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Issue A&A
Volume 419, Number 3, June I 2004
Page(s) L49 - L53
Section Letters
DOI http://dx.doi.org/10.1051/0004-6361:20040143



A&A 419, L49-L53 (2004)
DOI: 10.1051/0004-6361:20040143

Letter

Mid-infrared selection of AGN

M. Haas1, R. Siebenmorgen2, C. Leipski1, S. Ott3, B. Cunow4, H. Meusinger5, S. A. H. Müller1, R. Chini1 and N. Schartel6

1  Astronomisches Institut, Ruhr-Universität Bochum (AIRUB), Universitätsstr. 150/NA7, 44780 Bochum, Germany
2  European Southern Observatory (ESO), Karl-Schwarzschild-Str. 2, 85748 Garching, Germany
3  HERSCHEL Science Centre, ESA, Noordwijk, PO Box 299, 2200 AG Noordwijk, The Netherlands
4  Department of Mathematics, Appl. Math. & Astronomy, University of South Africa, PO Box 392, Pretoria 0003, South Africa
5  Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany
6  XMM-Newton Science Operation Center, ESA, Vilspa, Apartado 50727, 28080 Madrid, Spain

(Received 26 February 2004 / Accepted 15 April 2004 )

Abstract
Since a large fraction of active galactic nuclei (AGN) is missed in common UV-excess surveys and is even hard to find in radio, near-IR and X-ray surveys, we have used a new AGN selection technique which is expected to be not affected by extinction. Within the scientific verification of the ISOCAM Parallel Survey at $6.7~\mu$m we have discovered objects with exceptional mid-infrared (MIR) emission. They are essentially not detected on IRAS-ADDSCANs and only very few of them show up in the NVSS and FIRST radio surveys. Various colour criteria of the $6.7~\mu$m data with 2MASS and optical wavebands show that the sources reach more extreme IR colours than the sources in the Hubble Deep Field-South and the ELAIS survey. The comparison with known object types suggests that we have found AGN with a pronounced MIR emission, probably due to circum-nuclear dust. First results from optical spectroscopy of ten candidates corroborate this interpretation showing four AGN, two reddened LINER and four extremely reddened emission-line galaxies with MIR/FIR flux ratios higher than for known pure starburst galaxies. The results will make a significant contribution to the debate on the entire AGN population.


Key words: galaxies: fundamental parameters -- galaxies: photometry -- quasars: general -- infrared: galaxies

Offprint request: M. Haas, haas@astro.rub.de

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© ESO 2004


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