What Happens to a Dead Galaxy's Mass?

[brand1130]

TL;DR Summary

Does the gravitational pull of a dead galaxy diminish?

Summary: Does the gravitational pull of a dead galaxy diminish?

I have a question about dying galaxies. Most of the answers I've found thus far relate to them losing the ability to generate stars, or to general disbursement of mass caused by dark matter. This is not what I'm looking for. I am specifically curious about the mass of a dead galaxy. Here is my question: Does the matter in dead galaxies decay, and do dead galaxies lose their gravitational strength? Will the gravitational pull of a dead galaxy diminish over time as its matter converts to energy or other non-matter forms? Does it eventually vanish? Or does it become a bunch of cold rocks eternally floating through space? Thank you!

 

[phinds]

Does the matter in dead galaxies decay

No, why would it?

do dead galaxies lose their gravitational strength?

No, why would they?

Will the gravitational pull of a dead galaxy diminish over time as its matter converts to energy or other non-matter forms?

No, why would it "convert" to anything? What force would cause that?

Does it eventually vanish?

No

Or does it become a bunch of cold rocks eternally floating through space?

Yes

 

[brand1130]

Ahh, I was hoping for a more exciting answer than cold rocks. =(
Decay into pure energy, and then renew.

 

[phinds]

Decay into pure energy, and then renew.

That would be pop-sci nonsense.

 

[snorkack]

Yes. When a star burns out to a white dwarf, only under 1 % of its mass is converted to energy and radiated away.
But why do galaxies lose the ability to generate new stars?
Typical mass of white dwarfs is under 0,6 solar masses - Procyon B is 0,60, and must have been more massive than Sun or Procyon A when on main sequence.
Therefore Sun-like stars must shed a large part of their mass into planetary nebulae in order to turn white dwarfs. Old galaxies where all more massive stars have decayed and Sun-like but older stars are right now burning out should contain lots of planetary nebulae.
Expansion speeds of planetary nebulae are slow. Much slower than galaxy escape speeds.
Gas shed by old stars in their old ages should get stuck and collect in galaxies.
Where is the gas shed by stars of old galaxies? Why is it not forming new stars?

 

[mathman]

New stars come from condensation of gas clouds. A dying galaxy presumably has run out of gas.

 

[phinds]

New stars come from condensation of gas clouds. A dying galaxy presumably has run out of gas.

Perhaps not quite out, but with the gas having too low a density to accumulate enough to create stars/planets

 

[ohwilleke]

Will the gravitational pull of a dead galaxy diminish over time as its matter converts to energy or other non-matter forms?

Both energy and mass gravitate in proportion to E=Mc^2.

 

[stefan r]

We have to define what we mean by "dead".

A galaxy can stop forming stars and still eject many stars. Black holes can still merge. A white dwarf or brown dwarf can still collide with a neutron star and flair into a Thorne-Zytkow object. 2 "dead" galaxies could collide and become lively.

Black holes have hawking radiation. If the theory is correct then a supermassive black hole will decay in around 10⁶⁴ years. At the moment cosmic background radiation adds more mass to black holes than Hawking radiation takes away.

https://en.wikipedia.org/wiki/Hawking_radiation#Black_hole_evaporation
...Or does it become a bunch of cold rocks eternally floating through space?...

https://en.wikipedia.org/wiki/Hawking_radiation#Black_hole_evaporation
Not rocks in the long long run. Iron is the lowest energy state you can get through fusion. Falling into a black hole gives off a much larger amount of energy than nuclear fusion.

There are some ideas about proton decay. If proton decay exists then there will eventually be no rocks. 10³⁶ years is a really long time. Is quite fast when we compare it to the 10⁶⁴ years it takes for a supermassive black hole to decay by Hawking radiation.

All particles in a galaxy eventually either evaporate by 3 body interactions or fall into black hole. If a particle goes over the cosmological event horizon it will never come back. Light and gravity waves radiate out of a galaxy taking mass with them.

 

[snorkack]

Both energy and mass gravitate in proportion to E=Mc^2.

Yes, but energy travels at far over the escape speed of galaxy and promptly leaves it.

 

[pervect]

Eventually, old dead galaxies will probably evaporate, at least according to "The End of the Universe" <<link>>

This is not a peer reviewed paper, but I believe the author can be trusted. There is a list of the sources and references at the end of the article, though, for readers who want to find out more and who may be skeptical about the claims. I have to admit that I have not read the primary references at the end of the article.

It's not that the matter vanishes, is that stars just are slowly get ejected from the galaxies, in a process similar to how solids and liquids evaporate.

In about 10^14 years, all normal star formation processes will have ceased, and the universe will have a population of stars consisting of about 55% white dwarfs, 45% brown dwarfs and a small number of neutron stars and black holes. Star formation will continue at a very slow rate due to collisions between brown and/or white dwarfs.

The black holes will suck up some of the other stars they encounter. This is especially true for the big black holes at the galactic centers, which power radio galaxies if they swallow stars at a sufficiently rapid rate. But most of the stars, as well as interstellar gas and dust, will eventually be hurled into intergalactic space. This happens to a star whenever it accidentally reaches escape velocity through its random encounters with other stars. It's a slow process, but computer simulations show that about 90% of the mass of the galaxies will eventually "boil off" this way — while the rest becomes a big black hole.

