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Astron. Astrophys. 343, L65-L69 (1999)

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5. Conclusion

In our ultra-deep 15µm survey, deeper by almost a factor of two than other surveys, we do not detect any sign of the flattening of the 15µm counts at the faintest levels, expected from evolutionary models (Franceschini et al. 1994). We derive a resolved MIR cosmic background of 3.3 nW m-2 sr-1, with a median redshift of sources of 0.7 (Metcalfe et al. 1999), similar to [FORMULA] of ISOCAM sources in the HDF (Aussel et al. 1999). ISO counts demonstrate comparable forms of evolution in the mid-IR to those observed in the CFRS (Flores et al. 1999b), then a larger amount of energy was emitted above z=0.6 than in the local universe.

Deep 15 µm ISOCAM imaging is a good way to select star-forming/AGN galaxies at moderate to high redshifts which are not easy to identify in UV/optical surveys (Ivison et al. 1998, Soucail et al. 1999). A number of these sources are well correlated with faint galaxies in the visible, some of them having very red colours in the NIR. Caution must therefore be employed when inferring global star formation activity based only on UV-continuum or optical luminosities of high-z galaxies. A more detailed analysis of the SED of these MIR detected galaxies will be necessary to unveil the nature of these MIR sources and to give an estimate of their SFR. But our observations confirm already that abundant star formation activity occurs in very dusty environments at [FORMULA].

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© European Southern Observatory (ESO) 1999

Online publication: March 1, 1999