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OVERVIEW

Videos option_1,3,4,8 show 1.2 billion years in the lives of two interacting disk galaxies. The gravitational forces at work in each galaxy originate in two components: A central, spherical, bulge of 10¹⁰ solar masses and a concentric, spherical, dark matter (DM) halo five times as massive. The halos are 50 percent larger than the visible disks. A total of 5000 stars are plotted. During the interaction, each star responds to the gravitational forces of both bulges and halos, but not of any other stars.

The action is shown in the reference frame of galaxy G1, which is located at the centre of the coordinate system and features a disk of orange stars. Red stars show the extent of its bulge as the simulation begins; a few stars with large initial velocities are occasionally ejected. The initial motions of the red stars are distinct from those in the disk; as in actual galaxies they move in randomly oriented, elongated orbits. Disk stars orbit in circular paths.

The bulge of Galaxy G2 isn't traced by stars; you don't see a central globular array of stars in G2. But disk stars that initially orbit inside the G2 bulge are coloured yellow.

The results of the interaction - what you see - depend on a variety of initial conditions; there is a large range of possibilities. Most important are the initial location of G2 relative to G1, its initial relative velocity (speed along each of the X, Y, and Z axes), and the orientation of its disk relative to G1.

These videos show bound systems. In real life the galaxies would eventually merge, though the simulation does not show that happening, in part because of the limited time covered. More fundamentally, the simulations do not include an effect called Dynamical Friction, in which the orbital energy of a galaxy moving through a "background" of stars in another galaxy is gradually transferred into that background as random motions (heat). Refer to Option 8 below.

In options 1, 3, 4 the two disks are coplanar, and extended tidal tails develop. Their orientation, shape and size depend on the how the motion of one galaxy during flyby compares to the orbital motions of stars in the disk of the other. The following paragraph elaborates.

Option 1 shows a G1R-G2P encounter. "G1R" indicates a retrograde encounter; an observer riding on G1 sees G2 passing by moving against the orbital motions in its disk. G2 is unable to extract a robust tidal tail from G1. On the other hand "G2P" refers to a prograde encounter. G1 passes by G2 moving with the motions of its disk stars, allowing some of them to be drawn out into a spectacular tidal feature.

Using the above notation, options 3 and 4 show G1R-G2R and G1P-G2P encounters, respectively.

Option 8 shows what happens when the G2 disk is relatively inclined by 70 degrees. Tidal tails are much less prominent in this case, although the system remains bound. The eventual result would be a "star pile", which in real life would be classified as an elliptical galaxy - something like a galaxy-sized version of the Young Star Cluster simulation.