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stitt29
2010-Mar-24, 04:27 PM
Hi,

recently read that dark matter was invented to explain why stars at the edge of galaxies are moving too fast i.e. a ring of dark matter outside the galaxy, would result in a lower net pull to the centre of the galaxy so the stars then have a higher orbital speed.

The question is how come this ring of dark matter does not then affect the stars nearer the centre of the galaxy?

Hornblower
2010-Mar-24, 10:44 PM
Hi,

recently read that dark matter was invented to explain why stars at the edge of galaxies are moving too fast i.e. a ring of dark matter outside the galaxy, would result in a lower net pull to the centre of the galaxy so the stars then have a higher orbital speed.

The question is how come this ring of dark matter does not then affect the stars nearer the centre of the galaxy?

Whatever you read must have been poorly written. In a typical galaxy the reputed dark matter is distributed throughout a roughly spherical halo, not just a ring.

Let us start with a uniform spherical shell of matter of whatever type. Newton showed, as an exercise in calculus, that its gravitational attraction of outlying bodies is equal to that of the same mass concentrated in the center. For bodies inside the shell, there is no gravitational effect, as the attractions to different parts of the shell, distributed in all directions, cancel out.

Suppose we have a spherically symmetrical halo of diffuse non-luminous stuff that is not strongly concentrated toward the center the way the core stars are. We analyze if mathematically as a multitude of concentric shells. It will have very little effect on close-in stars even if it has several times the total mass of what we can see, but it will greatly increase the centripetal gravitation of outlying bodies over what they would get just from the visible stuff. Thus the outlying bodies orbit faster than they would have without the dark stuff.

We have modeled the system with stealthy "dark matter" because detailed observations have largely ruled out such things as additional dust or gas, zillions of planet-sized bodies, or outlying black holes.

Cougar
2010-Mar-25, 02:53 AM
recently read that dark matter was invented to explain why stars at the edge of galaxies are moving too fast...

Proposed or hypothesized or theorized, but not "invented." :) And it's not just stars at the edge of galaxies. It's pretty much all the stars in the galaxy are moving faster than the apparent mass interior to their orbits would allow. (Stars nearer the center of the galaxy are affected less.)


...i.e. a ring of dark matter outside the galaxy...

See, this is why I dislike the term "dark matter halo," which is unfortunately what has come to be used. But, as Hornblower says, dark matter is decidedly not "a ring of dark matter outside the galaxy." It's more like the galaxy is embedded in a huge glob of the stuff, which is significantly larger than the galaxy. So, yes, it extends well beyond the "edge" of the galaxy, and it also permeates all of the galaxy.

I don't think the exact distribution of DM is precisely known. I was just reading an older (1986) article by John Bahcall - not an unknown name in the field - and I was a bit surprised to see him write...



"The unseen material [he didn't call it dark matter] must be mostly in a disk form, i.e., dissipational. If all of the material were in a relatively round halo, [there's that word again :mad: ] then the rotation velocity at the solar position would have to be as large as 500 km s-1."

Since that time, I've typically heard the dark matter distribution is more like a solid sphere, but only very roughly so. And he apparently missed with the "dissipational," too, as most dissipational models have been ruled out by (lack of) observation.

kzb
2010-Mar-25, 12:58 PM
The model relies on the distribution of non-baryonic matter being clumped, and being denser towards the centre of galaxies.

I don't know why matter that hardly interacts with itself or anything else would clump in this way after the big bang, and do it ahead of the baryonic matter at that.

ngc3314
2010-Mar-25, 01:08 PM
The model relies on the distribution of non-baryonic matter being clumped, and being denser towards the centre of galaxies.

I don't know why matter that hardly interacts with itself or anything else would clump in this way after the big bang, and do it ahead of the baryonic matter at that.

