Letter to the Editor

Discovery of 10 μm silicate emission in quasars

Evidence of the AGN unification scheme

¹ European Southern Observatory, Karl-Schwarzschildstr. 2, 85748 Garching b. München, Germany e-mail: rsiebenm@eso.org
² Astronomisches Institut, Ruhr-Universität, Universitätsstr. 150, 44780 Bochum, Germany
³ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, Postfach 2024, 53010 Bonn, Germany
⁴ IPAC, California Institute of Technology (Caltech), Pasadena, CA 91125, USA

Received: 16 March 2005
Accepted: 10 April 2005

Abstract

According to the unified scheme, AGN are surrounded by a dust-torus, and the observed diversity of AGN properties results from the different orientations relative to our line of sight. The strong resonance of silicate dust at 10 μm is therefore, as expected, seen in absorption towards many type-2 AGN. In type-1 AGN, it should be seen in emission because the hot inner surface of the dust torus becomes visible. However, this has not been observed so far, thus challenging the unification scheme or leading to exotic modifications of the dust-torus model. Here we report the discovery of the 10 μm silicate feature in emission in two luminous quasars with the Infrared Spectrograph of the Spitzer Space Telescope.