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Astron. Astrophys. 360, 49-56 (2000)

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1. Introduction

The discovery and subsequent sub-parsec scale studies of H2O maser emission toward the nuclei of several galaxies have greatly improved our understanding of the circumnuclear regions of active galactic nuclei (AGN). Intense interferometric studies of the maser in the Seyfert/LINER galaxy NGC 4258, mainly using VLBI, provided a detailed description of a rotating circumnuclear disk in Keplerian motion with a radius of 0.13 pc, bound by a super massive black hole with a mass of 3.7[FORMULA] (Miyoshi et al. 1995; Greenhill et al. 1995; Nakai et al. 1993). Similar H2O masing disks to those found in NGC 4258 have been identified in other AGN: NGC 1068 (Greenhill et al. 1996), NGC 3079 (Trotter et al. 1998; Sawada-Satoh et al. 2000) and NGC 4945 (Greenhill et al. 1997). These results imply that a sub-parsec or parsec-scale molecular disk/torus surrounding the nucleus is common in some narrow-line AGN and that their existence agrees well with the scheme which describes the unification of different types of AGN, originally proposed by Antonucci (1993).

VLBI observation of nuclear absorption lines is another method of studying the kinematics of the innermost region of circumnuclear gas. Gallimore et al. (1996a) found that the spectral profile and location of the HI absorption on 100 pc scales in NGC 1068 are consistent with those of H2O masers which are seen around the nucleus and 30 pc further out along the jet. VLBI observations of NGC 1068 with sub-milliarcsecond angular resolution showed that nuclear H2O masers are distributed with a radius of 0.65 pc and exhibit sub-Keplerian rotation (Greenhill et al. 1996). These observational facts strongly suggest that the structure and kinematics of the circumnuclear gas might be traced by absorption-lines as well as water masers.

NGC 5793 is an edge-on disk galaxy with a bright compact nucleus seen in radio continuum emission. Baan et al. (1998) classified the optical emission of NGC 5793 as being Seyfert type 2, which implies AGN activity. Assuming [FORMULA] = 75 km s-1 Mpc-1, the galaxy lies at a distance of 46 Mpc and 1 milliarcsecond (mas) corresponds to 0.23 pc. Gardner et al. (1992) made a European VLBI Network (EVN) image of the 21 cm continuum emission and found that the radio nucleus of NGC 5793 is unresolved with a radius of 16 mas, or about 4 pc on a linear scale. NRAO Very Large Array (VLA) 1 observations of the intense HI and OH absorption toward the nucleus demonstrated that the absorbing gas is located in a compact region around the nucleus (Gardner & Whiteoak 1986) and the wide velocity range of the HI and OH absorption profiles, [FORMULA] 200 km s-1, is interpreted as being the result of a number of clouds of dense gas along the line of sight. Gardner et al. (1992) also performed HI absorption-line imaging with the EVN and concluded that three absorbing systems with different velocities are present, two of which are associated with the nucleus, while the third may be associated with gas flowing away from the nuclear region. However, insufficient spatial resolution made it difficult to determine whether or not the absorption really arises from the nucleus itself.

Single-dish observations detected intense H2O maser emission from NGC 5793 (Hagiwara et al. 1997). The spectrum shows strong high-velocity features displaced by [FORMULA] km s-1 on either side of the systemic velocity of the galaxy ([FORMULA] = 3442 km s-1), accompanying weaker features are observed near the systemic velocity. Hagiwara et al. (1997) showed that the two velocity peaks of the H2O systemic features seem to correspond to those of the OH absorption line peaks (Gardner & Whiteoak 1986), indicating a similar distribution of the H2O and OH molecular gas in the galaxy. The parsec scale molecular absorption associated with AGN is a powerful tool for probing the inner layer of circumnuclear molecular gas which is considered as an energy source in the vicinity of a central engine. The nucleus of NGC 5793 also contains large opacity ([FORMULA] [FORMULA] 2) HI absorption covering a wide velocity range. According to the most recent HI absorption observation with VLBI, Pihlström et al. (2000) find that the atomic HI gas is not associated with the [FORMULA] 10 pc region close to the nucleus and suggest that it is probably associated with the outer galactic disk imaged using CO emission by Hagiwara (1998). In contrast, the OH molecular gas on parsec scales could trace a compact molecular gas disk/torus surrounding nucleus.

In this paper we describe the VLBI observations in Sect. 2, present the results of both continuum and OH absorption imaging in Sect. 3, and discuss the structure and kinematics of the nuclear region in Sect. 4.


[TABLE]

Table 1. Adopted parameters of NGC 5793.
Notes:
a) Position of a radio core determined by VLA (Whiteoak & Gardner 1987). The coordinates are converted from those of B1950.
b) Assuming [FORMULA]= 75 km s-1 Mpc[FORMULA]
c) Hagiwara et al. (1997)
d) de Vaucouleurs et al. (1976)
e) Baan et al. (1998)
f) Whiteoak & Gardner (1987)
g) Roth (1994)


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Online publication: July 27, 2000
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