Near-Infrared Colors of Submillimeter-selected Galaxies

We report on deep near-infrared (NIR) observations of submillimeter-selected galaxies (SMGs) with the Near-Infrared Camera (NIRC) on the Keck I telescope. We have identified K-band candidate counterparts for 12 out of 15 sources in the SCUBA Cluster Lens Survey. Three SMGs remain nondetections with K-band limits of K > 23 mag, corrected for lensing. Compensating for lensing, we find a median magnitude of K = 22 ± 1 mag for the SMG population, but the range of NIR flux densities spans more than a factor of 400. For SMGs with confirmed counterparts based on accurate positions from radio, CO, and/or millimeter continuum interferometric observations, the median NIR color is J-K = 2.6 ± 0.6 mag. The NIR-bright SMGs (K < 19 mag) have colors of J-K ≃ 2 mag, while the faint SMGs tend to be extremely red in the NIR (J-K > 3 mag). We argue that a color selection criterion of J-K ≳ 3 mag can be used to help identify counterparts of SMGs that are undetected at optical and radio wavelengths. The number density of sources with J-K > 3 mag is 5 arcmin^-2 at K < 22.5 mag, greater than that of SMGs with S(850 μm) > 2 mJy. It is not clear if the excess represents less luminous infrared-bright galaxies with S(850 μm) ≲ 2 mJy, or if the faint extremely red NIR galaxies represent a different population of sources that could be spatially related to the SMGs.