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Paper: The Spectral Energy Distribution of the Central Parsecs Region of Galaxies
Volume: 373, The Central Engine of Active Galactic Nuclei
Page: 600
Authors: Prieto, M.A.; Reunanen, J.; Beckert, Th.; Tristram, K.; Neumayer, N.; Fernandez, J.A.; Acosta, J.
Abstract: Genuine spectral energy distributions (SED) of the central few parsec region of the nearest and brightest active galaxies in the southern hemi- sphere are presented. These are compiled from very high-spatial resolution data taken from radio (VLBA), IR adaptive optics and interferometry, and optical (HST) observations. The new SEDs are characterized by two main emission bumps, each peaking in the X-ray and IR, respectively, as it is known from op- tically obscured galactic nuclei. Yet, the shape of the IR SED largely departs from that derived from large-aperture data. They reveal two new features: (1) a very sharp decay at wavelengths shortward of 2 μm, plausibly a consequence of the heavy extinction towards the core region; and (2) a flattening in the 1020 μm range and a further turnover toward longer wavelengths. Moreover, the true bolometric luminosity of these few parsec core regions turns out to be about an order of magnitude lower than previously estimated on the basis of IRAS/ISO data. These findings indicate that large-aperture IR data may be largely domi- nated by the host galaxy contribution, and warn over interpretations of IR/X-ray and IR/optical correlations based on large-aperture IR data that have been used to differentiate AGN from normal galaxy populations.

The newly derived IR bolometric luminosities still exceed the output energy measured in the high energies by factors 3 to 60. On the expectation that both luminosities should be comparable within an order of magnitude, the reduced factors between both suggests that the derived IR luminosities are getting closer to the genuine power output of the core.

Due to the apparent SED emission turnover in the mid-IR region, an extrap- olation of the VLBA core emission towards shorter wavelengths closely meets the IR data. In Cen A, NGC 1068 and NGC 5506, this extrapolation fits a power law with exponent around 1/3. This indicates that the IR emission may not be so much dust-dominated as previously thought but includes an important nonthermal component.

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