Volume 530, June 2011
|Number of page(s)||11|
|Published online||26 May 2011|
Megamaser detection and nuclear obscuration in Seyfert galaxies⋆
Astronomisches Institut, Ruhr–Universität Bochum,
2 Department of Physics, University of California, Santa Barbara, CA 93106, USA
3 Facultad de Ciencias, Universidad Católica del Norte, Antofagasta, Chile
Accepted: 3 February 2011
We revisit the relation between H2O maser detection rate and nuclear obscuration for a sample of 114 Seyfert galaxies drawn from the CfA, 12 μm, and IRAS F25/F60 catalogs. These sources have mid-infrared spectra from the Spitzer Space Telescope and we search for accompanying X-ray and [O III] 5007 Å fluxes from the literature. We use the strength of the [O IV] 25.9 μm emission line as a tracer of the intrinsic AGN strength. After the normalization by [O IV], the observed X-ray flux provides information about X-ray absorption. The distribution of X-ray/[O IV] flux ratios is significantly different for masers and non-masers: The maser detected Seyfert-2s (Sy 1.8–2.0) populate a distinct X-ray/[O IV] range, which is, on average, about a factor of four lower than the range of Seyfert-2 non-masers and about a factor of ten lower than the range of Seyfert-1s (Sy 1.0–1.5). Non-masers are almost equally distributed over the entire X-ray/[O IV] range. This provides evidence that high nuclear obscuration plays a crucial role in determining the probability of maser detection. Furthermore, after normalization with [O IV], we find a similar but weaker trend for the distribution of the maser detection rate with the absorption of the 7 μm dust continuum. This suggests that the obscuration of the 7 μm continuum occurs on larger spatial scales than that of the X-rays. Hence, in the AGN unified model, at moderate inclinations away from edge-on, the 7 μm dust absorption may occur without any proportionate X-ray absorption. The absorption of [O III] appears unrelated to maser detections. The failure to detect masers in obscured AGN is most likely due to insufficient observational sensitivity.
Key words: galaxies: Seyfert / galaxies: nuclei / masers / X-rays: galaxies / infrared: galaxies
Full Table 1 is only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (22.214.171.124) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/530/A147
© ESO, 2011
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