The two spiral galaxies (in figure 1) IC 2163 (on the right) and NGC 2207 (on the left), have narrowly missed a collision in the constellation Canis Major, where they are located about 6 degrees southwest of the Dog Star, Sirius. Imaged by the Wide Field and Planetary Camera 2 on NASA's Hubble Space Telescope, these beautiful hurricane worlds can now be more closely examined to survey the damage.

IC 2163 has an unusual eye-shaped structure formed by fast-moving gas and star streams in the eyelids and a bright galactic nucleus in the pupil. The bright oval streams extend on either side to form long tidal arms that fling out stars and gas for 100 thousand light years. The tidal arm on the left is partially hidden by the foreground dust in NGC 2207. Computer simulations indicate that ovals like this are very short-lived features that appear when a companion galaxy orbits in the direction of rotation. This proved to be a valuable clue for reconstructing the past history of this pair.

Seven astronomers led by the husband and wife team of Bruce and Debra Elmegreen have been observing these galaxies and others like it for the last ten years. Neutral hydrogen velocities that were obtained by Elias Brinks, Michele Kaufman and Debra Elmegreen with the Very Large Array Radio Telescope in New Mexico were meticulously compared to computer models of the orbits, structures, and motions, made by Maria Sundin, Magnus Thomasson, and Bruce Elmegreen. By matching the models with the observations, they could determine that IC 2163 orbited around NGC 2207 from the left to the right, going behind NGC 2207 200 million years ago and making its closest approach 40 million years ago. The main response in IC 2163 to the tidal forces was the eye-shaped oval and tidal tails, while the disk of NGC 2207 developed a large warp.

The high resolution of the HST image reveals an impressive array of parallel dust filaments extending like fine brush strokes on the right side of IC 2163. Many of these are probably shock fronts in the region where the streaming gas meets its tidal tail. Other dust filaments cross in front of IC 2163, from spiral arms in the foreground galaxy. Gas streams funnel into the center of IC 2163 (see figure 2), possibly building a dense core for future bursts of star formation, while the nucleus of NGC 2207 contains irregular spirals in the dust (see figure 3). Studies of the extinction in NGC 2207 imply the forces from self-gravity are too small to make nuclear spirals in the normal way, but gas pressure alone could make them, as a sort of giant sound wave.

Despite the close encounter, no super-star clusters such as those in the Antennae merger are visible in the HST image of this galaxy pair. It is apparently too early for such clusters to form in any burst of star formation, and the tidal forces are too weak to compress the gas much. The encounter did produce extremely massive clouds in the disks, however, but only one of these has spawned a young stellar cluster so far. Detailed studies of the dust and star formation structures in this image should provide a wealth of information about the development of a starburst produced by an interaction.

What happens next? The models indicate that these two galaxies will separate again, so they can start to relax to more normal disk shapes, but they will probably come even closer to each other on the next orbit, and will eventually coalesce. Then there will be an enormous burst of young stars and even more debris flung out into space, in a system closely resembling the Antennae galaxy, a billion years from now.

Papers on IC 2163 & NGC 2207 and Other Near-Interacting Pairs of Galaxies

Search for electronic versions of these and other papers at the NASA Astrophysical Data System.

• B.G. Elmegreen, M. Kaufman, and M. Thomasson, "An Interaction Model for the Formation of Dwarf Galaxies and 100 Million Solar Mass Clouds in Spiral Disks," Astrophysical Journal, 412, 90 (1993).

• D.M. Elmegreen, M. Kaufman, E. Brinks, B.G. Elmegreen, and M. Sundin, "The Interaction Between Spiral Galaxies IC 2163 and NGC 2207, I. Observations," Astrophysical Journal, 453, 100 (1995).

• B.G. Elmegreen, M. Sundin, M. Kaufman, E. Brinks, and D.M. Elmegreen, "The Interaction Between Spiral Galaxies IC 2163 and NGC 2207, II. Models," Astrophysical Journal, 453, 139 (1995).

• Elmegreen, B.G., Elmegreen, D.M., Brinks, E., Yuan, C., Kaufman, M., Klaric, M., Montenegro, L., Struck, C., and Thomasson, M. "Dust Spirals and Acoustic Noise in the Nucleus of the Galaxy NGC 2207," Astrophysical Journal Letters, 503, L119 (1998)

• Kaufman, M., Brinks, E., Elmegreen, D.M., Thomasson, M., Elmegreen, B.G., Struck, C., and Klaric, M. "Observations of the Ocular Galaxy NGC 2535 and its Starburst Companion NGC 2536," Astronomical Journal, 114, 2323 (1997).

• Kaufman, M., Brinks, E., Elmegreen, B.G., Elmegreen, D.M., Klaric, M., Struck, C., Thomasson, M., and Vogel, S., 1999, "The Interacting Galaxies NGC 5394/5395: A Post--Ocular Galaxy and Its Ring/Spiral Companion," Astronomical Journal, accepted