OPTICALLY FAINT COUNTERPARTS TO THE ISO-FIRBACK 170μm POPULATION: THE DISCOVERY OF COLD, LUMINOUS GALAXIES AT HIGH REDSHIFT

We present Keck spectroscopy and UKIRT near-IR imaging observations of two 170μm-selected sources from the ISO-FIRBACK survey which have faint counterparts in the optical, and r − K ∼ 5. Both sources were expected to lie at z > 1 based on their far-infrared, submillimeter and radio fluxes, assuming a similar spectral energy distribution to the local ultra-luminous infrared galaxy (ULIRG) Arp 220. However, our spectroscopy indicates that the redshifts of these galaxies are z < 1: z = 0.91 for FN1-64 and z = 0.45 for FN1-40. While the bolometric luminosities of both galaxies are similar to Arp 220, it appears that the dust emission in these systems has a characteristic temperature of ∼30K, much cooler than the ∼50K seen in Arp 220. Neither optical spectrum shows evidence of AGN 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. Subject headings: cosmology: observations — galaxies: evolution — galaxies: formation — galaxies: starburst