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H. Aussel (Osservatorio Astronomico di Padova), L. Vigroux (SAp, CE-Saclay), A. Franceschini (Osservatorio Astronomico di Padova), D. Elbaz (SAp, CE-Saclay), M. Dennefeld (IAP), C.J. Cesarsky (ESO)
Extragalactic number counts obtained at 15~\mum in the ISOCAM deep surveys (Elbaz et al.,in press, astro-ph/9910406) present at the flux level of 0.4 mJy an excess of a factor of 10 with respect to the predictions of models deduced from IRAS counts with no evolution. We present investigations on the nature of the galaxies responsible for this excess.
Our study is focused on two fields, the Hubble Deep Field North and South, because 1) they span the range 0.1-0.5 mJy where the counts are dominated by the galaxies in excess ; 2) the large amount of data available on both fields enables a detailed multi-wavelength approach with a completeness in redshifts above 80%.
Combining optical colors and redshift of the 15~\mum sources, we show that they differ from the blue population responsible for the excess observed in the B-band counts, because they are intrinsically brighter and redder. Detailed study of spectroscopic star--formation indicators show that extinction is important in these objects.
Most of our detections have estimated bolometric luminosities in the range of 1011L\odot--1012L\odot, i.e they are Luminous InfraRed Galaxies. At these luminosities, local samples selected at 12~\mum with IRAS are dominated by AGN (Fang et al. 1998). Using spectra and radio spectral indexes, we show that the fraction of AGNs at higher redshifts in our sample is about 10%. This implies that most of our 15~\mum-selected galaxies are mainly starbursts, therefore suggesting a very strong evolution of dust enshrouded star-formation activity above a redshift of 0.5. Such activity is likely to be triggered by mergers, since our samples include numerous interacting galaxies. Extrapolating their far infrared emission, we show that these galaxies contribute for at least 30% and possibly all of the extragalactic infrared background detected at 140~\mum with DIRBE.
The author(s) of this abstract have provided an email address for comments about the abstract: aussel@pd.astro.it