Mid-Infrared Emission of Galaxies as a Tracer of Star Formation: 
Local Evidence and Implications at Intermediate and High Redshift

Herve Aussel
IfA


I will review new results obtained with the ISO satellite on samples
of local galaxies at mid-infrared wavelengths (MIR, from 5 to 20
microns), that lead to some modifications of the interpretation of the IRAS
color-color diagram.  While it was considered to be a measure of the
global dust temperature of galaxies, and hence of their star 
formation activity, it is now more seen as an indicator of the
relative importance of quiescent and star-bursting regions within the
galaxies.  Still, I will show that the MIR emission of galaxies can be
used as a measure of their global star formation rate, and that it is
now possible to build reliable templates of the emission of galaxies
from the UV to the radio domain.

This has important consequences for the interpretation of
extragalactic number counts, obtained recently at 15 microns with ISO,
that are in excess by a factor 10 with respect to the predictions of
models without evolution.  I will show that a large fraction of the
infrared extragalactic background detected between 100 and 200 microns
in the data of the COBE satellite has been resolved into discrete
sources by the deep ISO surveys, and that this background is produced
at intermediate redshifts.  I will show that SIRTF deep surveys will
resolve most of the background measured at longer wavelengths