The rest-frame optical spectrum of MS 1512-cB58

Moderate-resolution, near-IR spectroscopy of MS 1512-cB58 is presented, obtained during commissioning of the near-infrared spectrometer (NIRSPEC) on the Reck II telescope. The strong lensing of this z = 2.72 galaxy by the foreground cluster MS 1512+36 makes it the best candidate for detailed study of the rest-frame optical properties of Lyman-break galaxies. In 80 minutes of on-source integration, we have detected H alpha, [N II] lambda lambda 6583, 6548, [O I] lambda 6300, He I lambda 5876, [O III] lambda lambda 5007, 4959, H beta, H gamma, [O II] lambda 3727, and a strong continuum signal in the range of 1.29-2.46 mu m. A redshift of z = 2.7290 +/- 0.0007 is inferred from the emission lines, in contrast to the z = 2.7233 calculated from UV observations of interstellar absorption lines. Using the Balmer line ratios, we find an extinction of E(B-V) = 0.27. Using the line strengths? we infer a star formation rate (SFR) of 620 +/- 18 M. yr(-1) (H-0 = 75, q(0) = 0.1, and Lambda = 0), which is a factor of 2 higher than that measured from narrowband imaging observations of the galaxy but is a factor of almost 4 lower than the SFR inferred from the UV continuum luminosity. The width of the Balmer lines yields a mass of M-vir = 1.2 x 10(10) M.. We find that the oxygen abundance is 1/3 solar, in good agreement with other estimates of the metallicity. However, we infer a high nitrogen abundance, which may argue for the presence of an older stellar population.