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#12
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NET Angular Momentum of Globular cluster of stars
Steve Willner wrote:
In article , writes: Is the NET angular momentum of a globular cluster of stars, zero? I think that's the case for most clusters, but I vaguely remember that there are a tiny number that show rotation. I might be mistaken on either part of this. As I recall dimly from either reading or hearing talks about this, Omega Cen appears slightly oblate, although I do not know what if any radial velocity observations confirm that this is due to rotation. And it may be the core of a small galaxy captured by the MWG long ago, rather than being a "classical" globular. Mike Dworetsky Is the NET angular momentum of an elliptical galaxy and or the central bulge of a spiral, zero or close to zero For elliptical galaxies, a quick web search turned up https://academic-oup-com/mnras/artic...6.2011.18496.x The authors claim 86% of early type galaxies are "fast rotators." I haven't studied the paper to find out what that means or what sample they defined. I expect there are many more works on this subject; as I say, it was a very quick search. There are also lots of theory papers simulating major mergers, from which elliptical galaxies are supposed to form. The simulation results must include a final angular momentum. -- Mike Dworetsky (Remove pants sp*mbl*ck to reply) |
#13
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NET Angular Momentum of Globular cluster of stars
In article ,
(Eric Flesch) writes: If dark matter resides throughout an elliptical galaxy or globular cluster (or the Galactic halo) then it could raise the ambient background gravitational level to where a resident star no longer feels the gravitational effect from its neighbours or from the system centre. Instead, the star would follow thre contours (potentials) of the dark matter structure. So stars would just mingle throughout without following orbits as such. What distribution of dark matter are you postulating? If the distribution is uniform, then the dark matter has no effect regardless of how much of it there is. If the distribution is spherically symmetric, then we have the familiar case where each object responds to the amount of matter interior to that object's position relative to the center of the dark matter distribution. If the distribution is more complicated, then so are the effects, but in any case, stars are affected in the usual way by other stars. I suppose if you have clumps of dark matter (say tens or hundreds of solar masses each) whizzing around randomly, the overall effects could look random, but it's hard to see how such dark matter clumps could form or be held together or why they should inhabit elliptical but not spiral galaxies. A nice calculator for galactic rotation curves is at http://burro.astr.cwru.edu/JavaLab/R...eWeb/main.html but it didn't work when I tried it just now. (I suspect some kind of Java compatibility problem.) -- Help keep our newsgroup healthy; please don't feed the trolls. Steve Willner Phone 617-495-7123 Cambridge, MA 02138 USA |
#14
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NET Angular Momentum of Globular cluster of stars
On 04/04/2017 05:29, Mike Dworetsky wrote:
Steve Willner wrote: In article , writes: Is the NET angular momentum of a globular cluster of stars, zero? I think that's the case for most clusters, but I vaguely remember that there are a tiny number that show rotation. I might be mistaken on either part of this. As I recall dimly from either reading or hearing talks about this, Omega Cen appears slightly oblate, although I do not know what if any radial velocity observations confirm that this is due to rotation. And it may be the core of a small galaxy captured by the MWG long ago, rather than being a "classical" globular. There was something in ApJ early stellar dynamics measurements mid 1990's showing that the luminosity oblateness varies with radius more or less spherical near the middle and far out but oblate in between. https://arxiv.org/pdf/astro-ph/9612184.pdf They credit someone else in 1983 as having first measured the isophotes in 1983 and claim mean 0.121 minimum 0 up to 2', maximum 0.25 at 10' and then becoming rounder as you go further out. (see p6) -- Regards, Martin Brown |
#15
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NET Angular Momentum of Globular cluster of stars
On Tue, 04 Apr 2017, Steve Willner wrote:
(Eric Flesch) writes: the system centre. Instead, the star would follow thre contours (potentials) of the dark matter structure. So stars would just mingle throughout without following orbits as such. What distribution of dark matter are you postulating? If the distribution is uniform, then the dark matter has no effect My point is that "dark matter" could be gravitationally opaque even as it gravitates. We know it has different qualities than baryonic matter, but we're not yet at the point where "we know what we don't know". So it could act to muffle the gravitational environs of ellipticals (etc) while responding to those environs in ways that creates contours from it. Gravitational permeability & permittivity, anyone? From outside the system its gravitational effect would be indistinguishable from baryonic matter. [[Mod. note -- The notion of "gravitationally opaque" doesn't exist in general relativity, so if you want this then you need to come up with a new theory of gravity. -- jt]] |
#16
