The Counter Arc to MS1512-cB58 and a Companion Galaxy

Abstract

We present near-infrared spectra of ``A2'', the primary counter arc to the gravitationally lensed galaxy MS1512-cB58. The spectra showredshifted H-alpha, [NII], [OIII], and H-beta at z=2.728 +/- 0.001. We observe the same H-alpha/[OIII] ratio as cB58, which together with the redshift confirms that A2 is indeed another image of a single background galaxy. Published lensing reconstruction reports that A2 is a magnification of the entire source, while cB58 is an image of only a part. At marginal significance, A2 shows higher line to continuum ratios than cB58 (by a factor of about 2), suggesting a non-uniform ratio of young to old stars across the galaxy. We observe a second emission line source in the slit. This object, ``W5'', is predicted to be a lensed image of another galaxy at a redshift similar to cB58. W5 is blueshifted from cB58 by about 400 km/s and has a significantly lower H-alpha/[OIII] ratio, confirming that it is an image of a different background galaxy in a group with cB58. The H-alpha emission line in W5 implies a star formation rate of 6 Msun/yr (H_0 = 70 km/s/Mpc, Omega_M=0.3, Omega_L=0.7), after correcting for lensing magnification.