Questions about Dark Matter

[Fallingstar1971]

Observations made in the past concerning how all stars in a galaxy rotate the center at the same speed as proof of dark matter.

However, if this were true, how could a star ever achieve escape velocity?

If a star can achieve escape velocity, is there a barrier where the stars are allowed to rotate more quickly? How far away from the black hole at the center of the galaxy would you have to be to observe such an effect?

If a Black Hole consumes dark matter, would it display the same properties of a black hole consuming normal matter?

With the current models of Dark matter, what effect would it have on an escaping star?



Star

[MeteorWayne]

Pretty much, stars don't reach escape velocity from a galaxy. The only rare exceptions are those ejected from multiple star systems, or during asymmetric stellar explosions. The space between galaxies is VERY empty.

What gives you the idea such events happen?

[darkmatter4brains]

You could check out the Astronomy Magazine for September 2009 that had an article about this very thing.

What revved up the galaxy's hyperfast stars?, Sep , 2009
"Since 2003 astronomers have found 16 stars escaping the Milky Way's gravity. The slingshot could be our galaxy's central black hole."

Don't remember if they talked about dark matter or not, though.

[drwayne]

I wonder if there is an very loose analogy in here somewhere to a Bolzmann distribution. and the probability
that electrons can borrow enough thermal energy to jump a bandgap they shouldn't have the energy to
jump.

Or maybe I just have that on my mind for something I am working on.

[Fallingstar1971]

space.com ran an article some time ago about a neutron star speeding from the milky way

http://www.space.com/scienceastronomy/0 ... _star.html

Its just that if they are all going the same speed to begin with, I just dont understand how it was able to get its speed


Star

[ramparts]

Hi Fallingstar - As has been mentioned by MW-Wayne, almost any star with a velocity allowing it to leave the galaxy (and mind you, we've only found 16!) was ejected by, say, three star interactions, an orbit with a black hole, etc. I believe these are somewhat common in the galactic center, where the supermassive black hole and the large number of stars makes this a pretty plausible scenario. This has nothing to do with the galactic rotation curves - just because most stars are orbiting at a certain speed doesn't mean they all have to

[Fallingstar1971]

First, thanks for your responses. So a star can accumulate the speed required to escape the Milky way.

But what about dark matter? Could this be a real world test? How will this dark matter affect the escaping star? Will it deflect it from its original trajectory?

If it exists, and if they are right about its properties, if it can help hold a galaxy together then that is a lot of influence. I would think that there would be some kind of effect.

And yes, I was totally thinking of rotational curves here. I still cant get my head around how stars in the middle of a galaxy rotate at the same speed as stars on the edge of a galaxy. I cant understand how this works. Its like they are riding spokes on a wheel. A ridged structure connected to the middle firmly holding everything in its length. It that how dark matter works? Like the spoke of a wheel?

Could this be a new "force"? Does Dark matter only manifest on the grandest scales? Or is this something that we may someday be able to capture in a lab?

Star

[ramparts]

Hi Star - Actually, the thing about a galaxy is that there are no spokes. Nothing connects us to the stars closer in or further out except gravity. There's no rigid rod like a spoke holding everything together, so different parts of the galaxy can rotate at different rates. This is called differential rotation, and it's pretty common - even the Sun does it! There are different speeds at the poles and equator of the Sun - actually pulls it out into a slightly egg shape

[MeteorWayne]

Fallingstar...that spoke idea is exactly why there must be dark matter around galaxies. Take the SOlar system for example. The closer you are to the sun, the faster you move and the shorter your year. Mercury 88 days, Venus 243 days, earth 365 days, Jupiter 12 years, Saturn 30 years, Neptune 165 years (almost exactly 1 orbit since it was discovered), dwarf planet Pluto 248 years, etc. That's because most of the mass (over 99.5%) of the solar system is concentrated in the sun.
But this rule doesn't work for galaxies. The outer stars (while not on rigid spokes or at excatly the same rate as the inner stars) do not follow the rules if the only mass were the supermassive black hole and the visible stars and detectable gas. The outer stars rotate faster than they should. This shows that there is a large amount of mass that can not be seen by normal means, including a large amount outside of the limit of the visible stars. That is "dark matter"

Unfortuantely, for single stars ejected at high speed, I wouldn't think the deflection caused by that matter would be detectable over human lifetimes; though I'm sure there are folks trying to resolve such an effect.

