The Z = 2.51 Extremely Red Submillimeter Galaxy Smm J04431+0210

We report the redshift measurement for the submillimeter selected galaxy SMMJ04431+0210 (N4) using the Near Infrared Spectrograph on the Keck II telescope. The data show Hα, [NII] 6583,6548, and [OIII] 5007 lines at a redshift of z = 2.51. The high nuclear [NII]/Hα line ratio is consistent with a LINER or Type-II AGN. The Hα emission is spatially resolved, suggesting the presence of significant star-forming activity outside the nucleus. From imaging with the Near Infrared Camera on the Keck I telescope, we find an extremely red near-infrared color of J−K=3.2 for N4. Follow-up redshifted CO(3→2) observations with the Owens Valley Millimeter Array constrain the mass of molecular gas to be less than 4 × 10M⊙, after correcting for lensing. The CO to sub-mm flux limit, the spectroscopic line ratios, and the spectral energy distribution for N4 are all within the range of properties found in other high-redshift sub-mm sources and local ultraluminous infrared galaxies. After the correction for lensing, N4 is the weakest intrinsic sub-mm selected source with a known redshift and represents the first redshift for the < 2mJy 850μm sources which are responsible for the bulk of the emission from the sub-mm population as a whole. We argue that N4 contains either an AGN or LINER nucleus surrounded by an extended region of active star-formation. Subject headings: galaxies: active — galaxies: evolution — galaxies: formation — galaxies: individual (SMMJ04431+0210) — galaxies: starburst