148+041852 and HSCJ141136−010216, found by YattaLens but missed in Chitah's classification 4 , for X-shooter spectroscopic followup.…”

Chitah: STRONG-GRAVITATIONAL-LENS HUNTER IN IMAGING SURVEYS

“…They also measured the lens redshift to be z l = 0.551 from FORS1 observations and inferred a possible AGN source redshift of z s = 3.14 from the u* drop-out criterion and a candidate Ly-alpha line. Anguita et al (2009) found that an SIE with a small amount of external shear (γ = 0.038) provides an adequate fit to the observed positions of the quasar images in COSMOS 5921+0638.…”

GaiaGraL:GaiaDR2 gravitational lens systems

“…Falco et al (1999). References for full table available only at CDS: Morgan et al (2003Morgan et al ( , 2004, Johnston et al (2003), Biggs et al (2003), Inada et al (2003bInada et al ( , 2005Inada et al ( , 2006Inada et al ( , 2007Inada et al ( , 2010Inada et al ( , 2012Inada et al ( , 2014, Wisotzki et al (2004), Pindor et al (2004), Oguri et al (2005), McKean et al (2005, Eigenbrod et al (2006), Bolton et al (2006), Morokuma et al (2007), Blackburne et al (2008), Jackson (2008), Anguita et al (2009), Ghosh & Narasimha (2009)…”