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Milky Way Rotation Curve
Can anyone point me to up to date data or plots of the Milky Way's
rotation curve, or alternatively to information about either velocity data or data on the distribution of visible matter in the Milky way, from which I can plot the rotation curve? Regards -- Charles Francis substitute charles for NotI to email |
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Milky Way Rotation Curve
Can anyone point me to up to date data or plots of the Milky Way's
rotation curve, or alternatively to information about either velocity data or data on the distribution of visible matter in the Milky way, from which I can plot the rotation curve? You can find one sample rotation curve for our galaxy in my notes at http://spiff.rit.edu/classes/phys301...res/mw/mw.html That page also contains a link to a paper with rotation curves for a number of other spiral galaxies. And, of course, you can always go to the ADS http://adsabs.harvard.edu/abstract_service.html and type "Milky Way rotation curve" into the Abstract Words box, to get a large number of references. Michael Richmond |
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Milky Way Rotation Curve
Thus spake Stupendous_Man
Can anyone point me to up to date data or plots of the Milky Way's rotation curve, or alternatively to information about either velocity data or data on the distribution of visible matter in the Milky way, from which I can plot the rotation curve? You can find one sample rotation curve for our galaxy in my notes at http://spiff.rit.edu/classes/phys301...res/mw/mw.html Many thanks. That is useful. That page also contains a link to a paper with rotation curves for a number of other spiral galaxies. And, of course, you can always go to the ADS http://adsabs.harvard.edu/abstract_service.html and type "Milky Way rotation curve" into the Abstract Words box, to get a large number of references. Yes. 293 in fact. I have been trying this with arxiv, but this seems to be a rather better database. I have churned up some useful stuff thanks, including a couple more curves. In so far as I can tell from my searches so far, the mass distribution is not well constrained by observation. Is that right? This is rather important for a comparison between theory and observation. Regards -- Charles Francis substitute charles for NotI to email |
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Milky Way Rotation Curve
In article , Oh No
writes: In so far as I can tell from my searches so far, the mass distribution is not well constrained by observation. Is that right? This is rather important for a comparison between theory and observation. The whole concept of dark matter revolves (pun intended) around the concept that we can't detect it (i.e. "constrain the mass distribution by observation") directly, but only indirectly, through the effects it has on the motion of visible (in the broader sense of the term, i.e. including radio observations) objects. Of course, to determine the mass distribution at all, one has to specify some theory, be it conventional gravity, MOND, or something else. Or, perhaps, have some idea about the mass distribution from theory. In the foreseeable future, we won't be able to say "here's the matter, here are the observations, here's the theory, do they match?" but rather "here are observations of visible matter and here is the theory, do they match?". The answer is that they don't. Thus, one needs some non-visible matter (dark matter) or a modified theory (e.g. MOND). Personally, I'm rather agnostic on the matter. To some extent, MOND shaves close with Occam's razor, explaining a lot from a little. On the other hand, I don't think it would be particularly mysterious if most matter were dark. A rough analogy: almost everything in our day-to-day life is determined by the behaviour of electrons (chemistry, exclusion principle preventing everything from being in one place at one time, electricity, electronics etc), but we don't find it particularly puzzling that most of the mass is in the nucleus (even though it takes up much less space), not in the electrons. So, why should we be surprised if most of the mass of the universe is not in a form which interacts directly (i.e. electromagnetically) with our observational instruments? |
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Milky Way Rotation Curve
Phillip Helbig---remove CLOTHES to reply
wrote: [...] Personally, I'm rather agnostic on the matter. To some extent, MOND shaves close with Occam's razor, explaining a lot from a little. I'm not sure that's true. MOND typically involves an arbitrary function that extrapolates between the low acceleration and high acceleration regimes. The equation of motion is of the form F(a/a_0)a = -del Phi where F is a function such that F(x)~x for x1 and F(x)~1 for x1. It's interesting that one can find a function (or actually a collection of functions) for which this matches many observations; but freedom to choose an arbitrary function isn't exactly a small assumption. For a relativistic version like Bekenstein's TeVeS, it's even worse -- you need a scalar and a vector interaction on top of the metric, and an action that contains an arbitrary, and typically quite complicated, function of the scalar field. Bekenstein's original TeVeS action, for example, involves the function F(x) = 3/8 x^{-2}{x(4+2x-4x^2+x^3) + 2ln[(1-x)^2]} in the scalar fiels Lagrangian. I don't see why thr freedom to choose such a function in the action is an improvement over an assumption of dark matter. Steve Carlip |
