# Matter leaving a spinning galaxy

• I
By what means can a galaxy's spin increase whereby matter is expelled from the outer reaches of the galaxy?

Ibix
2020 Award
Do you have evidence that this has happened? Please provide a link.

Do you have evidence that this has happened? Please provide a link.
I'm trying to understand the process by which matter can leave a spinning galaxy.

Ibix
2020 Award
As far as I'm aware, occasional stars get flung out by lucky interactions with other ones, similar to the slingshot manoeuvre that we use with probes (I'd look that up). That will alter the angular momentum of the galaxy slightly. I think that's what you are talking about.

I'm trying to understand the process by which matter can leave a spinning galaxy.
The matter has a net angular momentum from the beginning.

sophiecentaur
Gold Member
2020 Award
The matter has a net angular momentum from the beginning.
Conservation of momentum doesn't preclude a random object being thrown out on a hyperbolic orbit, though i.e. to escape. The angular momentum of that object still contributes to the total angular momentum of the galaxy.

Conservation of momentum doesn't preclude a random object being thrown out on a hyperbolic orbit, though i.e. to escape.
No, it doesn't. But how likely is that? Reaching galactic escape velocity requires a lot of energy. I didn't do the math but I would be surprised if a single swing-by would be sufficient. I expect that a series of such encounters or a single event with much more energy (e.g. a supernova) would be necessary. Can something like that ever have a significant impact on the total angular momentum of the galaxy?

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Ibix
2020 Award
Can something like that ever have a significant impact on the total angular momentum of the galaxy?
I would expect not. Impact yes, significant no. That's why I asked the question in post #2, because #1 reads a bit like the OP is expecting some process that alters the spin of the galaxy and causes it to eject stars en masse. I've never heard of such a thing - as far as I know any matter escaping the galaxy does so by mechanisms as you described.

The matter has a net angular momentum from the beginning.
I am trying to understand how does a galaxy settle into its mass during its formation. Does the process of matter accretion during galaxy formation itself ensure that no matter needs to be expelled due to excess angular momentum? Or is it a process where any matter that may have been attracted during the galaxy formation process is eventually expelled if it has excess angular momentum?

Ibix
2020 Award
I think you are thinking about this wrong. There is some matter that undergoes gravitational collapse. Some of it ends up trapped and becomes a galaxy. Some escapes. No matter "needs" to be expelled. It's just that some will happen to have, or acquire, sufficient energy to escape and some won't.

sophiecentaur
Gold Member
2020 Award
Reaching galactic escape velocity requires a lot of energy.
Does the process of matter accretion during galaxy formation itself ensure that no matter needs to be expelled due to excess angular momentum
If it's a rare event then we can postulate some rare conditions. I'd suspect that, just as in a planetary system, the planets that are farthest out will have only slightly negative orbital energy. Energy needed to leave the Sun's orbit will only be just above this negative level of total energy. Waving my arms suggests that the limiting diameter of a galaxy would be affected in the same way. The final dimensions of a galaxy would be dominated by the original mass and rotational momentum of the nebula it was formed from.
Couldn't the rate of accretion be similar to the rate of ejection, in many cases?

Ranku
I am trying to understand how does a galaxy settle into its mass during its formation.
It collapses to a disc and than angular momentum is transferred from the center into the outer regions. In the result the disc expands and the center collapses further. During this process the total angular momentum remains almost unchanged.

Ranku
I'd suspect that, just as in a planetary system, the planets that are farthest out will have only slightly negative orbital energy.
Even from the edge of the Milky Way escaping still needs too much energy to be a common process. It is difficult to calculate the exact value because it strongly depends on the dark matter halo. The halo also causes the galaxy to be more compact compared to a solar system. Thus, they are not really comparable.

sophiecentaur
stefan r