(It may seem odd that first the galaxies form by gravitational attraction of matter and then fall apart again by "boiling off", but the point is, intergalactic matter is less dense now than it was when galaxies first formed, thanks to the expansion of the universe. When the galaxies first formed, there was lots of gas around. Now the galaxies are essentially isolated — intergalactic space is almost a vacuum. And you can show in the really long run, any isolated system consisting of sufficiently many point particles interacting gravitationally — even an apparently "gravitationally bound" system — will "boil off" as individual particles randomly happen to acquire enough kinetic energy to reach escape velocity. Computer calculations already suggest that the solar system will fall apart this way, barring other interventions. With the galaxies it's even more certain to happen, since there are more particles involved, so things are more chaotic.)

There is a lot more in the whole article - and even more in the references it cites - but this snippet seems the most relevant

 

[phinds]

It's a slow process, but computer simulations show that about 90% of the mass of the galaxies will eventually "boil off" this way — while the rest becomes a big black hole.

Which would imply that over a really really long time (I'm practicing gross understatement here) the whole thing would boil away since Hawking Radiation will evaporate the BH.

 

[brand1130]

Would you say that a galaxy that formed 10 billion years ago, still has 99%+ of its mass today?

 

[stefan r]

Would you say that a galaxy that formed 10 billion years ago, still has 99%+ of its mass today?

Is the intergalactic medium escaping or falling in? If it is still falling in then the Milky Way has not finished forming yet.

The temperature of hot IGM gas is 10⁵ to 10⁷ K. Even at 10⁷ it the average thermal velocity is not quite enough to escape the Milky Way from where the Sun is located. Gasses escape from atmospheres but that is because the Sun is adding energy and blowing out a solar wind. The intergalactic medium is hot because of particles falling out of the void. That makes me think it has to be falling in.

If we take 10³⁶ watts as luminosity then a galaxy should be losing 1.1 x 10¹⁹ kg/s. If the galaxy is 2.2 x 10⁴² kg then it should radiate 1% of it's mass in 2 x 10²¹ seconds. 6.3 x 10¹³ years. There is no reason to think the galaxy's radiation is constant. Starbursts and active nuclei are brighter. 6,000 times as bright would not likely last over 10 billion years.

The Magellanic clouds have a star burst and are also dumping gas toward the MilkyWay. I do not know how much mass is in the Magellanic stream. Some galaxies in dense galaxy clusters have ram striping removing gas. That could take more than 1% of a galaxy's mass. That is probably not relevant to the original question.

 

[mfb]

Would you say that a galaxy that formed 10 billion years ago, still has 99%+ of its mass today?

It depends on the history of that galaxy. Did it collide with something? Did smaller galaxies fall into it? If it was just isolated in space: Probably yes.

Yes, but energy travels at far over the escape speed of galaxy and promptly leaves it.

Energy isn't an object. If you mean radiation, then yes.

Light and gravity waves radiate out of a galaxy taking mass with them.

Gravitational waves. Gravity waves are things like water surface waves.

 

[snorkack]

Is the intergalactic medium escaping or falling in? If it is still falling in then the Milky Way has not finished forming yet.

The temperature of hot IGM gas is 10⁵ to 10⁷ K. Even at 10⁷ it the average thermal velocity is not quite enough to escape the Milky Way from where the Sun is located. Gasses escape from atmospheres but that is because the Sun is adding energy and blowing out a solar wind. The intergalactic medium is hot because of particles falling out of the void. That makes me think it has to be falling in.

If we take 10³⁶ watts as luminosity then a galaxy should be losing 1.1 x 10¹⁹ kg/s. If the galaxy is 2.2 x 10⁴² kg then it should radiate 1% of it's mass in 2 x 10²¹ seconds. 6.3 x 10¹³ years. There is no reason to think the galaxy's radiation is constant. Starbursts and active nuclei are brighter. 6,000 times as bright would not likely last over 10 billion years.

Look at conservation of energy aspect. Converting primordial gas into carbon and oxygen white dwarfs releases about 0,5...0,6 % as radiation, and is not completed in 10 Gyr.
Converting mass into neutron stars and black holes can release more.

The Magellanic clouds have a star burst and are also dumping gas toward the MilkyWay. I do not know how much mass is in the Magellanic stream. Some galaxies in dense galaxy clusters have ram striping removing gas. That could take more than 1% of a galaxy's mass. That is probably not relevant to the original question.

It is.
My reasoning is:
in 10 Gyrs, a substantial fraction of original stellar mass should be shed from red giants into interstellar gas.
It should then be converted to new stars.
Yet old and yellow elliptical galaxies that don´ t seem to have either new stars or large masses of interstellar gas seem to be a common observation.
How, considering the mass of gas that must have been shed from the stars that burnt out?
Is there any efficient process at work to eject interstellar gas into intergalactic gas and prevent it from accumulating in the galaxy and forming new stars?