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Astron. Astrophys. 359, 887-899 (2000)

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Mid-infrared diagnostics to distinguish AGNs from starbursts *

O. Laurent 1,2, I.F. Mirabel 1,3, V. Charmandaris 4,5, P. Gallais 1, S.C. Madden 1, M. Sauvage 1, L. Vigroux 1 and C. Cesarsky 1,6

1 CEA/DSM/DAPNIA Service d'Astrophysique, 91191 Gif-sur-Yvette, France
2 Max-Planck-Institut für Extraterrestrische Physik, Postfach 1603, 85740 Garching, Germany
3 Instituto de Astronomía y Física del Espacio. cc 67, suc 28. 1428 Buenos Aires, Argentina
4 Observatoire de Paris, DEMIRM, 61 Av. de l'Observatoire, 75014 Paris, France
5 Astronomy Department, Cornell University, Ithaca NY, 14853, USA
6 European Southern Observatory, 85748 Garching, Germany

Received 31 May 1999 / Accepted 18 May 2000


We present new mid-infrared (MIR) diagnostics to distinguish emission of active galactic nuclei (AGN) from that originating in starburst regions. Our method uses empirical spectroscopic criteria based on the fact that MIR emission from star forming or active galaxies arises mostly from HII regions, photo-dissociation regions (PDRs) and AGNs. The analysis of the strength of the 6.2 µm Unidentified Infrared Band (UIB) and the MIR continuum shows that UIBs are very faint or absent in regions harboring the intense and hard radiation fields of AGNs and pure HII regions, where the UIB carriers could be destroyed. The MIR signature of AGNs is the presence of an important continuum in the 3-10 µm band which originates from very hot dust heated by the intense AGN radiation field. Using these two distinct spectral properties found in our MIR templates, we build diagnostic diagrams which provide quantitative estimates of the AGN, PDR and HII region contribution in a given MIR spectrum. This new MIR classification can be used to reveal the presence of AGNs highly obscured by large columns of dust.

Key words: galaxies: active – galaxies: starburst – infrared: galaxies

* Based on observations made with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, the Netherlands and the United Kingdom) and with the participation of ISAS and NASA.

Send offprint requests to: O. Laurent (olaurent@mpe.mpg.de)

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© European Southern Observatory (ESO) 2000

Online publication: July 13, 2000