The Nuclear Region: HI Absorption Measurements

Absorption by neutral atomic hydrogen in luminous spiral galaxies provides an excellent tool to study the gas kinematics in their nuclear regions. Whereas from emission studies it is difficult to interpret the observed radial velocities, absorption measurements have the advantage that the detected lines must come from the near side of the nucleus. So, red-shifts or blue-shifts relative to the systemic velocity of the galaxy, $v_{\rm sys}$, unambiguously show infall or outflow of gas, respectively. Since the absorption lines are intrinsically narrow as a result of the low temperatures of the optically thick gas, typically 30 to 150 K (Dickey, Salpeter & Terzian 1979), it is surprising that numerous galaxies show HI absorption over a broad range of velocities (a few hundred kms^-1); see e.g., Fig. 1. Some examples are found in the absorption surveys by Mirabel (1982) and Mirabel & Sanders (1988), using the Arecibo telescope, and by Dickey (1982, 1986), obtained with the large configurations of the VLA.

Interferometric studies of individual bright galaxies, e.g., NGC 253, NGC 660, NGC 1068, NGC 1365, NGC 1808, M82, NGC 3079, NGC 3628, NGC 4945, etc. reveal HI emission and absorption over about the same velocity range. And many more galaxies can be added to the sample (see Table 1). It is remarkable that these are all barred galaxies with high central activity and far-infrared (FIR) luminosities of a few 10¹⁰L$_{\odot}$ (see Tables 2 and 3), often revealing outflow of gas from the central region. They also frequently contain megamasers and Seyfert nuclei although the starburst activity usually dominates the total power output.

Sérsic & Pastoriza (1965) had already noted that galaxies with peculiar nuclei, e.g., NGC 1097, NGC 1365, NGC 1808, and NGC 7552 (most of them containing `hot spots', i.e., extremely bright HII regions), are barred (SB or SAB), strongly indicating a relationship between the nuclei of galaxies and their whole structure. My main aim is to investigate further the nature of this relationship.

To understand the nuclear kinematics in these galaxies -- which are probably very much alike -- sensitive high-resolution HI observations need to be carried out which can at least partly resolve the central continuum. A summary of our high-resolution ATCA observations is given in Section 3. Section 4 contains a literature review of the sample of galaxies described above, including some of our new results, and a comparison with strongly interacting galaxies and others with broad HI absorption lines. As a summary I present a global model in Section 5.

Table: Some properties of the HI absorption and emission lines in the selected galaxies

Name	$v_{\rm abs}$ range	$\Delta v_{\rm abs}$	$v_{\rm em}$ range	$\Delta v_{\rm em}$	gradient	Ref.
	[kms-1]		[kms-1]		[kms-1arcsec-1]
NGC 253	75-330	255	75-400	325		(a)
NGC 660	640-1050	410	675-1050	375	87	(b)
NGC 1068	760-1300	540	900-1300	400		(c)
NGC 1365	1473-1786	313	1432-1840	408		(d)
NGC 1808	820-1180	360	810-1190	380	13	(e)
NGC 3034	66-375	309	50-380	330		(f)
NGC 3079	840-1420	580	860-1420	560	500	(g)
NGC 3628	650-1080	430	670-1020	350	27	(h)
NGC 4945	380-740	360	355-750	395		(j)
Circinus	350-550	200	285-590	295		(k)
NGC 6221			1300-1650	350		(l)
NGC 7552	1500-1700	200	1460-1760	300		(l)
NGC 7582	1380-1780	400	1380-1810	430		(l)

Column (1): Galaxy name. Columns (2)-(5): The velocity range observed in HI absorption and emission, respectively. Column (6): Lower limit to the velocity gradient of the HI absorption feature, when resolved. Column (7): References. (a) Dickey, Brinks & Puche 1992; (b) Gottesmann & Mahon 1990; Baan, Rhoads & Haschick 1992; (c) Gallimore et al. 1994; Brinks et al. 1995; (d) Jörsäter & van Moorsel 1995; (e) Koribalski, Dickey & Mebold 1993; (f) Weliachew, Fomalont & Greisen 1984; Crutcher, Rogstad & Chu 1978; (g) Irwin & Seaquist 1991; Baan & Irwin 1995; (h) Schmelz, Baan & Haschick 1987; Wilding, Alexander & Green 1993; (j) Ables et al. 1987; (k) Koribalski & Whiteoak 1996; (l) Koribalski, Lavezzi, Dickey & Whiteoak 1996.