NASA/IPAC EXTRAGALACTIC DATABASE
Date and Time of the Query: 2019-05-22 T10:03:51 PDT
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For refcode 1997ApJS..110..321B:
Retrieve 364 NED objects in this reference.
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1997ApJS..110..321B A Survey for H_2_O Megamasers in Active Galactic Nuclei. II. A Comparison of Detected and Undetected Galaxies J. A. BRAATZ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138 A. S. WILSON Department of Astronomy, University of Maryland, College Park, MD 20742; and Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218 AND C. HENKEL Max-Planck-Institut fur Radioastronomie, Auf dem Hugel 69, D-53121 Bonn, Germany Received 1996 August 1; accepted 1996 December 10 ABSTRACT A survey for H_2_O megamaser emission from 354 active galaxies has resulted in the detection of 10 new sources, making 16 known altogether. The galaxies surveyed include a distance-limited sample (covering Seyferts and LINERs with recession velocities less than 7000 km s^-1^) and a magnitude-limited sample (covering Seyferts and LINERs with m_B_ <= 14.5). In order to determine whether the H_2_O-detected galaxies are "typical" active galactic nuclei (AGNs) or have special properties that facilitate the production of powerful masers, we have accumulated a database of physical, morphological, and spectroscopic properties of the observed galaxies. The most significant finding is that H_2_O megamasers are detected only in Seyfert 2 and LINER galaxies, not Seyfert 1's. This lack of detection in Seyfert 1's indicates either that they do not have molecular gas in their nuclei with physical conditions appropriate to produce 1.3 cm H_2_O masers or that the masers are beamed away from Earth, presumably in the plane of the putative molecular torus that hides the Seyfert 1 nucleus in Seyfert 2's. LINERs are detected at a similar rate to Seyfert 2's, which constitutes a strong argument that at least some nuclear LINERs are AGNs rather than starbursts, since starbursts have not been detected as H_2_O megamasers. We preferentially detect H_2_O emission from the nearer galaxies and from those that are apparently brighter at mid- and far-infrared and centimeter radio wavelengths. There is also a possible trend for the H_2_O-detected galaxies to be more intrinsically luminous in nuclear 6 cm radio emission than the undetected ones, though these data are incomplete. We find evidence that Seyfert 2's with very high (N_H_ > 10^24^ cm^-2^) X-ray- absorbing columns of gas are more often detected as H_2_O maser emitters than Seyfert 2's with lower columns. It may be that the probability of detecting H_2_O maser emission in Seyfert galaxies increases with increasing column of cool gas to the nucleus, from Seyfert 1's through narrow-line X-ray galaxies to Seyfert 2's. Subject headings: galaxies: active-galaxies: nuclei-galaxies: Seyfert- ISM: molecules-masers -radio lines: galaxies
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