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

被引:56
作者
Kuraszkiewicz, JK
Wilkes, BJ
Hooper, EJ
McLeod, KK
Wood, K
Bjorkman, J
Delain, KM
Hughes, DH
Elvis, MS
Impey, CD
Lonsdale, CJ
Malkan, MA
McDowell, JC
Whitney, B
机构
[1] Harvard Smithsonian Ctr Astrophys, Cambridge, MA 02138 USA
[2] Univ Texas, Dept Astron, Austin, TX 78712 USA
[3] Wellesley Coll, Dept Astron, Wellesley, MA 02481 USA
[4] Univ St Andrews, Dept Phys & Astron, St Andrews, Fife, Scotland
[5] Univ Toledo, Dept Phys & Astron, Ritter Observ, Toledo, OH 43606 USA
[6] Univ Minnesota, Dept Astron, Minneapolis, MN 55455 USA
[7] Inst Natl Astrofis Opt & Elect, Mexico City, DF, Mexico
[8] Univ Arizona, Steward Observ, Tucson, AZ 85721 USA
[9] CALTECH, IPAC, Pasadena, CA 91125 USA
[10] Univ Calif Los Angeles, Dept Astron, Los Angeles, CA 90095 USA
[11] Space Sci Inst, Boulder, CO 80303 USA
关键词
galaxies : active; galaxies : nuclei; infrared : galaxies; X-rays : galaxies;
D O I
10.1086/374919
中图分类号
P1 [天文学];
学科分类号
0704 ;
摘要
Hard X-ray selection is, arguably, the optimal method for de. ning 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 (similar to20-1000 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 mum 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 (N-H similar to 10(22) cm(-2)) are inconsistent with standard optically thick torus models (NH similar to 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 similar to10(9) 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.
引用
收藏
页码:128 / 148
页数:21
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