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 Keck 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 Hα, [N II] λλ6583, 6548, [O I] λ6300, He I λ5876, [O III] λλ5007, 4959, Hβ, Hγ, [O II] λ3727, and a strong continuum signal in the range of 1.29-2.46 μm. 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 (H0 = 75, q0 = 0.1, and Λ = 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 Mvir = 1.2 × 1010 M☉. We find that the oxygen abundance is 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.