What causes galaxies to merge, rather than orbit indefinitely?
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2$\begingroup$ It might be helpful to do some prior research on this topic, and add details of what you know about the subject already, and what remains uncertain. In general the same rules of gravitation and angular momentum act on galaxies (in reality just collection of star systems) and on stars & planets. $\endgroup$– tuomasCommented Nov 10, 2019 at 20:49
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$\begingroup$ For example en.wikipedia.org/wiki/Andromeda%E2%80%93Milky_Way_collision , en.wikipedia.org/wiki/Local_Group If Andromeda was not heading straight for us, it would orbit indefinitely. Note the timescale, an orbit would take more than the age of the Universe. $\endgroup$– Keith McClaryCommented Nov 11, 2019 at 6:15
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Galaxies are not so far from each other compared to their sizes as you might think.
The typical distance between galaxies is a few Mpc (1 Mpc, or megaparsec, is roughly 3 million light-years). While the stellar disk of a galaxy like the Milky is only $\sim30\,\mathrm{kpc}$ across, its virial radius — determined primarily by its dark matter halo — is larger than the stellar disk by a factor of 20–30. And although the stars are mostly confined to the stellar disk (and a smaller, but important, halo component), hot gas partly fills the dark matter halo.
The virial radius is (often) defined as the radius of a sphere containing roughly $\Delta=200$ times the average density of the Universe, since this corresponds to the region inside which matter is virialized, i.e. gravitationally bound. With a mass of $M_\mathrm{vir} \simeq 10^{12}\,M_\odot$ (e.g. Huang et al. 2016) and a universal density of $\sim10^{-29}\,\mathrm{g}\,\mathrm{cm}^{-3}$ (Planck Collaboration et al. 2018) , the virial radius of the Milky Way is thus $$ \begin{array}{rcl} r_\mathrm{vir} & = & \left( \frac{3M_\mathrm{vir}}{4\pi\Delta\rho} \right)^{1/3}\\ & \simeq & 320\,\mathrm{kpc}. \end{array} $$
Less massive galaxies are smaller, but also more numerous. In other words, the typical distance between galaxies isn't much more than (a few times) an order of magnitude (in contrast, the typical distance between stars is some 7 orders of magnitude times their radius — that's why stars rarely collide).
A head-on collision between the stellar disks of two galaxies is not necessary for two galaxies to merge. Once a galaxy is roughly inside the virial radius of another galaxy, tidal forces will "warp" the galaxy, strip it of its matter, and drastically change its gravitational potential. Often a "bridge" of gas will develop between the two galaxies.
Although dark matter is collisionless, the gas is subject to hydrodynamic forces which will cause the galaxies exit their first encounter with less (kinetic) energy than they entered with. With each orbit, they will then move closer and closer, until they merge.
Most major galaxies have experienced one major merger (i.e. with a galaxy-to-galaxy mass ratio of $\sim$1:1) in their lifetime, as well as many minor mergers (with mass ratios $\lesssim$10:1).
