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

FIRBACK is a deep survey conducted with the ISOPHOT instrument aboard the I nfrared 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 Mar ano 1 which covers around 0.25 square degree. We find that the source densi ty for objects with a flux above 200 mJy exceeds the counts expected for so urces 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 infr ared background detected with the FIRAS and DIRBE instruments aboard the Co smic Background Explorer (COBE). These sources are likely to be redshifted infrared galaxies. The steep number counts indicate strong cosmological evo lution in this population. The detected sources account for only 10 % of th e 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.