[ramparts]

Bear in mind that when we calculate the escape velocity of the galaxy, we're including dark matter.... escape velocity has to do with an object's mass, and the point of dark matter is that it adds mass to a galaxy.

[Fallingstar1971]

So I guess my next question would have to be, does the milky ways central black hole have more mass than the sum of the surrounding stars?

Could the spiral arms be illuminating "lines of force" (gravitational or magnetic) from the galaxies central black hole? If so, then that could be your spokes. The arms could be pointing the way.

Do stars between the spiral arms exhibit this same property of rotating at the same speed as stars in the center? I suppose it doesn't matter either way, a line of force could be present and NOT illuminated.

If not lines of force, then perhaps just lines of gravity caused by the stars and gas lined up and down the arms?

I was thinking that if there is a large amount of dark matter in the halo, then the star may be deflected twords the highest concentration of it, unless its uniform of course.

And lastly, space and time near a black hole get weird. Could some sort of time dilation be causing the stars in the center to appear to be moving slower than they really are?

Could "Frame Dragging" be locking everything in at the same speed?

Please bear with me if the questions seem silly. I am just curious and wish to know more

Star

[origin]

So I guess my next question would have to be, does the milky ways central black hole have more mass than the sum of the surrounding stars?

No, not even close, if by surrounding stars you mean the galaxy.

Could the spiral arms be illuminating "lines of force" (gravitational or magnetic) from the galaxies central black hole? If so, then that could be your spokes. The arms could be pointing the way.

Not based on what we know about gravity and magnetism.

I suppose it doesn't matter either way, a line of force could be present and NOT illuminated.

If not lines of force, then perhaps just lines of gravity caused by the stars and gas lined up and down the arms?

I was thinking that if there is a large amount of dark matter in the halo, then the star may be deflected twords the highest concentration of it, unless its uniform of course.

?

[kg]

I hope this is a good place to post this...
http://www.skyandtelescope.com/news/66968132.html

"...If correct, their interpretation of the Fermi data would tie together a number of hints and puzzling observations that suggest dark matter is making itself visible through a process of annihilation. It also implies the existence of a new force to which only dark matter particles are attuned..."

Should I be suspicious of something that requires a "new force" incorporated into it's explanation?

[Saiph]

good questions and ideas, but in brief the answer is: no. Not on any of them (sorry!)

But I'll go into more detail for you

Spiral arms are actually just 'density waves' Places where the orbits of stars tend to cluster together. Imagine a bunch of circles and elipses, of various shapes. These are stellar orbits, and stars move on them at different speeds too (but still close in speed, and direction). Now, shift all the circles so that the...say left..edges match up. You'll notice that there, it's dark, clustered, dense with lines (orbits)...meaning you'll tend to see lots of stars there. Elsewhere they're spread out, and while you'll see the stars, they won't be a big bright cluster of them.

The stars themselves move much faster than the arms, and thus move through them regularly. I.e. the stars that make up the arms aren't the same stars from year to year (well, longer than a year, but you get the idea). The reason they cluster this way is the spiral arms dense, star packed arms pass through slower moving dust clouds...and cause all sorts of star formation. The stars that form have similar orbits to those in the arm that created them becaue those stars dragged the gases along in their wake.

The stars really close to the core orbit as you'd expect, with orbital speed actually increasing as you head out, since you're enclosing a lot more mass each time you step further out. Then it peaks, and begins to drop (you've exited the big dense central bulge) but instead of gently dropping towards zero (as you'd expect if all/most mass is in the core) it flattens out..meaning that you are encompassing a lot more mass than expected when you step further out...much more than we can account for visually.

