FIRBACK: I. A deep survey at 175 $\mu$m with ISO, preliminary results

TL;DR

The FIRBACK survey at 175 μm using ISO's ISOPHOT instrument reveals a higher-than-expected density of infrared sources, indicating strong cosmological evolution, but these sources only partially account for the cosmic infrared background.

Contribution

First deep 175 μm survey with ISO showing significant excess of infrared sources and evidence of strong cosmological evolution in this population.

Findings

Source density exceeds IRAS expectations by an order of magnitude.

Detected sources account for only 10% of the cosmic IR background.

Steep number counts suggest strong cosmological evolution.

Abstract

FIRBACK is a deep survey conducted with the ISOPHOT instrument aboard the Infrared Space Observatory (ISO) at an effective wavelength of 175 $\mu$m. We present here results we have obtained on the first field, the so-called Marano1 which covers around 0.25 square degree. We find that the source density for objects with a flux above 200 mJy exceeds the counts expected for sources found in the IRAS deep surveys with a similar flux by about an order of magnitude. Such an excess was expected on the basis of the high far infrared background detected with the FIRAS and DIRBE instruments aboard the Cosmic Background Explorer (COBE). These sources are likely to be redshifted infrared galaxies. The steep number counts indicate strong cosmological evolution in this population. The detected sources account for only 10 % of the cosmic IR background. An extrapolation of the counts down to about 10 mJy would be needeed to account for the whole background at this wavelength.