Narrow-line AGN in the ISO-2MASS survey

March 2007 • 2007A&A...464..895L

Authors • Leipski, C. • Haas, M. • Meusinger, H. • Siebenmorgen, R. • Chini, R. • Drass, H. • Albrecht, M. • Wilkes, B. J. • Huchra, J. P. • Ott, S. • Cesarsky, C. • Cutri, R.

Abstract • Context: A long-standing challenge of observational AGN research is to find type 2 quasars, the luminous analogues of Seyfert-2 galaxies.
Aims: We search for luminous narrow-line type 2 AGN, characterise their properties, and compare them with broad-line type 1 AGN.
Methods: Combining the ISOCAM parallel survey at 6.7 μm with 2MASS, we have selected AGN via near-mid-infrared colours caused by the hot nuclear dust emission. We performed spectroscopy in the optical and, for a subset of the sample, also in the mid-infrared with Spitzer.
Results: We find nine type 2 AGN at redshift 0.1<z<0.5, three of them have even quasar-like [O III] luminosities. At the given redshift and luminosity range the number of type 2 AGN is at least as high as that of type 1s. At z>0.5 we did not find type 2 AGN, probably because the hottest dust emission, still covered by the NIR filters, is obscured. The optical spectra of the type 2 host galaxies show young and old stellar populations. Only one object is an ultraluminous infrared galaxy with starburst. The 5-38 μm spectra of the two type 2 sources observed show a strong continuum with PAH emission in one case and silicate absorption in the other case.
Conclusions: The near-mid-infrared selection is a successful strategy to find luminous type 2 AGN at low z. The objects exhibit a large range of properties so that it is difficult to infer details by means of popular SED fitting with simple average templates.

Based on observations made

with ESO telescopes at

La Silla and Paranal (IDs 072.B-0144, 075.A-0345, and

075.A-0374), with the 4-m Blanco telescope at Cerro Tololo

Inter-American Observatory (CTIO is part of the National Optical Astronomy Observatories,

which are operated by the Association of Universities for Research in Astronomy,

under contract with the National Science Foundation),

with the 2.2-m telescope at the

Centro Astronómico Hispano Alemán (CAHA) Calar Alto,

operated jointly by the Max-Planck Institut für Astronomie and the Instituto de

Astrofísica de Andalucía (CSIC),

with the 1.9-m telescope at the South African Astrophysical