The far-infrared spectral energy distributions of X-ray-selected active galaxies

Joanna K. Kuraszkiewicz, Belinda J. Wilkes, Eric J. Hooper, Kim K. McLeod, Kenneth Wood, Jon Bjorkman, Kisha M. Delain, David H. Hughes, Martin S. Elvis, Chris D. Impey, Carol J. Lonsdale, Matt A. Malkan, Jonathan C. McDowell, Barbara Whitney

Research output: Contribution to journalArticlepeer-review

50 Scopus citations


Hard X-ray selection is, arguably, the optimal method for defining a representative sample of active galactic nuclei (AGNs). Hard X-rays are unbiased by the effects of obscuration and reprocessing along the line of sight intrinsic/external to the AGN, which result in unknown fractions of the population being missed from traditional optical/soft X-ray samples. We present the far-infrared (far-IR) observations of 21 hard X-ray-selected AGNs from the HEAO 1 A2 sample observed with Infrared Space Observatory (ISO). We characterize the far-IR continua of these X-ray-selected AGNs and compare them with those of various radio and optically selected AGN samples and with models for an AGN-heated, dusty disk. The X-ray-selected AGNs show broad, warm IR continua covering a wide temperature range (∼20-1000 K in a thermal emission scenario). Where a far-IR turnover is clearly observed, the slopes are less than 2.5 in all but three cases so that nonthermal emission remains a possibility, although the presence of cooler dust resulting in a turnover at wavelengths longward of the ISO range is considered more likely. The sample also shows a wider range of optical/UV shapes than the optical/radio-selected samples, extending to redder near-IR colors. The bluer objects are type 1 Seyfert galaxies, while the redder AGNs are mostly intermediate or type 2 Seyfert galaxies. This is consistent with a modified unification model in which obscuration increases as we move from a face-on toward a more edge-on line of sight. However, this relation does not extend to the mid-infrared as the 25/60 μm ratios are similar in Seyfert galaxies with differing type and optical/UV reddening. The resulting limits on the column density of obscuring material through which we are viewing the redder AGNs (NH ∼ 10 22 cm-2) are inconsistent with standard optically thick torus models (NH ∼ 1024 cm-2) and simple unification models. Instead our results support more complex models in which the amount of obscuring material increases with viewing angle and may be clumpy. Such a scenario, already suggested by differing optical/near-IR spectroscopic and X-ray AGN classifications, allows for different amounts of obscuration of the continuum emission in different wave bands and of the broad emission line region, which, in turn, results in a mixture of behaviors for AGNs with similar optical emission-line classifications. The resulting decrease in the optical depth of the obscuring material also allows the AGN to heat more dust at larger radial distances. We show that an AGN-heated, flared, dusty disk with mass of ∼109 M⊙ and size of approximately a few hundred parsecs is able to generate optical-far-IR spectral energy distributions (SEDs) that reproduce the wide range of SEDs present in our sample with no need for an additional starburst component to generate the long-wavelength, cooler part of the IR continuum.

Original languageEnglish (US)
Pages (from-to)128-148
Number of pages21
JournalAstrophysical Journal
Issue number1 I
StatePublished - Jun 10 2003


  • Galaxies: active
  • Galaxies: nuclei
  • Infrared: galaxies
  • X-rays: galaxies

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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