Journal Article

The spectral energy distribution of the central parsecs of the nearest AGN

M. A. Prieto, J. Reunanen, K. R. W. Tristram, N. Neumayer, J. A. Fernandez-Ontiveros, M. Orienti and K. Meisenheimer

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 402, issue 2, pages 724-744
Published in print February 2010 | ISSN: 0035-8711
Published online February 2010 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2009.15897.x
The spectral energy distribution of the central parsecs of the nearest AGN

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Spectral energy distributions (SEDs) of the central few tens of parsec region of some of the nearest, most well-studied, active galactic nuclei (AGN) are presented. These genuine AGN-core SEDs, mostly from Seyfert galaxies, are characterized by two main features: an infrared (IR) bump with the maximum in the 2–10 μm range and an increasing X-ray spectrum with frequency in the 1 to ∼200 keV region. These dominant features are common to Seyfert type 1 and 2 objects alike. In detail, type 1 AGN are clearly distinguished from type 2 by their high spatial resolution SEDs: type 2 AGN exhibit a sharp drop shortwards of 2 μm, with the optical to UV region being fully absorbed; type 1s instead show a gentle 2 μm drop ensued by a secondary, partially absorbed optical to UV emission bump. On the assumption that the bulk of optical to UV photons generated in these AGN is reprocessed by dust and re-emitted in the IR in an isotropic manner, the IR bump luminosity represents ≳70 per cent of the total energy output in these objects, and the second energetically important contribution is the high energies above 20 keV.

Galaxies selected by their warm IR colours, i.e. presenting a relatively flat flux distribution in the 12–60 μm range, have often being classified as AGN. The results from these high spatial resolution SEDs question this criterion as a general rule. It is found that the intrinsic shape of the infrared SED of an AGN and inferred bolometric luminosity largely depart from those derived from large aperture data. AGN luminosities can be overestimated by up to two orders of magnitude if relying on IR satellite data. We find these differences to be critical for AGN luminosities below or about 1044 erg s−1. Above this limit, AGN tend to dominate the light of their host galaxy regardless of the integration aperture size used. Although the number of objects presented in this work is small, we tentatively mark this luminosity as a threshold to identify galaxy-light-dominated versus AGN-dominated objects.

Keywords: techniques: high angular resolution; galaxies: nuclei; galaxies: Seyfert; infrared: galaxies; radio continuum: galaxies; X-rays: galaxies

Journal Article.  16704 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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