Galaxy Interaction

Following the previous post, I think I should clarify what galaxy interaction is.

Mice Galaxies

Galaxy interaction (galaxy collision) is the result of a disturbance of gravity of a galaxy over another. An example of lesser interaction is a satellite galaxy perturbing the spiral arm of the primary galaxy. An example of further disruption would be a galactic collision. A giant galaxy interacting with its satellite galaxy is fairly common. The gravitational force of the satellite galaxy can attract one of the spiral arms of the primary galaxy. Or the satellite galaxy can even sink into the primary (clear example is the previous post on the Sagittarius Dwarf Elliptical Galaxy also known as SagDEG).

This can cause a small amount of star formation. The satellite can absorb some of the stars of the primary or the opposite. The collision of galaxies is common in galactic evolution. Due to the extremely thin distribution of matter in galaxies, collision is not proper, but rather gravitational interaction. A collision may involve a fusion of galaxies. That happens when two galaxies collide and do not have enough momentum to continue the journey after the collision. Then, they gradually unite to form a single galaxy. If one of the colliding galaxies is much larger than the other one, it will remain largely intact after fusion, ie the larger galaxy will appear more or less as before, while the smaller one will be dismembered and will become part of the bigger one. If a galaxy passes through the other the disruption of the shape of the galaxy is less than in the fusion of galaxies, as the two galaxies maintain their material and shape after crossing each other. The most common result of gravitational fusion of two or more galaxies is an irregular galaxy; which may be either elliptical or spiral.

Remarkable examples of Galaxy Interaction

Name	Type	Distance million Light years	Magnitude	Notes
Whirlpool Galaxy (M51)	SAc (SB0-a)	37	+8.4	Satellite interacting with their primary
NGC 2207 and IC 2163	SAC / SABC	114	+11	Galaxies in the first phase of galactic collision
Mice Galaxies (IC 819/20)	S0 / SB (s) ab	300	+13.5	Galaxies in the second phase of galactic collision
NGC 1097	SB (s) bc (E6)	Four. Five	+9.5	Satellite interacting with their primary
Antennae galaxies (NGC 4038/9)	SAC / sbm	68	+10.3	Galaxies in the third phase  of galactic collision
NGC 520	S	100	+11.3	Galaxies in the third phase of galactic collision