The serendipitous observation of a gravitationally lensed galaxy at z = 0.9057 from the Blanco Cosmology Survey: The Elliot Arc

TL;DR

This paper reports the discovery and analysis of a gravitationally lensed galaxy at redshift 0.9057, using multiple methods to estimate the cluster mass and comparing it with theoretical predictions, while also measuring the galaxy's star formation rate.

Contribution

The study presents the first detailed analysis of a lensing galaxy at z=0.9057, combining weak and strong lensing, spectroscopy, and modeling to estimate cluster mass and concentration.

Findings

Cluster mass M_200 = (5.1 +- 1.3) x 10^14 M_Sun

NFW concentration c_200 = 5.4 (+1.4 -1.1)

Galaxy's star formation rate is modest

Abstract

We report on the serendipitous discovery in the Blanco Cosmology Survey (BCS) imaging data of a z = 0.9057 galaxy that is being strongly lensed by a massive galaxy cluster at a redshift of z=0.3838. The lens (BCS J2352-5452) was discovered while examining i- and z-band images being acquired in October 2006 during a BCS observing run. Follow-up spectroscopic observations with the GMOS instrument on the Gemini South 8m telescope confirmed the lensing nature of this system. Using weak plus strong lensing, velocity dispersion, cluster richness N_200, and fitting to an NFW cluster mass density profile, we have made three independent estimates of the mass M_200 which are all very consistent with each other. The combination of the results from the three methods gives M_200 = (5.1 +- 1.3) x 10^14 M_Sun, which is fully consistent with the individual measurements. The final NFW concentration c_200 from the combined fit is c_200 = 5.4(+1.4 -1.1). We have compared our measurements of M_200 and c_200 with predictions for (a) clusters from Lambda_CDM simulations, (b) lensing selected clusters from simulations, and (c) a real sample of cluster lenses. We find that we are most compatible with the predictions for Lambda_CDM simulations for lensing clusters, and we see no evidence based on this one system for an increased concentration compared to Lambda_CDM. Finally, using the flux measured from the [OII]3727 line we have determined the star formation rate (SFR) of the source galaxy and find it to be rather modest given the assumed lens magnification.