Introduction

In this lecture I will discuss some aspects of the relation between the large-scale gas dynamics, nuclear kinematics, and activity in spiral galaxies. The original idea which led to this project was the possibility of using HI absorption measurements as a tracer of nuclear rings in galaxies. The two keys which may help to understand the relationship are bars in galaxies and galaxy-galaxy interactions. A successful approach to this issue involves observations and theoretical N-body simulations of galaxies which complement each other.

The best tracer for the overall gas distribution and kinematics in galaxies is the 21-cm emission line of the neutral hydrogen atom (HI). Although the HI content and global spectrum of a galaxy can be measured with a single-dish telescope, for this project interferometers such as the Australia Telescope Compact Array (ATCA)¹in the Southern Hemisphere and the Very Large Array (VLA)²in the Northern Hemisphere are required to map galaxies in detail. For the brightest galaxies we can also use the HI absorption line to study the gas motions close to the centre by using the largest configurations of the existing interferometers. This method has several advantages: 1) at high angular resolution the HI emission is resolved out leaving absorption only (see Dickey 1979), 2) the HI absorption unambiguously traces the gas in front of the continuum sources, and 3) the velocity resolution is usually very high (a few kms^-1), much higher than in the studies of optical emission lines. The obvious disadvantage is that absorption studies are limited to bright galaxies, mostly starburst or otherwise active galaxies. And even in those galaxies one has to keep in mind that the absorption depends directly on the distribution and strength of the continuum sources in the nuclear region.

A description of one of the first interferometric studies of HI absorption lines is given by Clark, Radhakrishnan & Wilson (1962) and Clark (1965).

Figure 1: Major-axis position-velocity diagram of the starburst galaxy NGC 1808 (Koribalski, Dickey & Mebold 1993). We assumed a position angle of 320$^{\circ}$ (see also Fig. 5 and 6). The contour levels are -4.5, -3, -1.5, 1.5, 3, 4.5, 6, 7.5, and 9 mJybeam^-1. These data were taken with the VLA BnC array; the resolution is about $15\arcsec \times 20\arcsec \times 5$ kms^-1. -- Note that both HI emission (solid lines) and absorption (dotted lines) cover about the same velocity range.