It interacts gravitationally, and gravitationally interacting bodies can exchange energy (this is the basis of, for example, simulating galaxy dynamics with or without dark matter). There can be long-term efficient exchanges of energy and angular momentum between individual particles and large-scale properties of a distribution, leading to the amusingly named "violent relaxation" in which the distribution erases most of its initial conditions and ends up much like the density profile of an elliptical galaxy. Broadly speaking, dynamicists refer to this kind of density profile as resulting from dissipationless collapse - that is, only gravity acts and there are no direct collisions between particles. (The assumption that dark matter interacts with itself gravitationally, and not just with baryons, finds support from the observation that dark matter is strongly clumped on galaxy and cluster scales, from dynamics and gravitational lensing).

stitt29
2010-Mar-25, 04:05 PM
get it a bit better now. Its embedded in the galaxy so inner stars are within it so less affected outer stars have a greater pull to the centre because basically the centre is now more massive.

However as gravitationally interacting bodies can exchange energy I suppose then that there could also be a transfer of momentum from the centre to the outer stars- which could explain things without the need for dark matter

phunk
2010-Mar-25, 05:17 PM
However as gravitationally interacting bodies can exchange energy I suppose then that there could also be a transfer of momentum from the centre to the outer stars- which could explain things without the need for dark matter

The problem isn't how they got that momentum, the problem explained by dark matter is what's holding the outer stars in when they're moving fast enough to fly out of the galaxy.

Andrew D
2010-Mar-25, 05:35 PM
The problem isn't how they got that momentum, the problem explained by dark matter is what's holding the outer stars in when they're moving fast enough to fly out of the galaxy.

I thought the problem was that they were moving so fast in the first place?

I know there are alternatives to the DM hypothesis, but I haven't really read into any. Does anyone have any good references? MOND for example?

Cougar
2010-Mar-25, 05:48 PM
I don't know why matter that hardly interacts with itself or anything else would clump in this way after the big bang, and do it ahead of the baryonic matter at that.

It can do some gravitational clumping ahead of the baryonic matter because it doesn't or only weakly interacts via the electromagnetic and strong nuclear interactions. So while the baryonic stuff was in thermal equilibrium due to the very high temperature prior to 300,000 years after the "beginning," the dark matter was, hypothetically, unaffected by this smoothed baryonic brew, so could start clumping earlier.

Grey
2010-Mar-25, 07:44 PM
I don't think the exact distribution of DM is precisely known.You're right that it's not precisely known, but you can map out roughly what it needs to be for any given galaxy. It turns out that it has to be pretty much spherical, with a mass distribution that falls off like the inverse square of the distance from the center. Conveniently, it also turns out that if you do some basic kinematic analysis on a collection of particles that are gravitationally bound, but don't otherwise interact much, an inverse square mass distribution is exactly what you end up with. So that means that if there are some hypothetical particles that interact only through gravity (and maybe the weak force), it is reasonable for them to behave in a manner that would account for what we see. We can take that further and simulate things like galactic collisions, see how dark matter should behave (assuming again that it interacts significantly only through gravity), and then compare that to observations of real galactic collisions, and see if the distribution of dark matter we'd predict through theory matches the distribution we can infer from the movement of luminous matter. It again turns out that it works quite well.

forrest noble
2010-Mar-25, 08:20 PM
stitt29, Roobydo


recently read that dark matter was invented to explain why stars at the edge of galaxies are moving too fast i.e. a ring of dark matter outside the galaxy, would result in a lower net pull to the centre of the galaxy so the stars then have a higher orbital speed.

The question is how come this ring of dark matter does not then affect the stars nearer the centre of the galaxy?

Other formulations and dark matter:

There are a number of modified Newtonian formulations to explain the same observations that dark matter is trying to explain. MOND is the most well know of these but others might be classified under the same name if they involve modified Newtonian (dynamics) formulations. These formulations can explain more or less rotation curves of disc stars in spiral galaxies. Physicists have a problem with MOND and all formulations that do not have an accompanying logic. Newton with the inverse square law has strong logic concerning the fall off of the force with distance in the same way as light and magnetism. GR's logic is based upon warped space and similarities to proven equations of magnetism.

