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Observations

In addition to detailed studies of individual galaxies like NGC 253 (Koribalski, Whiteoak & Houghton 1995), NGC 1808 (Koribalski, Dickey & Mebold 1993; Koribalski 1993), NGC 4945 (Ables et al. 1987; Ott 1995), and Circinus (K. Jones et al. 1996, in prep.; Koribalski & Whiteoak 1996), a snap-shot survey of the brightest nearby galaxies ( $F_{\rm 100\mu m} \ga$ 20 Jy; $v_{\rm sys} \la$ 3000 kms^-1) has been carried out at declinations below $-35^{\circ}$ using the largest configuration of the ATCA (Koribalski et al. 1996). The 20-cm radio continuum emission and HI were observed simultaneously. Because of the high angular resolution ($\ga$6) the HI emission is resolved out in most cases and only HI absorption against the strongest background sources is detected. After Hanning smoothing the velocity resolution is 6.6 kms^-1. A large variety of nuclear structures was found (multiple sources, such as NGC 1792; double sources, such as NGC 3883; point sources plus extended emission, such as NGC 2442, etc.) as well as broad HI absorption lines against the strongest nuclear sources.

To investigate the relationship between the processes occurring in the nuclear regions of galaxies and their large-scale dynamics follow-up observations have been carried out of several of those galaxies showing HI absorption in the snap-shot survey. For that project the rather compact 375-m configuration (to pick up faint extended emission, e.g., tidal tails and bridges between neighbouring galaxies) and the intermediate 1.5-km configuration of the ATCA were chosen; some of the preliminary results are presented here. The angular resolution varies and is given in each of the figure descriptions whereas the velocity resolution is the same as before.

Figure: (top) HI distribution of the starburst galaxy NGC 253.

Figure: (bottom) HI mean velocity field of NGC 253 (for a full description see Koribalski, Whiteoak & Houghton 1995). The data were taken with three different arrays of the ATCA (12 h each).