Deep far-infrared surveys with ISO: evidence for strong evolution in the LIG population at intermediate redshift

Deep surveys at 90μm and 170μm carried out with the ISOPHOT detector onboard the Infrared Space Observatory (ISO) satellite have detected a substantial number of faint sources, consistent with strong evolution in the far-infrared number counts down to flux levels (4σ) of f90 ∼ 50mJy and f170 ∼ 100mJy. Initial ground-based follow-up observations at 20 cm with the VLA have been used to identify the most plausible optical counterparts to the far-infrared sources, and optical spectroscopy using the Keck II telescope has provided secure redshifts for a flux-limited sample of the brighter far-infrared objects. The ISOPHOT sources are found to span the redshift range z ∼ 0-1.6, and have luminosities in the range log Lir ∼ 10.5-13.52. The luminosity function of these intermediate-redshift, luminous infrared galaxies (LIGs) supports an earlier finding of strong evolution [i.e. Φ(z) ∝ (1 + z)6.5±3, assuming pure number density evolution] in the more local (z < 0.3) population of ultraluminous infrared galaxies (ULIGs) found at the faintest levels in the IRAS survey. The faint ISOPHOT sources also provide the first good evidence for a possible link between the local population of LIGs and the more distant population of faint submillimeter sources recently detected by SCUBA at 850μm.