Dark matter is considered by the mainstream to be a better explanation since GR would not have to be changed. MOND, as well as similar formulations, have been criticized since it has been claimed by some that such formulations cannot explain the rotation of galaxies in a cluster or the mechanics of interacting galaxies.

One of many possible arguments against dark matter is that it has to cluster surrounding galaxies in a halo fashion and then fall off in density moving toward the galactic bulge. Since it is trying to explain gravity, what kind of mechanism could cause this effect?(rhetorical). It is also realized that since the quantity, density, and distribution of dark matter would seemingly always be unknowns, predictions involving dark matter and GR would seemingly never be more than guess work.

Even if a dark matter of some kind is recognized/ discovered some day, for it to explain galactic rotations of disc stars, new explanations of the reasons for its clustering, in the manner proposed, also must be explained.

Cougar
2010-Mar-25, 10:08 PM
MOND, as well as similar formulations, have been criticized since it has been claimed by some that such formulations cannot explain the rotation of galaxies in a cluster or the mechanics of interacting galaxies.

Right. MOND may come close to modeling the orbits of stars within a galaxy, but it fails rather miserably in modeling how galaxies move and orbit in galaxy clusters. Then there's the bullet cluster (http://en.wikipedia.org/wiki/Bullet_Cluster): weak gravitational lensing shows the dark matter is there, providing strong support for the dark matter hypothesis.


One of many possible arguments against dark matter is that it has to cluster surrounding galaxies in a halo fashion and then fall off in density moving toward the galactic bulge. Since it is trying to explain gravity, what kind of mechanism could cause this effect?(rhetorical).

I don't know that it falls off in density. But it becomes less of an issue because if the radius of an orbit decreases by 2, say, doesn't the (roughly uniformly distributed) mass interior to that orbit fall of by a factor of 8?

Hornblower
2010-Mar-25, 10:10 PM
<snip>One of many possible arguments against dark matter is that it has to cluster surrounding galaxies in a halo fashion and then fall off in density moving toward the galactic bulge. Since it is trying to explain gravity, what kind of mechanism could cause this effect?(rhetorical). It is also realized that since the quantity, density, and distribution of dark matter would seemingly always be unknowns, predictions involving dark matter and GR would seemingly never be more than guess work.

Even if a dark matter of some kind is recognized/ discovered some day, for it to explain galactic rotations of disc stars, new explanations of the reasons for its clustering, as proposed, also must be explained.

My bold for reference. Who says it has to become less dense toward a galactic bulge. Can you give us some references?

forrest noble
2010-Mar-25, 10:36 PM
Cougar,


I don't know that it falls off in density. But it becomes less of an issue because if the radius of an orbit decreases by 2, say, doesn't the (roughly uniformly distributed) mass interior to that orbit fall of by a factor of 8?

This seems right to me.

I recall reading that a halo of dark matter alone surrounding a spiral galaxy cannot explain disc star rotation curves because if only a halo existed rotation velocities of disk stars would accordingly fall off too quickly at greater distances inward from the halo -- so accordingly dark matter densities seemingly would need to continue and progressively decrease inward toward the bulge since there seems to be no transition area/ volume between gravitational influences of the bulge and from the proposed halo.

http://en.wikipedia.org/wiki/Dark_matter

Cougar
2010-Mar-26, 12:57 AM
You're right that it's not precisely known, but you can map out roughly what it needs to be for any given galaxy. It turns out that it has to be pretty much spherical, with a mass distribution that falls off like the inverse square of the distance from the center. Conveniently, it also turns out that if you do some basic kinematic analysis on a collection of particles that are gravitationally bound, but don't otherwise interact much, an inverse square mass distribution is exactly what you end up with. So that means that if there are some hypothetical particles that interact only through gravity (and maybe the weak force), it is reasonable for them to behave in a manner that would account for what we see. We can take that further and simulate things like galactic collisions, see how dark matter should behave (assuming again that it interacts significantly only through gravity), and then compare that to observations of real galactic collisions, and see if the distribution of dark matter we'd predict through theory matches the distribution we can infer from the movement of luminous matter. It again turns out that it works quite well.

