  We introduce a new infrared diagnostic to separate galaxies on the basis of
their dominant infrared emission: stellar or nuclear. The main novelty with
respect to existing diagnostics, is the usage of a broad band encompassing at
the same time the 9.7micron Silicate absorption feature and one of the adjacent
broad PAH (polycyclic aromatic hydrocarbon) features. This provides a robust
estimate of the near- to mid-infrared continuum slope and enables a clear
distinction among different classes of galaxies up to a redshift z=2.5. The
diagnostic can be applied to a wealth of archival data from the ISO, Spitzer,
and Akari surveys as well as future JWST surveys. Based on data in the GOODS,
Lockman Hole, and North Ecliptic Pole (NEP) fields, we find out that
approximately 70% active galactic nuclei detected with X-ray and optical
spectroscopy dominate the total mid-infrared emission. Finally, we estimate
that AGN contribute less than 30% of the mid-infrared extragalactic integrated
emission.
