Evidence for a Molecular Disk in the Nucleus of LINER galaxy NGC 2639

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Session 57 -- Molecular Astrophysics/ISM
Oral presentation, Thursday, June 15, 1995, 2:00pm - 3:30pm

[57.03] Evidence for a Molecular Disk in the Nucleus of LINER galaxy NGC 2639

J. A. Braatz, A. S. Wilson (UMD), C. Henkel (MPIfR)

We report monitoring observations of the 22 GHz water vapor masers in the nucleus of the active galaxy NGC 2639. The 100-m Effelsberg dish was used to take spectra on 5 different days over the past 18 months. The maser spectrum in this galaxy consists of a bright peak near systemic velocity, with a broad underlying feature, and perhaps several weaker features. The strong maser peak has been progressing towards larger recessional velocities at a rate of 6.0 $\pm$ 0.4 km s$^{-1}$ yr$^{-1}$. The well-studied H$_2$O maser source in NGC 4258 exhibits similar behavior. Monitoring observations over the past decade show that the maser features in NGC 4258 also drift to larger recessional velocities, and recent VLBI maps clearly show that these masers originate from a circumnuclear disk. In light of this work, we suggest that NGC 2639 has a similar molecular disk in its nucleus. The observable maser features are thought to originate in clouds which are passing through the line of sight to the nucleus. A radio continuum source in the nucleus provides the ``seed emission'' for the maser. The velocity gradient of the maser features is then a direct measurement of the centripetal acceleration felt by the clouds as they orbit the central nuclear mass. It may be possible to determine the distance from these clouds to the nuclear source even without the necessity of VLBI observations (for which this maser is currently too weak) by finding ``satellite'' maser features which originate at the tangential points of the edge-on molecular disk. A search for these features is currently underway, and if it is successful, the mass of the central object may be determined.

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