Wow, that is news to me. But I trust your knowledge, Grey, to exceed my own, and your explanation makes perfect sense. For some reason, I was thinking the DM was pretty much equal density throughout the galactic region, but I can see that gravity alone would distribute it as you say.

forrest noble
2010-Mar-26, 04:47 AM
Hornblower,


Who says it has to become less dense toward a galactic bulge. Can you give us some references?

Dark Matter speculation. These URL's generally shows the pros and cons concerning dark matter galactic distribution.

Below are some variant quotations


...Computer modeling suggests that galaxies may have dark matter halos, but also have dark matter distributed throughout their structure – all representing up to 90% of a galaxy’s total mass.

...Cosmologists will have to go back to the drawing board on dark matter theories, the pair suggest, since the clumpy "cuspy" profiles are favored in many models. "A solution might include dark matter physics not ordinarily considered," they conclude.

...some of the assumptions that were previously made on the universe evolution predictions should be changed, as dark matter does not behave how scientists believed it did.


Further challenges to dark matter theory, or at least its most popular form - cold dark matter (CDM), came from analysis of the centres of low surface brightness galaxies. Numerical simulations based on CDM gave predictions of the shape of the rotation curves in the centre of dark-matter dominated systems, such as these galaxies. Observations of the actual rotation curves did not show the predicted shape. This so-called "cuspy halo problem" of cold dark matter is considered a tractable issue by theoretical cosmologists.

http://en.wikipedia.org/wiki/Cuspy_halo_problem

http://en.wikipedia.org/wiki/Galaxy_rotation_curve

http://www.examiner.com/x-30007-Space-News-Examiner~y2010m3d23-Supermassive-black-holes-hint-at-dark-matter-behavior

http://www.popsci.com/science/article/2010-03/dark-matter-reveals-itself-black-hole

Here are four links. It is generally believed that (if it exists) according to some, maybe up to 90% of a galaxy's mass may lie outside the observable galaxy. If that's the case then not too large a DM percentage could be within the visible galaxy or bulge. Simulation studies have indicated that conceivably a much lower density/ quantity of dark matter would be within the bulge since the black hole could conceivably consume it and after that eventually the entire galaxy.

A search will find a number of varying opinions on the entire DM issue as well as many seemingly conflicting studies and conclusions. If one is of a different opinion concerning DM, that also seems to fit in well.

Grey
2010-Mar-26, 04:45 PM
Wow, that is news to me. But I trust your knowledge, Grey, to exceed my own, and your explanation makes perfect sense. For some reason, I was thinking the DM was pretty much equal density throughout the galactic region, but I can see that gravity alone would distribute it as you say.It's actually a pretty straightforward calculation. We know that the rotational velocity curves of galaxies are roughly flat, and an inverse square distribution of mass is what you need to get a flat rotation curve. The spherical distribution is less certain, but movement of globular clusters (which tend to be distributed more or less spherically, rather than forming a disk), seems to suggest that the distribution of dark matter is likewise roughly spherically symmetric. Of course, to get a precise fit to a given galactic rotation curve is a little trickier, but the general pattern is pretty consistent.

Forrest, the reason that much of the mass of a dark matter halo (and I dislike the use of that term as well, for the same reason as Cougar) is outside of the visible galaxy is not that it gets less dense at the center. It's just that the dark matter extends well beyond the visible matter in the galaxy; it also permeates that visible portion of the galaxy as well.

Cougar
2010-Mar-26, 07:44 PM
We know that the rotational velocity curves of galaxies are roughly flat...

That I knew.


...and an inverse square distribution of mass is what you need to get a flat rotation curve.

That did not occur to me. :o


The spherical distribution is less certain, but movement of globular clusters (which tend to be distributed more or less spherically, rather than forming a disk), seems to suggest that the distribution of dark matter is likewise roughly spherically symmetric. Of course, to get a precise fit to a given galactic rotation curve is a little trickier, but the general pattern is pretty consistent.

Right. Obviously, a near miss, collision, or merger with another galaxy is going to skew this shape considerably for a while.