The Counterarc to MS 1512-cB58 and a Companion Galaxy

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/[O 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_solar yr^-1 (H₀=70 km s^-1 Mpc^-1, Ω_M=0.3,Ω_Λ=0.7), after correcting for lensing magnification.

Data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation.