UCLA

We present near-infrared spectra of "A2," the primary counterarc to the gravitationally lensed galaxy MS 1512-cB58. The spectra show redshifted Hα, [N II], [O III], and Hβ at z = 2.729 ± 0.001. We observe the same Hα/[O III] 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 ∼2), suggesting a nonuniform 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 ∼400 km s-1 and has a significantly lower Hα/[0 III] ratio, confirming that it is an image of a different background galaxy in a group with cB58. The Hα emission line in W5 implies a star formation rate of 6 M⊙ yr-1 (H 0 = 70 km s-1 Mpc-1, ΩM = 0.3, ΩΛ = 0.7), after correcting for lensing magnification.