Good current paper on mass / luminosity and rotation curves
in [Physics]
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From: Steve Willner on 14 Jul 2010 13:49 In article <0e1748ef-f14b-46e7-a552-82df5468b7cb(a)x1g2000prc.googlegroups.com>, dlzc <dlzc1(a)cox.net> writes: > http://arxiv.org/abs/1005.1085 This is what we were looking for. The authors find hot (20 MK) plasma out of the Galactic plane, but it's quite patchy. The maximum emission measure for any smooth component is something like 10^-3 cm^-6 pc. Any halo component is going to have an effective length of order a kpc, so the inferred density is sqrt(10^-6) cm^-3. That's about four orders of magnitude too small to be the dark matter. Or alternatively, if the density were four orders of magnitude higher, the surface brightness of the emission would be eight orders of magnitude larger. If there were enough ionized gas to make up more than a negligible fraction of the dark matter, it would be easily detectable by its X-ray emission. My speculation that a high enough temperature would make it undetectable was wrong. > http://arxiv.org/abs/0911.2192 Not sure of the relevance of this one. It seems to be concerned with the details of mixing of neutral and ionized gas. > http://arxiv.org/abs/0712.0476 There's hot gas expanding outward from the newborn stars in the Orion region. That's a nice result, but I don't see the relevance to the current discussion. > Hard to believe one could have gravitationally bound matter, that was > moving in excess of escape speed. Yes, if it's moving in excess of escape speed, it isn't bound. That was Eric's and my point. > I've already said they measured the temperature by the first > ionization, the first absorption band, of one of the minority > constituents. I'm not sure what measurement you're referring to, but usually the gas kinetic temperature is measured from the shape of the X-ray spectrum (using the exp(-E/kT) dependence). -- Help keep our newsgroup healthy; please don't feed the trolls. Steve Willner Phone 617-495-7123 swillner(a)cfa.harvard.edu Cambridge, MA 02138 USA
From: Yousuf Khan on 18 Jul 2010 02:58 On 7/11/2010 5:48 PM, nuny(a)bid.nes wrote: > On Jul 2, 10:41 pm, Yousuf Khan<bbb...(a)spammenot.yahoo.com> wrote: >> On 7/3/2010 6:49 AM, eric gisse wrote: >> >>> Turbulence typically refers to self-interactions within a fluid. I'm >>> pointing out the specific exception now rather than having to point out >>> later and then deal with 'but you said there's no turbulence!!!!' response. >> >>> Its' like people aren't listening when I say dark matter is modeled as a >>> perfect fluid. What the hell do folks think that implies? >> >> Then it's not a perfect fluid if there are self-interactions. You can >> say it's *close* enough to a perfect fluid at the galactic scale. At >> bigger scales, it is not. Second of all, you do realize that all of >> these "if...then...but if...else..." type behaviours of Dark Matter is >> exactly what Dark Fluid is supposed to address. >> >> Yousuf Khan > > Is Dark Fluid a superfluid, with quantized vortex lines and rings > around which galaxies and clusters form? Could such vortex lines map > to the observed really large scale foamy structure of the universe? > > > Mark L. Fergerson No, it's not a perfect fluid, otherwise there wouldn't be any self-interactions. It's the self-interactions that give the weird gravitational lensing features of the Train Wreck cluster, Abell 520. There isn't enough cosmological data yet to determine if Dark fluid can form vortexes and such, but it's a distinct possibility. Current Dark fluid theories are a simplified version of a model called the Generalized Chapylgin Gas model. But Chaplygin gas would be analysed with true fluid mechanics equations, and to do that you would need millions of high-quality data points taken throughout the universe. It's easy to do those kinds of measurements on fluids here on Earth, but to do the same thing on the whole universe is a bit difficult. So they replaced the fluid mechanical regimen with scalar, tensor, and vector simplifications. Yousuf Khan
From: eric gisse on 18 Jul 2010 03:39
Yousuf Khan wrote: > On 7/11/2010 5:48 PM, nuny(a)bid.nes wrote: >> On Jul 2, 10:41 pm, Yousuf Khan<bbb...(a)spammenot.yahoo.com> wrote: >>> On 7/3/2010 6:49 AM, eric gisse wrote: >>> >>>> Turbulence typically refers to self-interactions within a fluid. I'm >>>> pointing out the specific exception now rather than having to point out >>>> later and then deal with 'but you said there's no turbulence!!!!' >>>> response. >>> >>>> Its' like people aren't listening when I say dark matter is modeled as >>>> a perfect fluid. What the hell do folks think that implies? >>> >>> Then it's not a perfect fluid if there are self-interactions. You can >>> say it's *close* enough to a perfect fluid at the galactic scale. At >>> bigger scales, it is not. Second of all, you do realize that all of >>> these "if...then...but if...else..." type behaviours of Dark Matter is >>> exactly what Dark Fluid is supposed to address. >>> >>> Yousuf Khan >> >> Is Dark Fluid a superfluid, with quantized vortex lines and rings >> around which galaxies and clusters form? Could such vortex lines map >> to the observed really large scale foamy structure of the universe? >> >> >> Mark L. Fergerson > > No, it's not a perfect fluid, otherwise there wouldn't be any > self-interactions. It isn't a fluid at all. Look at the math, and stop guessing. > It's the self-interactions that give the weird > gravitational lensing features of the Train Wreck cluster, Abell 520. You are guessing. There are no published papers on this. > > There isn't enough cosmological data yet to determine if Dark fluid can > form vortexes and such, but it's a distinct possibility. There is nothing flowing that _CAN_ form 'vortexes'. Stop guessing. > Current Dark > fluid theories are a simplified version of a model called the > Generalized Chapylgin Gas model. Stop guessing. The dark fluid model is a generalization of the various tensor/vector/scalar/f(R) theories of gravitation. The papers on the subject *YOU HAVE CITED* do not say this. > But Chaplygin gas would be analysed > with true fluid mechanics equations, and to do that you would need > millions of high-quality data points taken throughout the universe. Millennium Simulation II - 2160^3 = dark matter particles. Its' been done. > It's > easy to do those kinds of measurements on fluids here on Earth, but to > do the same thing on the whole universe is a bit difficult. So they > replaced the fluid mechanical regimen with scalar, tensor, and vector > simplifications. > > Yousuf Khan |