H. Aussel 1, C. Cesarsky 1, F.X. Désert 2, J.L. Puget 3, D. Elbaz 1, J.L. Starck 1, & D. Clements 2
1 DSM/DAPNIA/SAp, CEA-Saclay, 91191 Gif sur Yvette Cedex FRANCE
2 Institut d'Astrophysique Spatiale, Orsay, France
Several extragalactic surveys have been performed with ISOCAM, especially in two bands, one covering the 5 to 8.5 microns wavelength range (LW2) and the other the 12 to 18 microns range (LW3). In this last band the number counts obtained above 1 milli-Jansky agree with a no-evolution model. On the contrary, the number counts below 1 mJy show a very strong evolution, more abrupt than the one observed in optical counts.
I will focus on two very deep observation, the ISOCAM observation of the Northern Hubble Deep Field and the Ultradeep Survey of the Marano field in the south, where optical, spectroscopic and radio data are available to characterize the strongly evolving population that is observed in the mid-infrared.
I will discuss whether the ISOCAM LW3 band is dominated by a hot dust continuum or by the diffuse emission of unidentified infrared bands (UIB). I will show that the 12-18 microns deep counts are dominated by a population of mergers and irregulars galaxies, that display spectroscopic evidence for being starburst or post-starburst galaxies from their [OII] and balmer lines. Using the Hubble Deep Field data, I will compare this population to the other galaxies in the field, focusing on the optical colors and radio properties, and show that the galaxies selected in the mid-infrared (MIR) are not the population of faint blues galaxies that appear in the faint optical counts. Moreover, the colors of MIR selected galaxies are not very different from the bulk of the galaxies of the field. Using these results, I will address the question of the extinction at redshift above 0.5 and its impact on the estimate of star formation rate of galaxies using UV data.
Last, using the constraints on the cosmic MIR background from gamma rays observations of Mrk 501, I will show that the observations of the Marano Field and Hubble Deep Field already resolve significant fraction of this background into sources. This, together with the slope of the counts at their faint end allow us to place constraint on the quantity of energy radiated in the infrared with respect to the visible at redshift greater than 0.5. If locally, IRAS has shown than about 30% of the energy is emitted in the infrared, I will show that this proportion increases at higher redshift, implying that extinction plays a crucial role in the emission of high redshift objects.