O. Laurent 1, I.F. Mirabel 1, V. Charmandaris 2, & P. Gallais 1
1 DSM/DAPNIA/SAp, CEA-Saclay, 91191 Gif sur Yvette Cedex FRANCE
2 DEMIRM, Observatoire de Paris, FRANCE
In the ultraluminous galaxies, it is still uncertain whether the physical phenomenon responsible for the infrared emission is a starburst and/or an AGN. The mid-infrared observations, in providing a new deeper view of such mechanisms, can be very useful to constrain the different contributions.
We present ISOCAM observations (GT program CAMACTIV (P.I. F.Mirabel)) of a large sample of galaxies harboring starburst and/or AGN activity. The comparison of pure starburst (M82) and AGN spectra (NGC 1068) allows us to constrain their typical mid-infrared emission (band and continuum features). Our data clearly show that the presence of an AGN significantly modifies the shape of the mid-IR spectrum.
A new diagnostic in the spectral range 4-16 microns using two different indicators is used to discriminate between the starburst emission and that of an AGN. This method based on the PAH strength and the continuum shape can be employed to detect the presence of a hidden AGN and to estimate the power of the AGN relative to the starburst in the mid-infrared range.
This study shows that the ultraluminous interacting galaxies in our sample (Arp 220, NGC 6240,IRAS 23128-5919) are classified as harboring a strong starburst activity with a non-detectable AGN feature except IRAS 19254-7245 which contains a non-negligible AGN emission mixed with the starburst emission.