A Grazing Encounter
Between Two Spiral Galaxies
Figure 1:NGC 2207 (on the left) and IC 2163 (on
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.
Figure 2: IC 2163. This image was made by Debra
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
Figure 3: NGC 2207. Zooming in on the nucleus
(colored yellow-orange in the Heritage image)
reveals spiral structure in the core of NGC 2207.
This black and white, unsharp masked image in
visual light (V band) was made by members of the
science team for the journal article Dust
Spirals and Acoustic Noise in the Nucleus of the
Galaxy NGC 2207.
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
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
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,
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,