21 - Galaxy Environments

Galaxies are _______ _______ of the Cosmic Web, as galaxies group together in various environments.

Biased tracers

A single halo might contain many galaxies, known as a ______ ______.

Galaxy cluster

____ of the baryonic mass of a cluster is in gas, just ___ in stars.

90%, 10%

Why cannot clusters cool down?

The gravity is so strong that gas heats up to incredibly high temperatures.

What temperature is the gas at (so high-energy, that its photons are emitted as X-rays)?

10-100 million Kelvin

Temperature measures the _______ energy of atoms.

Kinetic

Gas atoms can hold themselves up against _______ just like dark matter, via kinetic energy.

Gravity

What is the virial temperature?

The temperature gas needs to be stable against gravity

______ are a step down in size from clusters, containing most of the galaxies in the universe.

Groups

What is the virial temperature of groups?

1-10 million K

What are galaxies that have no comparable-size neighbours (sometimes called “isolated” galaxies) called?

Field galaxies

Where are these field or isolated galaxies sometimes, hence the name “void” galaxies?

Voids

Where do galaxy clusters live (largest density perturbations that collapse into nodes the fastest)?

The biggest nodes of the Cosmic Web

What galaxies are most common in clusters?

More mergers mean big ellipticals

What galaxies are most common in groups?

Mild interactions mean modest spiral galaxies

What galaxies are most common in fields?

Few interactions mean small spiral galaxies

What galaxies are most common in voids?

Isolation means small irregulars.

What is the name of the galaxy group we live in and what sizeable galaxies does it contain?

Local Group, Andromeda and the Milky Way

Lamda CDM predicts hundreds of tiny halos around the Milky Way, with each forming a tiny disc galaxy with stars. But we only see a couple dozen galaxies; what is this problem called?

The Missing Satellite Problem

What are 3 solutions to the Missing Satellite Problem?

Satellites not found yet, Galaxies don’t form in very small halos, Lamda CDM is wrong (DM is not cold, but “warm”)