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large-scale HI structure of the galaxy which covers a velocity range of about 920-1080 kms-1 in emission and absorption; the galaxy is viewed nearly face-on ( ). They find that the most striking feature of the Hsc i emission structure is a `fat' bar, or oval, which occupies the central 5 kpc. The distortions of the velocity field are most likely caused by the non-circular gas flows in the bar. No evidence for tidal interactions with the companions at a distance of 20 has been found. Pedlar et al. suggest that the massive, gas-rich bar of NGC 4151 could have caused the inflow of gas to the centre (similar to the model I discuss for the nearby starburst galaxies in Section 4.1), and that the neutral hydrogen gas present within the few hundred parsec of the nucleus (as shown by the HI absorption measurements) constitutes a fuelling reservoir for the AGN.
Using the VLA B-array HI absorption toward the nuclear region of
NGC 4151 was detected by Dickey (1986), with a centre velocity of
997 kms-1 and width of 5215 kms-1. To resolve the nuclear
region Mundell et al. (1995) obtained Merlin HI absorption
measurements with an angular resolution of 015. They found absorption
(FWHM 90 kms-1, centred at 9936 kms-1) against the
nucleus only. Several other, slightly weaker continuum sources which are part
of a jet pointing toward us show no absorption. The authors therefore conclude
that the neutral hydrogen disk in which the nucleus is embedded can be no
thicker than 50 pc.
The HI survey of Seyfert galaxies by Heckman, Balick & Sullivan (1978) shows how the low angular resolution of even the biggest single-dish telescopes makes it nearly impossible to detect HI absorption against the nuclear continuum sources of galaxies. Out of 58 Seyfert and Seyfert-like galaxies, 25 were detected in HI emission and only three in HI absorption.