2.4.2 Far Infrared

Obviously, far-IR selected samples are even more seriously affected by confusion. Figure 2.5 shows the counts from FIRBACK and the ISOPHOT Serendipitous Survey. The counts are consistent with models that include a strong evolving population of starbursts and a non-evolving population of spirals (Franceschini, 2001).

The integrated counts are an order of magnitude greater than the 100 $\mu$m IRAS counts at the same flux level. In the ELAIS area, 31 sources have a 90 $\mu$m counterpart. The average 175/90 flux density ratio is 2, as compared with 1 for a sample of local sources (Stickel, 1998). Oa partion of the Marano ISOPHOT field overlaps with the area observed by a deep ISOCAM survey. At most, 50% of the ISOPHOT sources have 15 $\mu$m counterparts (Elbaz, 1999). For those, the 175 $\mu$m/15 $\mu$m flux density ratios correspond to what one would expect for relatively nearby ($z\sim 0.5$) galaxies with Arp220-like spectra. Scott (2000) observed ten FIRBACK sources with SCUBA. The average 850 (and 450) $\mu$m/175 $\mu$m flux density ratios also favor a low mean redshift ($\sim 0.3$) although the degenerate dust temperature/redshift relation would also permit in principle solutions with $z$ up to 2. The far-IR sources not detected by ISOCAM could be at higher redshift. The interplay of the k-corrections is such that ISOCAM cannot see galaxies with an Arp220-like SED located at $z\sim 2$.

The integrated counts constitute less than 10% of the COBE far-IR background (Hauser, 1998). This suggests that ISOPHOT is detecting the tip of the iceberg of a new population of very luminous, moderate to high redshift galaxies. Most of the background is in sources with 175 $\mu$m flux densities between a few and 100mJy.

Figure 2.5: ISO 175 $\mu$m Integral Source Number Counts. Filled circles are from ISOPHOT FIRBACK survey (Dole, 2001) and empy squares from ISOPHOT serendipitous survey. The dashed and dot-dashed lines correspond to the non-evolving spirals the strongly evolving starbursts. The lowest curve is the expected contribution of AGN. The horizontal lines mark the estimated confusion limits for three telescopes sizes. The lines marked "60cm" and "360cm" correspond to ISO and Herschel (see Section B.8). From Franceschini (2001).