Can someone educate me about why the rotational speed of objects in outer reaches of galaxies infers or implies that there must be invisible matter far beyond the edge of the visible matter? Is it possible that this uniform rotation is due to some effect of the central black hole that we do not understand?

The operative physics term at play should be "conservation of angular momentum" but the galactic effect has a relatively flat rotation curve out the outer reaches of spiral arms, which is contra.

The stars at the periphery of a galaxy ought by our normal understanding of gravity to be revolving more slowly relative to the core than stars closer in, and by a predictable amount.

They're not. The appearance is that of a solid body instead of a collection of simply gravitationally bound bodies revolving about a massive core. So, either our understanding of gravity is flawed on these scales (but it's not on larger or smaller scales, at least not in the same way) OR something else is in play here.

That something else has come to be called Dark Matter, simply because something MUST be there, but we can't see it.

 Do we see this effect by examining and not seeing any variation in the red and blue shifts of the stars coming towards us and going away from us in the disks of edge on galaxies? If the gravitational effect of a super-massive black hole is many times the radius of a galaxy, wouldn't this cause the rotation curve to flatten more gradually, even approaching nearly apparent total flatness with respect to the relatively small diameter of the disk where the visible matter is, even if there is no matter of any kind outside the boundary of the visible matter?

Joep229 :you asked, quote, "  Do we see this effect by examining and not seeing any variation in the red and blue shifts of the stars coming towards us and going away from us in the disks of edge on galaxies? " , I can't answer with detail the mechanism they use to determine the velocities, masses and the distances of the stars in our galaxy, but I do know the size of our galaxy has been updated recently, and the original concern about dark matter was by Mr. Fritz Zwicky, which he refered to as missing mass, this was observed in the coma cluster, and later with our galaxy. 

You asked, quote, "If the gravitational effect of a super-massive black hole is many times the radius of a galaxy, wouldn't this cause the rotation curve to flatten more gradually, even approaching nearly apparent total flatness with respect to the relatively small diameter of the disk where the visible matter is, even if there is no matter of any kind outside the boundary of the visible matter?, No, the gravitational effect of a black hole is limited to it's influence as an equal number of visible stars necessary to account for the mass of the black hole, and I believe that would be about 4 million masses of our sun. I have heard the mass of the black hole is equil to .5 of 1 %  of the mass of the galaxy within where it resides, this was in a docudrama on the Science Channel.

Yes, its the doppler shift of stars in opposite arms of galaxies that are measured. Typically we see blueshifts on one side and redshifts on the orther when the galaxy is edge on to us or nearly so.

The surprise is that the velocity curve doesn't fall off as expected as you move outward along the arms. Effectively, those stars further out are moving faster than they should be. So fast, in fact, that they should be flung outward away from the galaxy, but they're not. So, something is "holding them in place."

While the overall gravitational effect of a supermassive black hole might extend well out into the galaxy, beyond a certain distance (as primordial has pointed out) it falls off pretty much according to the inverse-square law. After all, what's a few million solar masses compared to a few hundred billion, more or less. There simply isn't enough visible mass, or concentrated black hole mass, to make up for the observed effect on the periphery stars.

As they say, Zwicky Knew.

 Primordial, You said;

No, the gravitational effect of a black hole is limited to it's influence as an equal number of visible stars necessary to account for the mass of the black hole, and I believe that would be about 4 million masses of our sun. I have heard the mass of the black hole is equil to .5 of 1 %  of the mass of the galaxy within where it resides..."

That seems contradictory. The Milky Way black hole is supposedly just under 3.7 million times the mass of our sun. Are you saying that the visible Milky Way excluding the mass of the central black hole can only contain the same amount of mass as everything else in the galaxy combined and that is no more than 3.7 million times the mass of our sun? Is this a limitation and nothing else can be held and would fly off? Or are you saying that the mass of the black hole is 1/2 of 1% the mass of everything else in the galaxy which then seems like the mass of everything else can be up to 500 times the mass of the black hole? I'm confused.

