MERGING    DISKS

BACK

The phenomenon of Dynamical Friction (DF) was mentioned in the Interacting Disks page. These two videos illustrate an attempt to incorporate DF in a bound collision between two disks, i.e. a collision where the kinetic energy of the two players is less than their gravitational potential energy. They are doomed to merge.

The two videos show the same two galaxies, the difference being that only one incorporates DF. In all other respects the galaxies are duplicates of their counterpart in the other video. Their relative orientations and velocities at the start of the animations are the same. The view is from the perspective of the larger galaxy (G1), having a disk filled with green stars and a spherical bulge traced by red ones. A central supermassive black hole (SMBH) - a common feature in large galaxies - is marked in white; its motion shows the location of the bulge at any time. The bulge is truncated at the inner edge of the disk. The companion (G2) has six tenths of the mass of G1 but the same structural components. Each galaxy is held together by the gravity of its bulge, but also by a concentric dark matter halo about half again as large as the disk and five times as massive as the bulge. The assumed bulge masses are 1x10¹⁰ and 0.6x10¹⁰ solar masses for G1 and G2.

ANIMATION PROPERTIES: The time step is 500,000 years and the animation runs for 4000 steps or 2 billion years. Extensive DF computations at each step introduce somewhat jerky motions.

DF is a process wherein the kinetic energy of a mass moving through a "background" of stars is transferred to the random motions of the stars. Thus the intruding mass looses speed while the stars gain speed. Here the intruding mass is the bulge of G2 and the background is the bulge of G1, both delineated by the plotted stars. The result: the two bulges and their central black holes slowly merge as the G1 bulge stars spread out. The yellow G2 bulge stars also spread out as the merger progresses, but not as a direct consequence of DF.

Reality check: In the real universe both galaxies experience DF, both SMBHs - if present - spiral towards each other and both bulges expand due to energy input. This animation, in which the kinetic energy of G2 is transferred to the bulge of G1 but not vice-versa, gives a simplified impression of the resulting starpile as it develops over 2 billion years of simulated time. Simulated bulges don't change their masses, sizes, or shapes, which certainly happens in reality. What happens to the DM halos remains unclear; a map of the gravitational field surrounding the Bullet Cluster seems consistent with the notion that the DM components of the two clusters of galaxies are unaffected by their collision.

The end product of a disk-disk merger will likely be an elliptical galaxy. Indeed, a number of actual ellipticals appear to be caught in the late stages of merging; they're called Shell Galaxies because the smooth star distributions of "normal" ellipticals are comlplicated by a variety of arc- and jet-like structures. A catalog comprising 137 shell galaxies was published in 1983 and is freely available online - search for "shell galaxy catalog". The formal reference is: Malin, D.F. and Carter, D. 1983, Astrophysical Journal, 274, 534-540.