The Spitzer discovery of a galaxy with infrared emission solely due to AGN activity^⋆

¹ Laboratoire AIM Paris-Saclay, CNRS/INSU CEA/Irfu Université Paris Diderot, Bât. 709, Orme des Merisiers, 91191 Gif sur Yvette Cedex, France
e-mail: sacha.hony@cea.fr
² Academia Sinica Institute of Astronomy and Astrophysics, PO Box 23-141, Taipei 10617, Taiwan, ROC
³ Jodrell Bank Centre for Astrophysics, Alan Turing Building, School of Physics and Astronomy, University of Manchester, Oxford Road, Manchester M13 9PL, UK
⁴ Astrophysics Group, Lennard-Jones Laboratories, Keele University, Staffordshire ST5 5BG, UK
⁵ MS 169-506, Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA
⁶ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
⁷ Department of Astronomy, PO Box 400325, University of Virginia, Charlottesville, VA 22904, USA
⁸ Department of Physics and Astronomy, Iowa State University, A313E Zaffarano, Ames, IA 50010, USA
⁹ Center for Radiophysics and Space Research, Cornell University, Ithaca, NY 14853, USA
¹⁰ Leiden Observatory, PO Box 9513, 2300 RA Leiden, The Netherlands

Received: 7 March 2011
Accepted: 12 May 2011

Abstract

Aims. We present an analysis of a galaxy (SAGE1CJ053634.78-722658.5) at a redshift of 0.14 of which the infrared (IR) emission is entirely dominated by emission associated with the active galactic nucleus.

Methods. We present the 5−37 μm Spitzer/IRS spectrum and broad wavelength spectral energy distribution (SED) of SAGE1CJ053634.78-722658.5, an IR point-source detected by Spitzer/SAGE. The source was observed in the SAGE-Spec program and was included to determine the nature of sources with deviant IR colours. The spectrum shows a redshifted (z = 0.14 ± 0.005) silicate emission feature with an exceptionally high feature-to-continuum ratio and weak polycyclic aromatic hydrocarbon (PAH) emission bands. We compare the source with models of emission from dusty tori around AGNs. We present a diagnostic diagram that will help to identify similar sources based on Spitzer/MIPS and Herschel/PACS photometry.

Results. The SED of SAGE1CJ053634.78-722658.5 is peculiar because it lacks far-IR emissiondue to cold dust and a clear stellar counterpart. We find that the SED and the IR spectrum can be understood as emission originating from the inner  ~10 pc around an accreting black hole. There is no need to invoke emission from the host galaxy, either from the stars or from the interstellar medium, although a possible early-type host galaxy cannot be excluded based on the SED analysis. The hot dust around the accretion disk gives rise to a continuum, which peaks at 4 μm, whereas the strong silicate features may arise from optically thin emission of dusty clouds within  ~10 pc around the black hole. The weak PAH emission does not appear to be linked to star formation, as star formation templates strongly over-predict the measured far-IR flux levels.

Conclusions. The SED of SAGE1CJ053634.78-722658.5 is rare in the local universe but may be more common in the more distant universe. The conspicuous absence of host-galaxy IR emission places limits on the far-IR emission arising from the dusty torus alone.