2010MNRAS.402..724P

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 2-10µm range and an increasing X-ray spectrum with frequency in the 1 to ∼200keV 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 2µm, with the optical to UV region being fully absorbed; type 1s instead show a gentle 2µm 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 20keV.

Galaxies selected by their warm IR colours, i.e. presenting a relatively flat flux distribution in the 12-60µm 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 10⁴⁴erg/s. 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.