An Extremely Thin Gravitational Arc Formed by the Distant Cluster CL 0016+1609: Lensing of a Compact HDF-Type Object?

Abstract

Hubble Space Telescope WFPC2 images of the core of the distant cluster of galaxies CL 0016+1609 (z =0.545) have revealed a single extremely thin faint arc, likely produced by gravitational lensing of a very distant background galaxy. The arc, which we refer to as Arc-1, is ~3.5 arcsec in extent, has a radius of curvature of 25+/-5 arcsec, and subtends an angle of ~8^deg^ with respect to the center of the cluster. Arc-1 has total magnitudes of I = 23.30 and V = 24.65. As Arc-1 is unresolved in the Wide Field Camera CCD, with a FWHM <= 0.20 arcsec, the source galaxy is magnified by at least a factor of 7.8(2.2 mag). Therefore, the magnitudes for the unmagnified source galaxy are I >= 25.5 and V >= 26.85, similar to the very compact objects seen in the Hubble Deep Field. If the source galaxy is at a redshift of 3, it would be close to L^*^ in luminosity, decreasing in luminosity for larger magnification factors. The surface brightness of Arc-1 is not uniform. Rather, it is asymmetric, having several bright knots located on one side. This suggests Arc-1 is a single lensed image of a background galaxy and that the surface brightness variations along Arc-1 result from intrinsic structure in the source galaxy. This arc is an excellent demonstration of how the combination of the HST with a "gravitational telescope" can provide the most detailed structural information on distant compact galaxies and protogalaxies. Assuming Arc-1 lies close to the Einstein radius of the cluster, we find the V-band mass-to-light ratio of the cluster core to be M/L~ 370h (in solar units).