 Chip; so if I am understanding correctly, given the mass of what is visible plus the mass of the black hole, no matter how massive the black hole is, and how far it's reach, there is no way that the galaxy can be empty space outside of the visible portion. There has to be invisible mass of some kind out beyond the visible portion to keep the whole visible disk orbiting like a solid object.

If a solar system had 2 Jupiter sized planets separated by 100 million miles they could not possibly both orbit the sun at the same speed (the outer one would obviously have to orbit much slower) and even if this were possible and if their sun suddenly became say 100 times more massive, they would just as quickly have to speed up their orbital speeds by a factor of 100 (?) to stay in orbit. So comparing that to the black hole situation, It doesn't matter how massive the black hole is and how far out it warps space, the only thing that matters is the distance objects are from it.

I'm still stumped ( I guess I've joined the club!) about how bodies can orbit faster and faster as their distances from the center of the galaxy increases and orbit together as an apparent solid disk, regardless of whether there is invisible mass present. Getting back to the solar system with the 2 jupiter sized planets. If the outer one was made of dark matter, and therefore invisible. We would have no way of seeing it or knowing how fast it was orbiting. What we would see is a star and 1 orbiting planet. I suppose we could see that the star was wobbling too much and infer that something else was out there. If the star had 10 inner planets and another jupiter sized dark matter one very far out from the 10 inner planets. Wouldn't the 10 inner planets still orbit the star at the same decreasing speeds as the distance from the sun increases whether the dark matter mass was there or not?

Am I driving you crazy and should go do something else?

Joep229 : This proportionality is an approximation derived during the study of galaxies and their suppermassive black holes,it is not fully understood what this may indicate, with the exception that there appears to be some relative factor involved between the mass of a galaxy and the mass of the suppermassive black hole at it's nucleus. It would appear to me that the galaxy and the suppermassive black hole nucleus grow with a proportionality due to the mechanism of the combination. I have presented an idea as to how this may happen, with my concept, which is in conflect with Mr. Hawking's Concept of the quantum exchange( Hawking Radiation) of energy through the concept of virtual pairs first presented by Mr. Richard Feynman and Initially conceived by Mr. P.A.M. Dirac with his negative sea concept, however Mr.Hawking presents his concept in a format that should reduce the mass (energy) of the black hole, and I can only see the black hole as increasing in mass (energy). It's sort of long.

 I'm sure it's long, and beyond my understanding. Would you say that as gravity decreases between bodies over extremely long distances, some other force takes over. This force is undetectable up to a certain distance and then it starts to act like gravity, and increases as distance increases and this is what keeps super fast moving objects in orbit in distant reaches of a galaxy where there is not nearly enough gravity to hold them? Do you think that any 2 objects separated by this kind of distance experience this force, or is this just a black hole generated effect?

Joep229 :  To answer this question "Would you say that as gravity decreases between bodies over extremely long distances" , yes, proportional to the inverse square law.  You added " and some other force takes over" , yes, the gravitational effect that is attributed to dark matter or a need to understand the law of gravity that can explain this circumstance.

To answer this question "This force is undetectable up to a certain distance and then it starts to act like gravity, and increases as distance increases and this is what keeps super fast moving objects in orbit in distant reaches of a galaxy where there is not nearly enough gravity to hold them? " , yes, it has not been detected other than the fact that it has a gravitational interaction with baryonic matter in the characteristic of matter orbiting at a unusual high velocity and by gravitational lensing of light, as it passes massive star systems. 

To answer this question "Do you think that any 2 objects separated by this kind of distance experience this force, or is this just a black hole generated effect?", I can't give an observation that would lead to an absolute answer to this question. With the exception of somewhere the statement was made that gas nebula also have the same properties of rotation.

Joep229:

 Chip; so if I am understanding correctly, given the mass of what is visible plus the mass of the black hole, no matter how massive the black hole is, and how far it's reach, there is no way that the galaxy can be empty space outside of the visible portion. There has to be invisible mass of some kind out beyond the visible portion to keep the whole visible disk orbiting like a solid object.