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NET Angular Momentum of Globular cluster of stars
On Fri, 07 Apr 2017 11:52:34 EDT, Eric Flesch wrote:
My point is that "dark matter" could be gravitationally opaque even as it gravitates. We know it has different qualities than baryonic ... [[Mod. note -- The notion of "gravitationally opaque" doesn't exist in general relativity, so if you want this then you need to come up with a new theory of gravity. -- jt]] I'm pretty sure that general relativity is not a theory of gravity. [Moderator's note: I've kept this bit in because the rest of the post might prompt some interesting discussion. I don't think that this is the place to debate whether GR is a theory of gravity. -P.H.] I can however rephrase my point, Think of dark matter as a medium. Compare stars in a dark matter environment to fish in a lake. Fish don't fall because they are buoyant in the water. In a dark matter lake, stars would similarly have some gravitational buoyancy. The gravitational medium would modify or nullify the inverse square law. How could it not? And that's pretty much all of my point. |
#17
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NET Angular Momentum of Globular cluster of stars
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#18
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NET Angular Momentum of Globular cluster of stars
On Tue, 11 Apr 2017, Phillip Helbig wrote:
... (Eric Flesch) writes: gravitational medium would modify or nullify the inverse square law. How could it not? And that's pretty much all of my point. ... But the kind of dark matter we are talking about has little if any interaction with baryonic matter. Um, sorry Phil, but dark matter is supposed to account for (1) spiral galaxy rotational profiles, and (2) "great attractors". Given the existence of dark matter (for this argument), it definitely interacts with baryonic matter. And if it does so, it must have a major effect on the dynamics of galactic haloes, elliptical galaxies and globular clusters. Maybe fish-in-water isn't the right analogy, but the presence of a gravitational scalar diminishes (or nullifies) the gravitational influence of near neighbours. So the boundary of our solar system may simply be where our Sun's gravitational influence is supplanted by that of the "dark matter" medium, with neighbouring stars not in the equation. That's the idea, anyway. I also don't think that it would work quantitatively. Can you show that this idea (even neglecting the point I mentioned above) results in the very simple MOND law? I'm not up with MOND. Is it simple? Simple is good but not sufficient. I expect that MOND is a "top down" system designed to best account for observed behaviour, as opposed to a "bottom up" system which builds on known physical law. A top-down system must, of human necessity, be simple, else nobody will pay attention. thanks, Eric [[Mod. note -- 1. I think Phillip Helbig was referring to dark matter having little if any *non-gravitational* interaction with baryonic matter. 2. There's an extensive history of people studying what the consequences of a gravitational scalar field might be. See, for example, section 5.3 of Clifford M Will, "Theory and Experiment in Gravitational Physics" (Cambridge University Press, 1981, 1985), or section 3.3.2 of Will's review paper "The Confrontation between General Relativity and Experiment" (open-access at http://www.livingreviews.org/lrr-2014-4 ). To summarize, compatability with various solar-system experiments constrains the free parameters in these theories so as to give results almost identical to general relativity. -- jt]] |
#19
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NET Angular Momentum of Globular cluster of stars
In article , Eric Flesch
writes: On Tue, 11 Apr 2017, Phillip Helbig wrote: ... (Eric Flesch) writes: gravitational medium would modify or nullify the inverse square law. How could it not? And that's pretty much all of my point. ... But the kind of dark matter we are talking about has little if any interaction with baryonic matter. Um, sorry Phil, but dark matter is supposed to account for (1) spiral galaxy rotational profiles, and (2) "great attractors". Given the existence of dark matter (for this argument), it definitely interacts with baryonic matter. Sorry: it has little if any interaction with baryonic matter OTHER THAN GRAVITATIONAL INTERACTION. Thus, the buoyancy analogy doesn't work. I'm not up with MOND. Is it simple? Phenomenologically? Yes. 1. I think Phillip Helbig was referring to dark matter having little if any *non-gravitational* interaction with baryonic matter. Indeed. |
#20
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NET Angular Momentum of Globular cluster of stars
On Sat, 15 Apr 2017 21:29:20 EDT, Eric Flesch wrote:
[[Mod. note -- 2. There's an extensive history of people studying what the consequences of a gravitational scalar field might be. See, for example, section 5.3 of Clifford M Will, "Theory and Experiment in Gravitational Physics" (Cambridge University Press, 1981, 1985), or section 3.3.2 of Will's review paper "The Confrontation between General Relativity and Experiment" (open-access at http://www.livingreviews.org/lrr-2014-4 ). Thank you for this outstanding reference which I'll be reading for days. I wasn't aware of Clifford Will's work. [[Mod. note -- Will is also the author of the excellent popular book "Was Einstein Right?" (Basic Books, paperback 2nd Edition 1993) on experimental tests of general relativity. -- jt]] |
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