The z=2.51 extremely red submillimeter galaxy SMM J04431+0210

We report the redshift measurement for the submillimeter selected galaxy SMM J04431+0210 (N4) using the Near Infrared Spectrograph on the Keck II Telescope. The data show Halpha, [N II]lambdalambda6583, 6548, and [O III] lambda5007 lines at a redshift of z = 2.51. The high nuclear [N II]/Halpha line ratio is consistent with a LINER or type II active galactic nucleus (AGN). The Halpha 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 x 10(10) M., after correcting for lensing. The CO-to-submillimeter 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 submillimeter sources and local ultraluminous infrared galaxies. After the correction for lensing, N4 is the weakest intrinsic submillimeter-selected source with a known redshift and represents the first redshift for the under 2 mJy 850 mum sources that are responsible for the bulk of the emission from the submillimeter population as a whole. We argue that N4 contains either an AGN or LINER nucleus surrounded by an extended region of active star formation.