Introduction
What is a Galaxy?
Mostly a ball of dark matter, but really a whole bunch more
Gas, dust, stars, black holes and other dead stars
As a very rough gauge it’s dark matter, 1/10th stars, 1/100th gas, 1/1000th dust
Often galaxies travel together, with one or a few big galaxies and lots of smaller ones swirling around.
What does it mean to kill a galaxy?
We think of galaxies as being active/alive if they are forming new stars. However the biggest galaxies grow mostly by eating other galaxies.
“Red and dead” is real terminology
Option 1: Just Wait:
What’s the easiest way to destroy a galaxy?
Well, if you wait long enough, they tend to do it to themselves.
Galaxies need constant new sources of gas, or else they will use it up and run out.
If too many stars build up in the center of a galaxy it can change the gravitational potential enough that gas no longer efficiently collapses to form new stars.
Dwarf galaxies are sensitive and the very stars they form can turn off new star formation by heating up the gas.
Option 2: Galaxy Collisions
Galaxy collisions are the ultimate case of “slow motion train crash”
Satellite galaxies Small galaxies orbiting a larger one.
If they pass too close they can get torn up. Our milky way has done this many times. Some of the remains can be seen as “streams” swirling around the sky
This cycle ultimately keeps the milky way alive longer by supplying fresh gas. Cosmic recycling.
Milky-Way: The milky-way will collide with Andromeda in ~4billion years
The collision has a chance of being so violent that the combined “milkdromeda” will boost and cese star formation.
Cluster galaxies Gas can be stripped out of galaxies.
While passing through a cluster, a galaxy may have its gas heated and stripped away by the hot gas trapped in the cluster.
Option 3: Active Black Hole Feedback
What is an AGN?
Essentially it is a black hole that is currently gobbling up a lot of material. Material doesn’t fall straight onto a black hole, it swirls around picking up speed as it gets closer (like a tetherball, as it gets closer to the pole). This increased speed means that it bumps into nearby material much faster, heating up and creating intense magnetic fields. The intense heat can create high energy radiation like X-rays and gamma rays. The magnetic fields can be powerful enough to funnel material away from the black hole before it falls in.
The jets are often pointed out of a galaxy, so it’s not guaranteed that an AGN will kill a galaxy. This is a good thing since most galaxies likely go through many AGN phases as the black hole gets more/less material funneled to it.
Special thanks to Colin Vendromin for the music, also thanks to Zac Kenny for the logo!