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For refcode 1995A&A...304...21G:
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Copyright by European Southern Observatory (ESO). Reproduced by permission
1995A&A...304...21G Centripetal acceleration within the subparsec nuclear maser disk of NGC 4258 L.J. Greenhill, C. Henkel, R. Becker, T.L. Wilson, and J.G.A. Wouterloot Center for Astrophysics, 60 Garden St, Mail Stop 42, Cambridge, MA 02138, USA Max-Planck-Institut fur Radioastronomie, Auf dem Hugel 69, D-53121 Bonn, Germany GKSS Forschungszentrum, Institut fur Physik, Max-Planck-Strasse, D-21502 Geesthacht, Germany I. Physikalisches Institut, Universitat zu Koln, Zulpicher Str. 77, D- 50937 Koln, Germany Received 8 December 1994 / Accepted 21 July 1995 Abstract. From our monitoring of the H_2_O megamaser spectrum of NGC 4258 (M 106) during 1984-1986 and 1993-1994, we measure a mean linear drift of 9.5 +/- 1.1 km s^-1^ yr^-1^, using an automated analysis, in the velocities of 12 spectral features distributed throughout the `low- velocity' complex, |V-V_0_| < 150 km s^-1^, where the systemic velocity is V_0_. Features in the high-velocity complexes |V - V_0_| > 700 km s^- 1^) drifted by less than ~1 km s^-1^ yr^-1^. Also, no perceptible velocity drift was observed in the megamaser spectra of NGC 3079 and NGC 1068. The NGC 4258 spectra reported here, in combination with VLBI data, give strong support to a model in which the H_2_O emission arises in an edge-on Keplerian disk of inner radius 0.13 pc and rotation velocity ~1100 km s^-1^ that is bound by a mass of at least 10^7^ M_sun_. We confirm three predictions of the Keplerian disk model. (1) The high-velocity maser lines do not show significant acceleration and the emitting material must lie within 6^deg^ of the midline of the disk. (2) The low-velocity maser lines all exhibit very similar accelerations and the emitting material lies in a radially-thin arc that subtends ~10^deg^, along the near side inner edge of the disk. (3) In spite of the acceleration of low-velocity maser features, the overall velocity range of the complex remains stationary over time. The distribution of velocity drifts for the components of the `low-velocity' complex indicate that there are two groups of clumps, at slightly different galactocentric radii, which emit the observed maser radiation (r_blue_/r_red_ ~ 1.1). Models that invoke elliptical orbits to explain the distribution of drift rates do not explain characteristics of the low- and high-velocity emission. The relative intensities of the low- and high-velocity maser components and the absence of observed negative velocity drifts can be explained for unsaturated maser emission and maser amplification of the radio continuum emission associated with the central object whose gravity binds the disk. The positional alignment of H_2_O and radio continuum emission, in NGC 4258 and other sources, is also suggestive of a link between line and continuum fluxes in nuclear masers (e.g., amplification) Also reported are results from a search for new H_2_O maser sources ({delta} > -20^deg^) in far-infrared bright galaxies and active galaxies with compact nuclear radio structure, as well as M 31 and M 101. In view of the apparent association of H_2_O megamaser emission with an ultra- compact nuclear structure in NGC 4258 and other sources, we speculate that the presence of H_2_O megamasers is directly related to this ultra- compact structure, rather than to the galaxy's infrared luminosity. Thus H_2_O (and not CO) might become an important tracer of the dense cool circumnuclear gas in distant radio loud active galaxies. Key words: galaxies: individual: NGC 4258 - galaxies: nuclei - galaxies: ISM - masers
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