A SEARCH FOR SUBMILLIMETER H<sub>2</sub>O MASERS IN ACTIVE GALAXIES: THE DETECTION OF 321 GHz H<sub>2</sub>O MASER EMISSION IN NGC 4945

Yoshiaki Hagiwara, Shinji Horiuchi, Akihiro Doi, Makoto Miyoshi, Philip G. Edwards

doi:10.48550/arxiv.1604.07937

<jats:title>ABSTRACT</jats:title> <jats:p>We present further results of a search for extragalactic submillimeter H<jats:sub>2</jats:sub>O masers using the Atacama Large Millimeter/submillimeter Array (ALMA). The detection of a 321 GHz H<jats:sub>2</jats:sub>O maser in the nearby type 2 Seyfert galaxy, the Circinus galaxy, has previously been reported, and here the spectral analysis of four other galaxies is described. We have discovered H<jats:sub>2</jats:sub>O maser emission at 321 GHz toward the center of NGC 4945, a nearby type 2 Seyfert. The maser emission shows Doppler-shifted velocity features with velocity ranges similar to those of the previously reported 22 GHz H<jats:sub>2</jats:sub>O masers; however, the non-contemporaneous observations also show differences in velocity offsets. The subparsec-scale distribution of the 22 GHz H<jats:sub>2</jats:sub>O masers revealed by earlier very long baseline interferometry observations suggests that the submillimeter masers could arise in an edge-on rotating disk. The maser features remain unresolved by the synthesized beam of ∼0.″54 (∼30 pc) and are located toward the 321 GHz continuum peak within errors. A marginally detected (3<jats:italic>σ</jats:italic>) high-velocity feature is redshifted by 579 km <jats:inline-formula> <jats:tex-math> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjaa2e0aieqn1.gif" xlink:type="simple"/> </jats:inline-formula> with respect to the systemic velocity of the galaxy. Assuming that this feature is real and arises from a Keplerian rotating disk in this galaxy, it is located at a radius of ∼0.020 pc (∼1.5 × 10<jats:sup>5</jats:sup> Schwarzschild radii), which would enable molecular material closer to the central engine to be probed than the 22 GHz H<jats:sub>2</jats:sub>O masers. This detection confirms that submillimeter H<jats:sub>2</jats:sub>O masers are a potential tracer of the circumnuclear regions of active galaxies, which will benefit from higher angular resolution studies with ALMA.</jats:p>

A SEARCH FOR SUBMILLIMETER H<sub>2</sub>O MASERS IN ACTIVE GALAXIES: THE DETECTION OF 321 GHz H<sub>2</sub>O MASER EMISSION IN NGC 4945

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