Galaxy Transformations

Observations show interacting galaxies apparently frozen in the midst of billion-year-long collisions. Simulations transcend our limited view of time by indicating the past and future of such encounters. Below are some images from numerical simulations of colliding galaxies. Follow the links for more frames and videos.

Transformations of Galaxies. I.

Videos illustrating dynamical interactions of equal-mass disk galaxies (Barnes 1992).

MPEG movie (160 kb)

1. Initial Conditions. Before their collision, two disk galaxies are rotated about their vertical axis. Each galaxy contains a thin disk (blue), a central bulge (yellow), and a dark halo (red). One disk lies in the orbit plane, and spins in the same direction that the galaxies circle each other. The other disk is inclined by an angle of 71 degrees.

MPEG movie (392 kb)

2. Time Evolution. The galaxies approach each other, tidally interact as they pass each other, and fling out tidal bridges and tails. Subsequently they reach maximum separation, fall back together, and merge. The counter in the upper right shows time in units of 250 million years.

MPEG movie (152 kb)

3. Rotation, t = 1.5. About 125 million years after first approach, the galaxies have developed extended bridges and tails. These are shown to advantage by rotating about the vertical axis as in this video.

MPEG movie (208 kb)

4. Direct Disk. To show the reaction of the ``direct'' or in-plane disk, this video views this galaxy face-on while representing its partner as a dot.

MPEG movie (176 kb)

5. Inclined Disk. Complementing the video above, this one views the inclined disk face-on while representing its partner as a dot.

MPEG movie (736 kb)

6. Time Evolution. Following up the 2nd video above, this one shows the second approach, merger, and subsequent relaxation of the remnant.

MPEG movie (536 kb)

7. Rotation, t = 3. Just after the merger, the remnant is rotated about the vertical axis to illustrate the shells, plumes, and tails typical of very young merger remnants.

MPEG movie (512 kb)

8. Rotation, t = 6. About 750 million years after the merger, the remnant has substantially relaxed and appears more smooth and regular. Note the faint loops and tails which still surround it.

Transformations of Galaxies. II.

Videos produced with Lars Hernquist in our work on mergers of gas-rich spiral galaxies (Barnes & Hernquist 1996).

selected frames (118 kb); MPEG movie (440 kb)

1. Encounter Overview. Time-evolution of a galactic encounter, viewed along the orbital axis. Here dark halo matter is shown in red, bulge stars are yellow, disk stars in blue, and the gas in green.

selected frames (70 kb); MPEG movie (320 kb)

2. Gas Only. Same encounter at a larger scale. Moreover, here only galactic gas is shown.

selected frames (318 kb); MPEG movie (1752 kb)

3. Disk Response. A disk from the above encounter, viewed along its spin axis with the same color scheme as Video 1.

selected frames (75 kb); MPEG movie (368 kb)

4. Final Encounter. Close-up of the late stages of the collision, again showing only gas.

selected frames (66 kb); MPEG movie (216 kb)

5. Dwarf Formation. Formation of a `dwarf galaxy' -- a small, bound object -- in a tidal tail. Here stars and gas which fall into the dwarf are shown in blue and green, respectively, other disk and bulge material is shown in red, and halo matter is invisible.

Dynamical Effects of Mergers

This simulation illustrates some consequences of mergers between disk galaxies of unequal masses (Barnes 1996).

selected frames (119 kb)

Retrograde 3:1 Merger. Parabolic encounter and merger of two disk galaxies with mass ratios of 3:1. The larger disk is only partly disrupted by the collision; it survives as a plausible S0 galaxy.

SAAS-FEE Lectures

These videos were created for lectures at the 26th SAAS-FEE winter school on Galaxies: Interactions and Induced Star Formation (Barnes 1998).

Encounter Survey

selected frames (255 kb); MPEG movie (744 kb)

Encounter D. Close parabolic encounter of identical bulge/disk/halo galaxies. One disk is inclined by 109 degrees, the other by 180 degrees.

selected frames (250 kb); MPEG movie (728 kb); AVI movie (840 kb)

Encounter F. Close parabolic encounter of identical bulge/disk/halo galaxies. One disk is inclined by 71 degrees, the other by 109 degrees.

selected frames (219 kb)

Encounter D2. Close parabolic encounter of similar bulge/disk/halo galaxies with a three-to-one mass ratio. The small disk is inclined by 71 degrees, the large disk by 109 degrees.

Sneaking Up On The Mice

Optical image of `The Mice', NGC 4676. North is right, east is up. Image courtesy John Hibbard.

MPEG movie (448 kb)

Simulation of The Mice. A close encounter between two identical disk galaxies produces a configuration resembling The Mice (Hibbard & Barnes, in preparation). Here the encounter is seen from our viewpoint, almost edge-on to the orbital plane. The numbers at upper right show elapsed time in units of about 160 million years. Pericenter occurs at time 1.0; the best match to NGC 4676, shown above, is attained about one time unit later.

MPEG movie (368 kb)

Rotation about NS axis. The three-dimensional structure of this model, at the time best matching the observations, is revealed by rotation about the north-south axis. As the view above shows, the northern tail appears straight because we view it almost edge-on.

Orbit Decay

MPEG movie (416 kb)

Encounter of core/halo models. A close, parabolic encounter of two identical galaxy models, each consisting of a Jaffe (1981) model core and a Dehnen (1993) model halo. The core is shown in blue-while, while the halo is shown in red.

Orbit Structure

Example orbits identified in a strongly triaxial merger remnant and classified by the crossing-pattern algorithm (Fulton & Barnes, in preparation). All movies follow the same format. In each one, the major axis is indicated by a blue line; the minor axis is vertical. The particle is a white point which leaves a green trail as it orbits in the potential. At the same time, the view point circles around to show the orbit in three dimensions. Note that the potential is not rotating; only the point of view changes!

MPEG movie (352 kb)

Box orbit. Boxes are the main orbit family fully consistent with triaxial density distributions.

MPEG movie (664 kb)

X-Tube orbit. X-tube orbits loop around the major axis; they are the only family present in strictly prolate potentials.

MPEG movie (576 kb)

Z-Tube orbit. Z-tube orbits loop around the minor axis; they are the only family present in strictly oblate potentials.

MPEG movie(328 kb)

Fish orbit. In triaxial potentials with sufficiently deep potential wells, 3:2 resonances become stable; the result is fish-shaped orbit.

MPEG movie (480 kb)

Pretzel orbit. Likewise, 4:3 resonances may also become stable; the result is pretzel-shaped.

Theories of Spiral Structure

Simulations created for a lecture on spiral structure. Unlike the examples above, here only one galaxy is present!

selected frames (680 kb)

Disk Galaxy Simulations. These N-body models illustrate the development of spiral structures in isolated and tidally-perturbed disk galaxies.

Thanks to Joel Welling and Anjana Kar of the Pittsburgh Supercomputing Center for help with the videos. Thanks also to Narayan Raja for advice and comments on this site. Numerical simulations were run at the Pittsburgh Supercomputing Center and the Maui High Performance Computing Center.

Joshua E. Barnes (

Last modified: February 1, 2000