Lensing in the Hercules Supercluster

Abstract

We report Keck LRIS observations of an arclike background galaxy near the center of Abell 2152 (z = 0.043), one of the three clusters comprising the Hercules supercluster. The background object has a redshift z = 0.1423 and is situated 25'' north of the primary component of the A2152 brightest cluster galaxy (BCG). The object is about 15'' in total length and has a reddening-corrected R-band magnitude of mR = 18.55 ± 0.03. Its spectrum shows numerous strong emission lines, as well as absorption features. The strength of the Hα emission would imply a star formation rate SFR ≈ 3 h-2 M⊙ yr-1 in the absence of any lensing. However, the curved shaped of this object and its tangential orientation along the major axis of the BCG suggest lensing. We model the A2152 core mass distribution including the two BCG components and the cluster potential. We present velocity and velocity dispersion profile measurements for the two BCG components and use these to help constrain the potential. The lens modeling indicates a likely magnification factor of ~1.9 for the lensed galaxy, making A2152 the nearest cluster in which such significant lensing of a background source has been observed. Finally, we see evidence for a concentration of early-type galaxies at z = 0.13 near the centroid of the X-ray emission previously attributed to A2152. We suggest that emission from this background concentration is the cause of the offset of the X-ray center from the A2152 BCG. The background concentration and the dispersed mass of the Hercules supercluster could add further to the lensing strength of the A2152 cluster.