The Canada-United Kingdom Deep Submillimeter Survey. II. First Identifications, Redshifts, and Implications for Galaxy Evolution

Identifications are sought for 12 submillimeter sources detected in a deep submillimeter survey. Six are securely identified, two have probable identifications, and four remain unidentified with IAB > 25. Spectroscopic and estimated photometric redshifts indicate that four of the sources have z < 1 and four have 1 < z < 3, with the remaining four empty-field sources probably lying at z > 3. The spectral energy distributions of the identifications, as defined by measurements or upper limits to the flux densities at 8000 Å, at 15, 450, 850 μm, and at 6 cm, are consistent with the spectral energy distributions of high-extinction starbursts such as Arp 220. The far-IR luminosities of the sources at z > 0.5 are of order 3 × 1012 h-250 L☉, i.e., slightly larger than that of Arp 220. As with local ultraluminous infrared galaxies, the optical luminosities of the identified galaxies are comparable to present-day L*, and the optical morphologies of many of the galaxies show evidence for mergers or highly disruptive interactions. Based on this small sample, the cumulative bolometric luminosity function shows strong evolution to z~1, but weaker or possibly even negative evolution beyond. The redshift dependence of the far-IR luminosity density does not appear, at this early stage, to be inconsistent with that seen in the ultraviolet luminosity density. Although the computation of bolometric luminosities is quite uncertain, the population of very luminous galaxies that is detected in the surveys at z > 1 is already matching, in the far-IR, the bolometric output in the ultraviolet of the whole optically selected population. Assuming that the energy source in the far-IR is massive stars, this suggests that the total luminous output from star formation in the universe will be dominated by the far-IR emission once the lower luminosity sources, below the current far-IR detection threshold, are included. Furthermore, the detected systems have individual star formation rates (exceeding 300 h-250 M☉ yr-1) that are much higher than seen in the ultraviolet-selected samples and that are sufficient to form substantial stellar populations on dynamical timescales of 108 yr. The association with mergerlike morphologies and the obvious presence of dust makes it attractive to identify these systems as forming the metal-rich spheroid population, in which case we would infer that much of this activity has occurred relatively recently, at z~2.