Introduction: The two spiral galaxies NGC 2207 and IC 2163 became prominent some years ago after being photographed by the Hubble Space Telescope(HST). Although their position (R.A. 06h 16m 24.9s; Dec. -21° 22' 26") is just about 6 degrees southwest of the well known bright star Sirius in the constellation Canis Major they are invisible to the naked eye and to smaller telescopes. The two giants encounter in a distance of about 114 million light years (35 megaparsecs). NGC 2207 has a diameter of 143,000 light years (44 kpc or 4') and IC 2163 has a diameter of 101,000 light years (31 kpc or 3'). The image is 2.5 arcminutes on the vertical side. The exposure time using Hubbles Wide Field Planetary Camera 2 was about 4.5 hours to create the big version of this picture. The data gained from this exposure was intensely analysed and reported by the Hubble Heritage Team .
Description: The clockwise rotating NGC 2207, the bigger one, is in front of IC 2163 and covers the western part of the smaller one. NGC 2207 shows the typical oval structure of a big spiral galaxy like the Milky Way with long spiral arms and a big bright center. The spiral structure consists of two very bright shorter arms and two longer arms, that mainly dominate the disc. As mentioned, the diameter of the disc is about 143,000 light years (green marking).
IC 2163 (blue marking) has an eye-shaped or ocular structure with two symmetrical long tidal arms, that reach out about 100,000 from the center. One of the tidal arms (yellow marking lines) is covered by NGC 2207. This galaxy is rotating counterclockwise and moving in western direction behind NGC 2207.
It's not always easy to imagine the threedimensionality of the formation and movement of the two dancers. The sketches may illustrate the situation.
The researchers think that IC 2163 crossed the extrapolated plane of NGC 2207 in the west about 240,000,000 years ago, and that both systems reached their nearest position to each other about 40,000,000 years ago. The nearby parts of the galaxies may be falling into each other in the future.
NGC 2207 is 2.4 times brighter than IC 2163 (Magnitudes: NGC 2207: 11.59 mag; IC 2163: 12.55 mag). In the western parts of the spiral arms of NGC 2207 one can see about 15 bright starclusters. In IC 2163 there are two very bright starclusters in the central regions of the two tidal arms. Astronomers predict, that NGC 2207 may incorporate IC 2163 in a billion years from now. This would lead to an enormous amount of star formation in the high density regions as known from the famous antennae galaxy (see Fig. 14).
Tidal forces: When two rotating galaxies encounter and begin to rotate around each other, respectively around their common center of mass, two kinds of forces occur: the gravitational forces and the centripetal forces . Inside the huge galactic discs these forces show different directions and also different intensities, depending on rotation velocities and on the distribution of mass. This results in tidal forces comparable to those which create the tides on earth.
Well understood since Newton, the gravitational forces between earth and moon, together with the rotational forces on earth create two tidal "hills of water". One on the side of earth that faces the moon, the other on the opposite side. As the earth rotates these "hills of water", known as tidals waves, periodically reach every coast. On the moon there once have been tidal forces too, but nowadays it has become a kind of solid rock without any movable liquids, and therefore without tides.
The masses of earth and moon as well as their velocities are quite easy to determine and to calculate. But in the case of galaxies these calculations are very difficult for several reasons:
The rotation of galaxies seems to be very, very slow to us because of their huge sizes and their far away positions. The velocities of the rotation and other movements have to be calculated from the colors using advanced spectroscopy-methods. Some parts apparently move faster than others. This creates the spiral structure.
The bigger part of the mass of a galaxy may not exist in its stars, but invisible in the huge space between them. The amount of interstellar gases and so-called dark matter has to be estimated from the spectra data, the images and the threedimensional structure of a galaxie. A galaxy as a whole behaves more like a liquid body than a planet or a rock. At school we learn to calculate the path of a thrown rock which we take as a point of mass. This leads us to the calculation of the orbit of a planet around a sun. This kind of math does not work, if we want to calculate the path of three bodies interacting with each other. But a galaxy consists of thousands and millions of points of mass which all interact with each other!
Modern astrophysiscists use two very interesting methods to find answers to their questions:
1. The interpretation of images generated for example by the Hubble Space Telescope (HST) and the new Very Large Telescope (VLT). Structures like those in IGC 2163 and other galaxies usually are interpreted as tidal arms created in a way comparable to the tidal waves on earth. This allows conclusions on the distribution of mass in the galaxies.
2. The simulation of galaxy encounters in computers: Super computers can calculate lots of gravitational interactions at the same time using numeric mathematical models and visualize these models in images and films. The parameters of the models can be varied systematically and their results can be compared to images taken from nature. This allows to improve the models.
