Optically faint counterparts to the Infrared Space Observatory FIRBACK 170 micron population:: Discovery of cold, luminous galaxies at high redshift

We present Keck spectroscopy and UKIRT near-IR imaging observations of two 170 mum-selected sources from the Infrared Space Observatory (ISO) FIRBACK survey that have faint counterparts in the optical and r-K similar to 5. Both sources were expected to lie at z > 1, based on their far-infrared, submillimeter, and radio fluxes, assuming a spectral energy distribution similar to that of the local ultraluminous infrared galaxy (ULIRG) Arp 220. However, our spectroscopy indicates that the redshifts of these galaxies are less than 1: z = 0.91 for FIRBACK FN1-64, and z = 0.45 for FIRBACK FN1-40. While the bolometric luminosities of both galaxies are similar to that of Arp 220, it appears that the dust emission in these systems has a characteristic temperature of similar to30 K, much cooler than the similar to50 K seen in Arp 220. Neither optical spectrum shows evidence of active galactic nucleus activity. If these galaxies are characteristic of the optically faint FIRBACK population, then evolutionary models of the far-infrared background must include a substantial population of cold, luminous galaxies. These galaxies provide an important intermediate comparison between the local luminous IR galaxies and the high-redshift, submillimeter-selected galaxies, for which there is very little information available.