Galactic density in the low-mid distances from core

[Gerinski]

Hi, may I ask about this:

In a normal orbiting system, if the orbiting object has the right velocity, it will stay in a stable orbit around the more massive one (to be precise, they will both orbit around their common center of mass, but for a galaxy this coincides with the galactic center). Unless there is friction or collisions or other gravitational influences, the orbiting object will stay there for a huge time. But if there is friction it will gradually and slowly loose velocity and its orbit will gradually fall down to an ever lower orbit.

Now, I would guess that the density of intergalactic gas and dust is higher in the inner regions of the galaxy and much smaller, more diffuse, at the outer edges. So the friction of the medium is high close to the center and gradually smaller going towards the outer edge.

This would mean that the stars at closer distances from the center loose velocity by friction and fall faster than stars orbiting in the outer edges which travel through low-friction medium.

If that was the case it seems that we should see old galaxies as having a very dense core, then a rather empty low-distance disk, a diffuse mid-distance disk, and the outer edges still populated. But this is not what we observe, it seems that the density of stars decreases gradually towards the edges, without empty disks at short distance from the center.

Related to this, in the scenario I guessed in which close-distance stars fall quickly towards the center and leave an empty region, by conservation of momentum the core should start to spin faster and faster, while the outer edges would remain orbiting at their original slow speed. Is that correct and what is it that we actually observe?

Thanks

 

[Buzz Bloom]

This would mean that the stars at closer distances from the center loose velocity by friction and fall faster than stars orbiting in the outer edges which travel through low-friction medium.

Hi Gerinski:

Since this thread was created quite a while ago without any responses (except Greg's automatic one), I thought I would try to say something useful based on my very limited knowledge.

I think the "friction" you describe as the mechanism for having ordinary matter stuff change its orbits towards those with lower energy is mostly a phenomenon related to gasses (and maybe dust too) rather than to stars after they form. I think that the friction relates to interactions of small objects like molecules (or maybe dust particles too) which cause photons to be created which remove kinetic energy and angular momentum from an orbit. My guess is that the rate at which this kind of interaction takes place during an early state of galaxy formation is much greater than the rate at which the energy and angular momentum of stars in their orbits (around the centers of their galaxies) is removed after the stars form.

You also may want to look at post #7 in the thread
https://www.physicsforums.com/threads/the-dark-sky-ahead.836784/

Regards,
Buzz

 

[Gerinski]

Thanks Buzz, I really appreciate the answer !

 

[newjerseyrunner]

That friction you spoke of also affects the gas itself, the gas rubs on itself and loses moment, slowly falling towards the center. The black hole either eats it, or spits it back out into space at nearly the speed of light. This would quickly remove a lot of gas. It's also throwing stars around like crazy, and they get fairly close to each other. Their solar winds and radiation pressure should force any interstellar gas either away or into the black hole.