ISOCAM extragalactic mid-infrared deep surveys

Mathematics – Logic

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Cosmological Surveys, Mid-Infrared, Galaxy Evolution

Scientific paper

ISOCAM extragalactic mid-infrared deep surveys have detected a population of strong infrared emitters ten times more numerous than expected if there were no evolution from z < 0.2 (IRAS) up to the maximum redshift of these galaxies (z =1.5). The mid-infrared cosmic background produced by these galaxies ( = 2.35+/- 0.8 nW m-2 sr-1, at 15 μm above 50 μJy) is larger than 30 per cent of the energy radiated in the I band by the optical galaxies detected in the HDF, which are two orders of magnitude more numerous. This fraction is much higher than in the local universe (z<0.2) as probed by IRAS, where all integrated infrared emission from 8 to 1000 μm makes 30 per cent of the optical starlight (Soifer & Neugebauer 1991). Even assuming a conservative spectral energy distribution (SED), they produce a major contribution to the 140 μm DIRBE background measured by Hauser et al. (1998, = 25.1+/-7 nW m-2 sr-1) and Lagache et al. (1999, = 15.3+/-9.5 nW m-2 sr-1). This is both a confirmation of the strong infrared cosmic background that was first detected by Puget et al. (1996) and the first identification of the galaxies responsible for a large fraction of this background at 140 μm. We were able to identify the galaxies responsible for this strong infrared emission in the region of the Hubble Deep Field (HDF plus its flanking fields) due to the large number of ground-based observations of HDF galaxies. We find that these bright (several times 1011 Lodot) and massive ( = 1.5x 1011 Modot) infrared galaxies have a typical redshift of z =0.7 and optical colors similar to field galaxies. The fraction of galaxies with morphological signs of interactions is larger at higher redshift. This is consistent with the nature of local luminous infrared galaxies (LIGs, with Lbol = LIR>1011 Lodot) found by IRAS, which often exhibit evidence of galaxy interaction or merging (Sanders & Mirabel 1996) and indicates that interactions should play a crucial role in the evolution of galaxies as expected from bottom-up scenarios.

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