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Milky Way Rotation Curve
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Milky Way Rotation Curve
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Milky Way Rotation Curve
In article , Oh No
writes: I have two problems with cold dark matter. I don't have any problem with the idea that dark matter exists in forms which we know about from elementary particle physics. For example the neutrino only needs a small mass to account for much more than observable baryonic mass. But we know that the neutrino is not responsible for the phenomena associated with CDM. My problem here is not simply that we have no theory in elementary particle physics for CDM, but that the constraints on particles which can exist are quite severe, and they do not even appear to allow a theory of CDM. Dark matter doesn't have to be "elementary particles" in the narrower sense of the term. (Of course, it would be composed of elementary particles in some sense, like everything else.) Some people have suggested primordial black holes. (Although formed from baryonic matter, they would have formed before baryogenesis, thus there is no conflict with primordial nucleosynthesis etc.) However, this idea seems inconsistent with microlensing observations. I think the dark matter could even be bricks (except for the problem that bricks---as we know them, Jim---are baryonic). Of course, this is not probable, there is no mechanism etc. The point is, there are lots of forms the dark matter could take which are not ruled out by current observations. |
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Milky Way Rotation Curve
[...] Personally, I'm rather agnostic on the matter. To some extent, MOND shaves close with Occam's razor, explaining a lot from a little. I'm not sure that's true. MOND typically involves an arbitrary function that extrapolates between the low acceleration and high acceleration regimes. For a relativistic version like Bekenstein's TeVeS, it's even worse That's correct, of course. However, on the purely astronomical side, it does explain a lot from a little. As you point out, the theory itself might be inelegant. On the other hand, that's true of dark matter as well: saying "there must be dark matter" is at the level of the phenomenological MOND, whereas it remains to be seen whether the theory which explains this dark matter is elegant. Probably you already know this , but MOND/TeVeS is NOT the only alternative theory of gravity which explains galactic rotation curves (without DM). If you see the recent conference on alternate theories of gravity (which I sent a link at sci.physics.relativity) you will note that both Moffat and Mannheim(and am sure others) have diff. gravitational potentials to fit galactic rotation curves and these can be derived from a much more elegant theory My guess is that if the Pioneer anomaly is confirmed , then it may point to new gravitational physics explanation for the dark matter problem. anyhow see astro-ph/0505266 for more details. [Mod. note: reformatted -- mjh] |
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Milky Way Rotation Curve
Thus spake Phillip Helbig---remove CLOTHES to reply
LOTHESvax.de In article , Oh No writes: I have two problems with cold dark matter. I don't have any problem with the idea that dark matter exists in forms which we know about from elementary particle physics. For example the neutrino only needs a small mass to account for much more than observable baryonic mass. But we know that the neutrino is not responsible for the phenomena associated with CDM. My problem here is not simply that we have no theory in elementary particle physics for CDM, but that the constraints on particles which can exist are quite severe, and they do not even appear to allow a theory of CDM. Dark matter doesn't have to be "elementary particles" in the narrower sense of the term. (Of course, it would be composed of elementary particles in some sense, like everything else.) Yes, but therein lies a problem, because the equations of sr and qm only appear to allow the sorts of matte which we know about. Some people have suggested primordial black holes. (Although formed from baryonic matter, they would have formed before baryogenesis, thus there is no conflict with primordial nucleosynthesis etc.) However, this idea seems inconsistent with microlensing observations. Again, I have no philosophical problem with primordial black holes, but as you say, observation is negative. I think the dark matter could even be bricks (except for the problem that bricks---as we know them, Jim---are baryonic). Of course, this is not probable, there is no mechanism etc. The point is, there are lots of forms the dark matter could take which are not ruled out by current observations. One form which does seem to be ruled out, however, is that lensing observations do not permit haloes with profiles required by galactic rotation curves. One finds an increasing number of papers commenting on this salient fact, and yet no answer to it from those advocating CDM. Regards -- Charles Francis substitute charles for NotI to email |
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