Strongly Interacting Galaxies

Table 4 lists a few strongly interacting or merging galaxies which also show broad HI absorption lines. These galaxies happen to be some of the nearest and more famous examples of very luminous FIR-galaxies with far-infrared luminosities of more than 10¹²L$_{\odot}$ (see also Heckman et al. 1990). They are the sites of exceedingly powerful bursts of star formation. The RC3-Type is added to show how peculiar these systems are. Several of these galaxies have also been detected in hydroxyl absorption (see e.g., Schmelz et al. 1986).

I could easily add a few more galaxies to this group (see e.g., Dickey 1982, 1986; Mirabel 1982; Heckman et al. 1983). Mirabel & Sanders (1988) find that 18 out of their 92 luminous far-infrared galaxies show HI absorption with line widths of 100-600 kms^-1, although some cases are very weak (e.g., U 9618, Mrk 331), and many of the detections were known before (see the references in Table 4). Interferometric studies need to be carried out to separate the HI absorption from the emission to determine more accurate line widths and centre velocities. For example, the study fails to pick up the broad absorption in the nearby polar-ring galaxy NGC 660, and there may well be others.

Table: Strongly interacting pairs of spirals

Name	D	RC3-Type	abs. line width	References
	[Mpc]		[km/s]
NGC 520	30	.P.....	100	1,5,7
Mkn 231	160	.SAT5$P	150	1
NGC 2623	74	.P.....	525	2,5,7
NGC 3256	37	.P.....	300	10
NGC 3690/IC 694	42	.IB.9P.	300	1,5,8
IC 4553 (=Arp 220)	72	.S?....	743	2,(4),5,6,7
IC 883	92	.I..9*P	460	3,7
NGC 6240	100	.I.0.*P	700	1,3,(9)

References: (1) Dickey 1982; (2) Mirabel 1982; (3) Heckman et al. 1983; (4) Norris et al. 1985; (5) Dickey 1986; (6) Baan et al. 1987; (7) Mirabel & Sanders 1988; (8) Baan & Haschick 1990; (9) van der Werf et al. 1993; (10) English 1994.

What is happening in these galaxies? Are they scaled-up versions of the group of nearby luminous galaxies discussed in the previous section? I think the answer is yes. But in these galaxies the cold gas which feeds the starburst is driven inwards by strong tidal interaction or merging between galaxies (see e.g., Rieke et al. 1985), whereas in the nearby galaxies, bars and/or weak galaxy interactions may be responsible.

Are those broad absorption lines also caused by fast-rotating nuclear rings? Interferometer maps of the integrated CO emission have shown that cold gas is strongly concentrated toward the `active' nuclei (starburst nuclei or AGN) of those systems (Sargent & Scoville 1991; Scoville et al. 1991). The kinematics, which are more clearly revealed by high-resolution HI absorption measurements, indicate that at least part of the gas rotates around the nuclei (see e.g., Baan & Haschick 1990). A very nice example is the peculiar galaxy IC 4553, where the HI and OH absorption measurements clearly indicate a fast-rotating edge-on disk close to the nucleus; the total velocity shift across the source is 145 kms^-1 over a distance of 19 or $\sim$700 pc (Baan et al. 1987, and references therein). But there are also inward and outward motions of the cold gas. The central activity of IC 4553, which is also the best studied OH megamaser galaxy, is caused by a Seyfert2 or starburst nucleus (see e.g., Heckman et al. 1983).

The gas distribution and kinematics in interacting systems could provide information about the merger evolution. As the interaction progresses, a large fraction of the interstellar medium (ISM) of each galaxy is expected to sink into a common centre of mass. I suggest that the cold gas is accumulated in a ring near the central source from where it is spiralling inwards, eaten up by the hungry starburst, and then partly ejected into the halo.