Letter to the Editor

The ISO-2MASS AGN survey: on the type-1 sources

¹ Astronomisches Institut Ruhr-Universität Bochum (AIRUB), Universitätsstraße 150, 44780 Bochum, Germany e-mail: leipski@astro.rub.de
² Thüringer Landessternwarte Tautenburg (TLS), Sternwarte 5, 07778 Tautenburg, Germany
³ European Southern Observatory (ESO), Karl-Schwarzschild-Str. 2, 85748 Garching, Germany
⁴ Instituto de Astronomía, Universidad Católica del Norte (UCN), Avenida Angamos 0610, Antofagasta, Chile
⁵ Harvard-Smithsonian Center for Astrophysics (CfA), 60 Garden Street, Cambridge, MA 02138, USA
⁶ HERSCHEL Science Centre, ESA, Noordwijk, PO Box 299, 2200 AG Noordwijk, The Netherlands
⁷ IPAC, California Institute of Technology (Caltech), 770 South Wilson Avenue, Pasadena, CA 91125, USA

Received: 8 June 2005
Accepted: 9 July 2005

Abstract

We combined the ISOCAM Parallel Mode Survey at 6.7 μm ( filter) with the Two Micron All Sky Survey in order to obtain a powerful tool to search for AGN independent of dust extinction. Using moderate colour criteria and we have selected a sample of 77 AGN candidates in an effective area of ~10 square degrees. By means of optical spectroscopy we find 24 (~) type-1 QSOs at redshifts ; nine of them have . About one third of the ISO-2MASS QSOs show so red optical colours, that they are missed in optical and UV AGN surveys like SDSS, 2DF, or HES. With a surface density of about 2 deg down to mag the ISO-2MASS QSOs outnumber the 1.35 deg of the SDSS quasar survey by 50%; we find a combined optical-IR QSO surface density of 2.7 deg^-2. Since only two of the ISO-2MASS QSOs have also , the inclusion of the ISO mid-infrared photometry significantly extends the capabilities of the pure 2MASS red AGN survey. We suggest that the newly found red AGN resemble young members of the quasar population, and that quasars spend much of their lifetime in a dust enshrouded phase.