The Counterarc to MS 1512−cB58 and a Companion Galaxy

We present near-infrared spectra of ``A2'', the primary counter arc to thegravitationally lensed galaxy MS1512-cB58. The spectra showredshifted H-alpha,[NII], [OIII], and H-beta at z=2.728 +/- 0.001. We observe the sameH-alpha/[OIII] ratio as cB58, which together with the redshift confirms that A2is indeed another image of a single background galaxy. Published lensingreconstruction reports that A2 is a magnification of the entire source, whilecB58 is an image of only a part. At marginal significance, A2 shows higher lineto continuum ratios than cB58 (by a factor of about 2), suggesting anon-uniform ratio of young to old stars across the galaxy. We observe a secondemission line source in the slit. This object, ``W5'', is predicted to be alensed image of another galaxy at a redshift similar to cB58. W5 is blueshiftedfrom 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 withcB58. The H-alpha emission line in W5 implies a star formation rate of 6Msun/yr (H_0 = 70 km/s/Mpc, Omega_M=0.3, Omega_L=0.7), after correcting forlensing magnification.Comment: Accepted by ApJ. 18 pages including 5 figure