#211 What is a dwarf galaxy?

I learned this today. A dwarf galaxy is a galaxy with 1000 to a few billion stars.

Our galaxy, the Milky Way, has between 200 and 400 billion stars. Dwarf galaxies tend to orbit larger galaxies. There are at least 14 satellite dwarf galaxies orbiting the Milky Way right now. They are the most abundant type of galaxy in the universe, but they are difficult to detect because they are not as bright as larger galaxies.

The Small Magellanic Cloud that orbits the Milky Way is a good example of a dwarf galaxy. It is 200,000 light-years from here and it contains several hundred million stars. It is 7000 light-years across and has a mass of 7 billion solar masses. The Milky Way, for comparison, is 105,700 light-years across and has a mass of about 1.9 trillion solar masses. It was once a spiral galaxy, like the Milky Way, but had its shape altered by contact with the Milky Way at some point in the past.

There is a theory that large galaxies started out as dwarf galaxies. In the same way that planets that start to form suck in all of the dust, gas, and debris in the space around them, galaxies probably do the same. Dwarf galaxies could run into other dwarf galaxies, become larger, and then start to attract more dwarf galaxies. The Milky Way and the Andromeda Galaxy are both being orbited by many dwarf galaxies. As these dwarf galaxies slowly run out of energy, they will get closer and “fall in” to the larger galaxies.

The Sagittarius Dwarf Galaxy is currently on a collision course with the Milky Way. In about 100 million years, it will intersect the Milky Way and, over the following billion years, slowly be absorbed into the Milky Way. However, the Sagittarius Dwarf Galaxy has been orbiting the Milky Way for billions of years, and they have probably come into contact at some point in the past. Sagittarius’s mass has decreased by about half of its original mass. The dwarf galaxy has also had an impact on the larger Milky Way galaxy and has caused rippling movements of stars.

The theory that all galaxies start as dwarf galaxies but slowly get assimilated into larger galaxies is called the Cold Dark Matter Theory. One of the problems with this theory is that it predicts between 100 and 500 dwarf galaxies in the neighborhood of the Milky Way and only 38 have been found. There are two reasons for this. One is that all of the other galaxies have already been assimilated into the Milky Way, but that is not likely. The second possibility is that the dwarf galaxies are mostly made up of dark matter and are too dark to be seen.

It is thought that dwarf galaxies must contain a far larger amount of dark matter than larger spiral galaxies do. Dark matter exists and it can be proven to exist, but nobody knows what it actually is. There are theories, but no one knows for definite. This is obviously because it is too dark to be seen. Dark matter exerts a gravitational pull, but it doesn’t interact with regular matter in any other way. It can be seen because it bends light and it has an effect on stars and gas, but that’s it.

And dwarf galaxies have to have a lot of dark matter because the motion of their stars cannot be explained without it. A dwarf galaxy with the mass of 7 billion solar masses, like the Small Magellanic Cloud, wouldn’t have enough gravity to hold itself together. Its outer stars would be flung off into space and it would slowly disintegrate. The fact that it doesn’t, means there must be a lot more mass in the galaxy that we just can’t see. The only thing this could be is dark matter. And the ration of dark matter must be far higher than it is in a large galaxy.

There are several different types of dwarf galaxy. Here are some examples:

Blue compact dwarf galaxies. They are relatively small and contain clusters of hot, young stars. Some of them produce new stars at a very fast rate.

Ultra-faint dwarf galaxies. These are, as the name implies, dwarf galaxies that are very hard to see. They contain a few hundred to a few hundred thousand stars and are most likely to have more dark matter than any other type of dwarf galaxy.

Ultra-compact dwarf galaxies. They have about 100 million stars and are about 200 light-years across. They could be the cores of dwarf galaxies that have lost all of their outer stars to other galaxies.

Dwarf elliptical galaxies. These are very much like regular galaxies, just a lot smaller. It is likely that these are the kind of dwarf galaxies that merge to form larger ones.

Irregular dwarf galaxies. These are irregular shapes and they make up about a quarter of all dwarf galaxies. The theory is that they were regular elliptical galaxies that were stretched or bent out of shape by contact with a larger galaxy.

So, there are many types of dwarf galaxy, and it is likely that they contain large amounts of dark matter. They also occasionally, come together to form larger galaxies. And this is what I learned today.

Sources:

https://en.wikipedia.org/wiki/Dwarf_galaxy

https://en.wikipedia.org/wiki/Dwarf_elliptical_galaxy

https://en.wikipedia.org/wiki/Irregular_galaxy#dwarf_irregular_anchor

https://en.wikipedia.org/wiki/Satellite_galaxies_of_the_Milky_Way

https://en.wikipedia.org/wiki/Sagittarius_Dwarf_Spheroidal_Galaxy

https://en.wikipedia.org/wiki/Small_Magellanic_Cloud

https://esahubble.org/wordbank/dwarf-galaxy/

https://astronomy.swin.edu.au/cosmos/D/dwarf+galaxy