Also the 'lines of force' thing you see in books is an illustration only. They don't really exist. The forces are fields, more like blankets than discrete lines. The lines are used to help visualize what's going on, and analyze the forces involved. If you are thinking about the way iron filings group around magnets...forming 'lines of force' that's actually due to how the iron filings become slighly magnetic in the field, and attract other iron filings...creating irregularities in the field (i.e. the main magnet isn't the only magnet!). The magnetic field exists in between, and the lines positions are determined by how the iron was scattered beforehand.


If there was a large clump of DM in the halo, you could detect it by observing the deflection of stars moving past it...but single stars would be hard to use, and you don't really need 'escaping stars' to do it. It's actually being monitored in studies now-adays using the globular star clusters that reside in the halo. Same sorta idea, just a bigger scale.

as for time dilation and frame dragging being a factor...way to small, way to local to have this affect. You'd have to be within only a few thousand miles of a BH to really notice it. And we're talking thousands of light years here.

[amaterasu]

great post, Saiph

[Saiph]

thanks

It comes up often enough I really should just write up a really nice version and direct people there, or paste it when required.

I don't mind that it comes up a lot (it's a good question!), just saying it does

[Fallingstar1971]

Thanks for the info, great response. Now I am left with even more questions.

The Keck telescope has some great closeups of the galactic core.

Gravitational Lensing causes multiple galaxies to appear to surround a strong gravity source, such as an Einstein Cross.

Now, what effects will gravitational lensing have when viewing these stars in the galactic core? According to the images from Keck, we see single stars with plotted trajectories. Not multiple images of the same star.

Is the same corrective lensing technique used to correct for atmospheric blurring canceling out the gravitational lensing effects? Keck is looking pretty close to that "unseen" mass that these stars are orbiting. I am just curious as to how distorted our picture is due to the warping of space in that close of a proximity.

And if what we are seeing is distorted, then perhaps our measurements are distorted.

IMO, when looking this close, its like looking at the rocks at the bottom of a shallow pond. They are not quite where they appear due to the lensing effects of the water. A black hole would produce an even greater distortion, but not just with light. Space and time are distorted as well, at the core.

So your looking at distorted spacetime with Keck.

What does this have to do with my first question? Well, if our measurements of the core are distorted, then would that not effect the comparison of stars in the centers of galaxies moving at the same speed as stars on the outskirts?

I know you said stars inside the bulge behave as expected, do these distortions stop before you get outside the bulge?

In the models, do they start with a black hole and work out from there? Its amazing how these SuperComputers can take into effect the combined gravity and its effects on over a million stars orbiting a warped section of space, with each star warping space a bit on its own.

At what speed are the dwarf galaxies orbiting, at what range from the center does gravity resume its "explainable" "predictable" orbital mechanics? Does the speeds of these "satellite" galaxies fall off like the planets in the solar system?

Or maybe I just got this completely wrong......I dunno

Again, just curious, not trying to be stupid, just trying to understand.

Star

[MeteorWayne]

There probably are small distortions in position if stars pass behind the black hole, but I am sure that is taken into account..most observations take place far enough away that the effect would be undetectably small.

[Fallingstar1971]

Thanks again guys, I think I was thinking of gravity like a chain, like the black hole holds on to these stars, these stars hold on to the next stars farther out, and so on. With gravity acting like a chain or web connecting all the stars together and forcing them to move in unision due to the momentum of the spinning black hole in the center of the galaxy. Inflexable and absolute.

O well. Thanks again


Star

[Jerromy]

I may be totally wrong, but here is my input... I think of individual galaxies in our universe as toilets on our planet. Our planet spins and causes water in a drain to spin because of the rotation of the foundation affects all the water in the toilet equally. I think the universe is the same way... since the collective mass of the universe spins, it causes all the stars in a galaxy to spin together as a group. The mass of even the biggest galaxies is a drop in the bucket compared to the mass of our observable universe as a whole. That I believe is the mystery of dark matter in a nutshell.