Except for "no matter how massive" -- which could be interpreted out of all proportion to what is observed -- yes.

... So ... It doesn't matter how massive the black hole is and how far out it warps space, the only thing that matters is the distance objects are from it.

No, those things would matter very much. But they're not what is observed. You could get some very weird scenarios if you increased the mass of the BH greatly beyond what is observed to be the case for black holes ... and I don't know whether any models support the galaxy hanging together beyond a certain (nonsensically high) mass for its core BH.

I'm still stumped ( I guess I've joined the club!) about how bodies can orbit faster and faster as their distances from the center of the galaxy increases and orbit together as an apparent solid disk, regardless of whether there is invisible mass present.

Yes, that's very nearly the appearance of what we get ... it's as if the galaxies we observe were encased in some big invisible block ...

Getting back to the solar system with the 2 jupiter sized planets. If the outer one was made of dark matter, and therefore invisible. We would have no way of seeing it or knowing how fast it was orbiting. What we would see is a star and 1 orbiting planet. I suppose we could see that the star was wobbling too much and infer that something else was out there.

It's a matter of scale. Dark Matter doesn't seem to be noticeably at planetary scales (i.e., I don't know that we can say a "planet" is large enough, or even a solar system is large enough, to exhibit dark-matter properties. I have not read that anywhere.

If the star had 10 inner planets and another jupiter sized dark matter one very far out from the 10 inner planets. Wouldn't the 10 inner planets still orbit the star at the same decreasing speeds as the distance from the sun increases whether the dark matter mass was there or not?

That is what we see ... things appear "normal" at solar system scales ... however, there IS that Pioneer anomaly yet to be explained ...

 

It is very interesting to see regular announcements from the astronomy professionals about new observations of what might be dark matter. On the other hand, the new ideas about gravitation (MOND) as an explanation for the missing mass in galaxies are also widely discussed. It seems to me, (as a chemist just getting to grips with astronomy journals), that both sides are choosing to ignore significant evidence about the nature of the matter making up galaxies. Data from the late 1990’s (the Infrared Space Observatory) and, more recently, from the Spitzer space telescope have shown that spiral galaxies contain massive amounts of hitherto undetected molecular hydrogen (H2) in such quantities that may very well account for earlier estimates of missing galactic mass which led to the birth of both dark matter and the MOND ideas. Whereas atomic hydrogen, (H), is readily detected by radioastronomy, molecular H2 can only be detected in relatively warm environments, ie reasonably close to stars. Wherever there is interstellar dust, (and galaxies have lots of it), H2 is formed and there must be massive quantities throughout the universe which we are unaware of. Using a quasar located about 12 billion light years away as a beacon, molecular hydrogen has recently been detected by the ESO’s VLT at the very edge of the known universe, corresponding to the time when the universe was about a tenth of its present age, about 1.5 billion years old. All this evidence can be found in respected astronomy journals. Comments please?

The original issue with this missing matter involved the general observation by Fritz Zwicky in 1933 concerning rotational velocities of galaxy clusters. This was reviewed in detail by Vera Rubin in 1950 with her master thesis. Her results were not treated with seriousness by the American Astronomical Society, and were ignored in the mainstream community. In 1970, along with W. Kent Ford, she published an article entitled " Rotation of the Andromeda Nebula from a Spectroscopic  Survey of Emission Regions" in the Astrophysical Journal. Her observations suggested that in order for the rotational velocities to be true, there had to be ten times more apparent matter than was observable. It wasn't until 1980 that the astronomical community came around and affirmed her thesis and subsequent publication(s).

---this information came from a book entitled "Dark Cosmos" by Dan Hooper; copyright 2006 by Smithsonian Books.

I like this posting concerning diatomic hydrogen molecules in having greater abundance than is generally recognized. I have postulated that mono-atomic hydrogen does not seem to be able to suffer gravitational collapse at all because of it not possessing any inherent superconductivity. I also have my eye on what we call quantum tunneling...there is more to this than we are aware of at this moment. I suspect there is a duality in quantum tunneling that has not been considered.

