Abstract
The galaxies around us frequently interact with each other: tidal arms, bridges between galaxies. Galaxies are born in groups and substructures, and interact more than if their positions in the sky were random. Halton Arp's catalog of 338 special (interacting) galaxies was published in 1966, just after the Atlas of Normal Galaxies in 1961. Until 1972, astronomers thought that these interactions were magnetic, as matter was channelled into force tubes. But rudimentary simulations by the Toomre brothers in 1972 showed that tidal interactions, applied to loosely bound sets of galaxies, could form such fine bridges and spiral arms. Tidal forces at a distance can be decomposed into two Fourier terms: the second-order term, cos(2 theta), in the plane of the galaxy disk, and the first-order term, cos(theta), perpendicular to the plane. In this way, two spiral arms will wrap around the plane, which in turn will be warped like a pancake.
Numerical simulations have shown how tidal interactions can remarkably reproduce the morphologies of interacting galaxies. Depending on the impact parameter, the spiral arms can close in a ring, and so head-on collisions can explain Cartwheel-like galaxies. In the Local Group, the Magellanic Clouds are interacting with each other and with the Milky Way. They will soon merge with our Galaxy. Similarly, Andromeda is interacting with the Milky Way, and the two most massive spirals in the Local Group are likely to merge in 2-3 billion years to form a redder, or elliptical, galaxy. More generally, we can explain the formation of polar rings around galaxies, the formation of shells around elliptical galaxies, the formation of star-forming outbreaks when gas-rich spirals merge, and so on. Finally, the history of star formation, which has a peak around z = 2, can be explained by the greater rate of interaction and mergers between galaxies, when clusters become virialized.
