Annu. Rev. Astron. Astrophys. 1991. 29: 581-625 Copyright © 1991 by Annual Reviews. All rights reserved

8.2 Interaction Morphology

One of the interacting systems for which the most complete data exist and for which extensive dynamic modeling has been done is the ``antennae'' system, NGC 4038 / 39 (D = 20 Mpc for H₀ = 75 km/s/Mpc). Knots of intense H emission are found throughout both galaxies (Rubin et al 1970), and a total molecular gas mass of 5.3 x 10⁹ M (Young et al 1989) is seen distributed throughout this system. Aperture synthesis at 6" resolution of the CO emission has recently been reported by Stanford et al (1990). The integrated CO intensity contours are shown in Figure 12 superposed on a 6500 Å image of the inner region of this system. Three concentrations of CO emission are evident: Two are centered near the nuclei of NGC 4038 and NGC 4039, closely correlated with H and radio continuum peaks; a third CO emission region lies about 25" northeast of the NGC 4039 nucleus where two galaxies overlap. The masses of these molecular gas concentrations are in the range 0.2-1.2 x 10⁹ M. If the results obtained in this system are representative, it appears that a galactic collision such as is occurring in this system may result in a large mass of gas concentrating in the nucleus and in the region of direct overlap of the galactic disks. Similar characteristics are found for the CO emission in Arp 299 (IC 694 / NGC 3690) (Sargent et al 1987, Sargent & Scoville 1991).

Figure 12. CO integrated intensity maps in two interferometer fields are overlaid on an R band image of Arp 244 (NGC 4038 / 39), the ``Antennae'' system (Stanford et al 1990). The inset is a large scale, deep exposure of the galaxies. The northern and southern CO concentrations coincide with the nuclei of NGC 4038 and NGC 4039; the eastern emission complex is at the interaction interface of the two galaxies. Contour levels are 5, 10, 20, 30, 40, 50, 60, 70, 80, and 90% of the peak flux (89 Jy km s-1 beam-1).

Both the ``Antennae'' system and Arp 299 may be in an intermediate phase of galactic interaction; examples of more advanced mergers may be found in the ultraluminous IRAS galaxies (Section 9.3).