Abstract :
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.
