Computer Science
Scientific paper
Mar 1998
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998lict..........c&link_type=abstract
Licentiate Thesis, Technical Report No. 281L, Chalmers University of Technology, Gothenburg, Sweden
Computer Science
Seyfert Galaxies, Galactic Dynamics And Modelling, Water Masers
Scientific paper
In this report we investigate the structure of the molecular gas within the central few hundred parsecs of the Circinus galaxy, a type 2 Seyfert. Since the structure of the gas is influenced by the central engine of the galaxy, we briefly review the observations of water maser emission which give the compelling evidence that this central engine is in fact a super-massive black hole, which is, in turn, sustained through its accretion of this gas. In order to determine the dynamics of the molecular gas, we mapped the 2-1 and 3-2 rotational transitions of CO in the inner galaxy, and the results suggest a toroidal structure in orbit around the nucleus. In the main body of the report, we describe the use of a computer model to simulate the observed structure. For this purpose we chose to compare the model with the 2-1 data since, from our investigations, these appear to provide a more accurate tracer of the bulk molecular gas than the 3-2 data. Upon obtaining a satisfactory fit to the observed data, the model indicates that the nuclear feature does indeed appear to be a torus of inner radius approximately 140pc and outer radius >560 pc. The torus is inclined at 78 degrees and shares the same position angle as the main galaxy body. The CO gas is unevenly distributed and we have reason to believe that there exists a significant amount of gas of near systemic velocity lying along the line of sight to the inner edge of the south-west portion of the torus. This may may associated with a molecular outflow which also appears to constitute part of the nuclear feature. The outflow is conjectured to be perpendicular to the torus extending to 600 pc in either direction at a speed of < 350 km/s. The kinematics of the gas in the torus are not dominated by a compact object and we calculate the value of the central dynamical mass to be 3.2±0.8×108 solar masses. We have also calculated the dynamical mass within the central 560 pc and find this to be only double the molecular hydrogen mass of 1.6×109 solar masses. This suggests that a large fraction of the material within this region is in the form of molecular gas, and/or that applying the Galactic CO to molecular hydrogen conversion factor of 2.3×1020 cm-2 (K km s-1)-1 to this active nucleus causes us to overestimate the mass of the molecular gas by up to a factor of ~5.
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