This researcher, Dan Hooper, works at Fermilab I think (according to the back page) and he says that he has a position at Oxford in which research into dark matter/energy is one of his endeavors. He described a professor Silk at Oxford who pointed out to results from the Integer satellite data showing gamma ray equivalence to electron-positron emissions coming from the center of the Milky Way. This is supposed because the energy level of this emission is at 511,00eV...electron mass equivalence. However, he points out, these emissions come from a spherical halo from the center, and the production of the positrons seem to be able only to come from unstable stars and the emission pattern because of this should be in a "pancake" pattern and not spherical. A researcher at Oxford is investigating currently whether this can be due to dark matter annilihation, and of course he is joining her in this effort. Her, and now his, efforts are focusing on supersymmetric small particles that may fit the bill of producing these gamma rays.

Joep229:

so if I am understanding correctly, given the mass of what is visible plus the mass of the black hole, no matter how massive the black hole is, and how far it's reach, there is no way that the galaxy can be empty space outside of the visible portion. There has to be invisible mass of some kind out beyond the visible portion to keep the whole visible disk orbiting like a solid object.

No, this is not the understanding for how dark matter works. Picture dark matter as an invisible, huge cloud that completely encompasses every galaxy. (They do have pictures which show the affects of dark matter in the Nov. 2009 Astronomy issue.)

If you are within the galaxy, such as we are, then you don’t notice it’s affects because you are completely surrounded by it. You are inside this cloud and so it’s gravitational affects are fairly even and you don’t sense it. But as you start to leave the galaxy, you not only have the galaxy’s own gravitational pull, but now you start to feel the gravity from this cloud as you get farther away from it’s center.

Because dark matter is so dispersed, it’s gravitational force becomes more and more noticeable only as you get further away from the center of a galaxy, hence the flattening out of the rotational curve.

 

kentmere:

molecular H2 can only be detected in relatively warm environments, ie reasonably close to stars. Wherever there is interstellar dust, (and galaxies have lots of it), H2 is formed and there must be massive quantities throughout the universe which we are unaware of. Using a quasar located about 12 billion light years away as a beacon, molecular hydrogen has recently been detected by the ESO’s VLT at the very edge of the known universe, corresponding to the time when the universe was about a tenth of its present age, about 1.5 billion years old. All this evidence can be found in respected astronomy journals. Comments please?

Mond is a mathematical tool that mimics the rotational flattening out of a galaxy, but it doesn’t explain what the cause is, or anything alse. It also fails in that it doesn’t explain why light is bent around galaxies and galaxy clusters. Gravity does. So Mond fails where dark matter explains what we see, and now we have pictures to prove dark matter exists. (Again in the Nov. 2009 Astronomy magazine.)

Molecular H2 should be detected in the very early universe. It was more plentiful back then, before galaxies and stars started to absorb most of it. If there were huge clouds of molecular H2 surrounding our galaxy and other galxies today, (these are considered warm environments) then we would have detected them. The hydrogen clouds though, would do us no good if they existed outside of our galaxies, because they would not have the same affects as what we are seeing. The extra mass has to come from within the system being studied.

Again, I am postulating in this forum, that there are probably different forms of dark matter. Here is why.

First we see that galaxy clusters and super clusters are bound together. Because these galaxy clusters rotate too quickly for baryonic matter alone to hold them together, and the bending of light belies that they are much more massive than we can account for with just the ordinary elements, dark matter is now accepted.

The question I have is: does this dark matter envelope complete clusters of galaxies? I’m not sure. (I haven’t received my January, 2010 issue yet.) If the light traveling past the cluster is bent by the entire cluster, then the whole cluster is enveloped. If the light is bent gravitationally by the individual galaxies within the cluster, then it’s just the individual galaxies that have dark matter surrounding them. Either way, that should be easily determined by examining how the light is warped by galaxy clusters. I am wagering that the entire cluster bends light in a uniform enough fashion to indicate a form of dark matter that envelopes the entire cluster of galaxies.