1. Chris Mihos, Case Western Reserve University, Cleveland, Ohio: "This movie shows part of the evolution of two colliding disk galaxies -- one rotating in the direction of the encounter orbit, and the other rotating against the encounter. The galaxies are affected differently as a result of the combination of rotation and tidal effects. The blue particles in the movie represent stars in the galactic disks and are collisionless, while yellow represents gas, which can collide, shock, and dissipate energy. The effects of star formation are not shown in this movie." (621 KB, click on Fig. 9.)
2. Frank Summers (Space Telescope Science Institute), Chris Mihos (Case Western Reserve University) and Lars Hernquist (Harvard University): "In this visualization of a computer simulation, two spiral galaxies are set on a collision course. As one slices through the other, both are disrupted. The tidal forces of gravity produce long tails of material streaming away from the collision. The central regions relatively quickly fall together and merge. The visualization is based on a supercomputer simulation, which calculated the motions of 262,144 particles under the forces of gravity and hydrodynamics. The gas particles are shown in blue and the stars in yellow. Each particle is visualized with a size corresponding to its effective size in the calculation." (8,2 MB, click on Fig. 10.)
3. John Dubinsky, University of Toronto, Canada:
"A Simulation of the Milky Way/Andromeda Collision:
Here is the sequence of images from a numerical simulation of the collision
computed using Blue Horizon a 1152 processor IBM SP3 at the
San Diego Supercomputing Centre.
Each spiral galaxy is represented by about 40 million stars and is surrounded
by a 10 million particle dark matter halo for a total of more than 100 million particles
for the galaxy pair. These simulations reveal a
tremendous amount of detail in the destruction and unravelling of the
galaxies as they collide and merge to form an elliptical galaxy.
The Milky Way is shown face-on and moves from the bottom up to the left of
Andromeda and then to the upper right. Andromeda is tilted
from this perspective. The images are 1 million light years across.
After the initial collision,
a open spiral pattern is excited in both the Milky
Way and Andromeda and long tidal tails and a connecting bridge of stars
form are apparent.
The galaxies move apart and then fall back together for a second
collision and then after a few convulsions which throw off more stars
in complex ripple patterns they settle into something looking like
an elliptical galaxy."
(2,9 MB, click on Fig. 11.)
4. John Dubinsky, University of Toronto, Ontario, Canada "A high-resolution movie of the Milky Way/Andromeda Collision." (640x480 pixels, 10 MB. This is the most beautiful mpeg-movie we found on the internet! Click on Fig. 12.)
5. John Dubinsky, University of Toronto, Ontario, Canada:
"NGC 4038/4039 (Click on Fig. 14.) are perhaps the most famous pair of nearby interacting
galaxies known for their symmetrically placed tidal tails resembling insect
antennae. N-body simulations of the formation of the Antennae's tidal
tails allow us to set limits on the total amount of dark matter in the
interacting galaxies. Shown below are two simulations using different
amounts of dark matter in the galaxy models. The low mass model
only contains 4 times as much mass in dark matter as stars while the high
mass model contains 30 times as much dark matter. The low mass model
easily produces long tidal tails like the real pair while the high mass
model fails, producing only short, stubby features. Simulations using
intermediate masses show that long tails only arise when the mass
of dark matter is less than 10 times the mass of the stars.
The simulation spans about 2 billion
years." (Click on Fig. 13.)
Gravitation shareware: Macintosh-users can get a very, very basic impression on how these simulation programs may work after downloading the shareware-program
Gravitation Ltd. by Jeff Rommereide : "This program animates a two dimensional dynamical system modeling n bodies with given masses and initial velocities. You can specify the mass (reflected in the size of the body), initial velocity, and initial position of each "planet" in your system. Then let it run. Some especially nice features are that you can have the planet trace its path, you can set the action up to run "instant replay" (or even run it backwards), and you can keep track of the length of "time" using a frame counter. There is an editor to edit the planets (Fig. 15), so that every aspect of the program is quite visual. The program comes with a collection of examples ready to run."
We played around with the examples coming with the program and then tried to create the encounter of two planetary systems, taking them as a very simple model of galaxies. Each of the two identical systems consists of ten little planets orbiting around a central mass. One system is fixed in the center, the other one moves from the left to the right before it is caught by the second one. The result is seen in Fig. 16. Some of the planets belonging to the moving system are thrown far away, when they come under the gravitational influence of the stationery system. Rommereide named this effect "slingshot", but basically it is nothing else than tidal forces. The settings of our example can be received via e-mail.