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Astron. Astrophys. 357, 7-12 (2000)

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VLBA H I absorption observations of the water megamaser galaxy NGC 5793

Y.M. Pihlström 1, J.E. Conway 1, R.S. Booth 1, P.J. Diamond 2 and B.S. Koribalski 3

1 Onsala Space Observatory, 439 92 Onsala, Sweden
2 MERLIN/VLBI National Facility, Jodrell Bank Observatory, Macclesfield, Cheshire, SK11 9DL, UK
3 Australia Telescope National Facility, CSIRO, P.O. Box 76, Epping NSW 2121, Australia

Received 6 May 1999 / Accepted 17 February 2000

Abstract

We report parsec-scale resolution observations of neutral hydrogen seen in absorption in the [FORMULA] cm line in the H2O megamaser galaxy NGC 5793. We find that the total H I column density of [FORMULA] cm-2 is divided into three narrow (FWHM [FORMULA] km s-1) and one broad component (FWHM 50 km s-1). The narrow components have H I column densities of [FORMULA] cm-2 and there is evidence that they have sizes [FORMULA] pc and H I densities of [FORMULA] cm-3. The broad absorption component shows a velocity gradient in the same direction as the CO disk observed in this source. We argue that this broad component could be due to a circumnuclear disk or torus. The observed velocity gradient implies that this structure has radius [FORMULA] pc.

Key words: galaxies: active – galaxies: individual: NGC 5793 – galaxies: ISM – radio lines: galaxies

Send offprint requests to: Y.M. Pihlström (ylva@oso.chalmers.se)

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© European Southern Observatory (ESO) 2000

Online publication: May 3, 2000
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