TY - JOUR
T1 - The far-infrared spectral energy distributions of X-ray-selected active galaxies
AU - Kuraszkiewicz, Joanna K.
AU - Wilkes, Belinda J.
AU - Hooper, Eric J.
AU - McLeod, Kim K.
AU - Wood, Kenneth
AU - Bjorkman, Jon
AU - Delain, Kisha M.
AU - Hughes, David H.
AU - Elvis, Martin S.
AU - Impey, Chris D.
AU - Lonsdale, Carol J.
AU - Malkan, Matt A.
AU - McDowell, Jonathan C.
AU - Whitney, Barbara
PY - 2003/6/10
Y1 - 2003/6/10
N2 - 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.
AB - 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.
KW - Galaxies: active
KW - Galaxies: nuclei
KW - Infrared: galaxies
KW - X-rays: galaxies
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U2 - 10.1086/374919
DO - 10.1086/374919
M3 - Article
AN - SCOPUS:0041663270
VL - 590
SP - 128
EP - 148
JO - Astrophysical Journal
JF - Astrophysical Journal
SN - 0004-637X
IS - 1 I
ER -