But we also know for sure that individual galaxies are too massive in of themselves. They rotate to quickly to hold themselves together. We also have the pictures which shows the gravitational affects of dark matter at the scales of individual galaxy sizes. So there is a form of dark matter for sure, that can accumulate into galaxy size clouds.

Now this has been brought up before with the voyager and pioneer probes slowing down as they leave the solar system. Our solar system is not nearly as uniform on the same scale as a galaxy is. We’ve got small planets in the inner part, big giant ones in the middle and slighter smaller as you get farther out. A real small one if you count Pluto. But the point I was inferring is that we really don’t have an accurate way to ascertain the outer planets mass‘, with enough certainty to see if perhaps there is a form of dark matter that accumulates at solar system size levels. If we could know, to the ton, what each planets mass was(for sure), then we could see if dark matter inhabits individual solar systems. We don’t, but we do have another way to see if dark matter exists. We have space probes that are leaving our solar system. I think that the new horizon’s space craft which is headed to Pluto with very few course changes, has been quietly designed to try and answer the question of when does this slowdown begin to occur. I don’t know if anybody has attributed this slowdown to dark matter from within our own solar system, but I think it’s a valid concept. There maybe much more to learn about dark matter than we’ve ever imagined. I think there are different forms of this non-baryonic material.

Dusty Matter wrote:  If there were huge clouds of molecular H2 surrounding our galaxy and other galxies today, (these are considered warm environments) then we would have detected them.

Kentmere replies:

Two Dutch astronomers (E. A. Valentijn and P. P. van der Werf, Astrophysical Journal, 1999, vol 522, L29-35), using the Infrared space observatory (ISO), were able to detect molecular hydrogen in the spiral galaxy NGC 891, observing some very weak infrared signals at 17 and 28 microns, arising from the rotational motion of the molecule. The results revealed the existence of large amounts of H₂ in the dusty centre of the galaxy. It was estimated that there is enough molecular hydrogen, some 5 -15 times as much as atomic hydrogen, to account for the missing mass of this galaxy! Similar studies have since been made using data from ISO for other galaxies and much of the work on molecular hydrogen has been summarised recently by Emilie Habart and colleagues from Italy (INAF) and the Institut d’Astrophysique Spatiale, Université Paris-Sud, in ‘Space Science Reviews’ (2005, vol.119, 71-91).  If this applies to all galaxies, then surely the missing mass argument starts to weaken.

Huge clouds of molecular hydrogen associated with galaxies will also contribute towards lensing phenomena. The point about molecular H2 is that is the prime example of cold, Baryonic dark matter ie real stuff that is difficult to detect. As to whether people have detected non-Baryonic dark matter is an interesting point. Observations of strange things, eg the Bullet galaxy, have certainly been made, but such claims for the detection of non-Baryonic dark matter are as yet speculative. Physicists have been sitting at the bottom of a deep mine in northern England for several years now, spending millions on equipment for detection of WIMPs, but so far without result! Not all cosmologists believe in the non-Baryonic dark matter idea. One prominent UK astronomer has likened it unto 'the emperor's new clothes' and  'fairies at the bottom of the garden'.

kentmere:

It was estimated that there is enough molecular hydrogen, some 5 -15 times as much as atomic hydrogen, to account for the missing mass of this galaxy!

People can make all kinds of estimates, but how accurate are they? If it was true, then our own galaxy should comprise about 80% of it’s own mass to molecular H2. We don’t see this however.

kentmere:

...such claims for the detection of non-Baryonic dark matter are as yet speculative.

Not true. I take it that you subscribe to Astronomy magazine, which had a main article on dark matter, in the Nov. 2009 issue. While it hasn’t been detected by earthbound experiments, they have detected it from a astronomical point of view. They have pictures of it’s gravitational lensing affects, from when two galaxies collided. Far from being invisible clothes it is real. Do you think we should all just ignore this evidence, and believe the estimates of molecular H2 in other galaxies, while our own galaxy does not indicate this?

It is the estimates of the amounts of molecular H2 that is speculative.