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#51
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Beyond IDCS J1426.5+3508
On Wednesday, October 3, 2012 9:58:43 AM UTC+2, Phillip Helbig
wrote: Why do you expect the density to be as you described it? I expect that a galaxy cluster more or less is the same as a dwarf eliptical galaxy like: NGC 147 See: http://en.wikipedia.org/wiki/NGC147 That means the highest density is in the center and slowly decreases. Those distributions are the same I have used to study the Virial Theorem This description is identical for the bulge of our Milky Way. The following link is "interesting": http://en.wikipedia.org/wiki/Virial_...#Virial_radius They use the equation rho c = 3*H^2/8*pi*G to study the behaviour of a single galaxy cluster while at the same type that same equation is used to study the whole Universe. Nicolaas Vroom http://users.telenet.be/nicvroom |
#52
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Beyond IDCS J1426.5+3508
On 10/1/12 2:17 AM, Phillip Helbig---undress to reply wrote:
H^2 = \frac{8\pi G\rho}{3} Thus the critical density is, by definition, 3H^2/(8\pi G) since the value of the density is always the critical density in the Einstein-de Sitter universe. So H(z) is proportional to the square root of the density. (If lamnda and/or k are not zero, then the expression for H(z) is of course more complicated.) In a purely mathematical sense critical density rho_c is proportional to H^2/G and critical density rho_c is proportional to H^2 if G is a constant. but alternatively critical density rho_c is proportional to H if H/G is a constant. As previously posted by Jonathan Thornburg, lunar ranging in our local bound system indicates (d^2G/dt^2)/G = (4 ± 5) * 10^{-15}/year^2 but this does not negate a delta G in galactic cluster systems in near equilibrium with expanding critical density rho_c. http://arxiv.org/abs/1106.4052 states the traditional physics: critical density rho_c is proportional to H^2 with G constant but no attempt is made in using this relationship to dimensionally explain the Table C1 data represented by the slope 1.91 ~ 2 graphed in Figure C1 that can be dimensionally explained by: critical density rho_c is proportional to H with H/G constant. |
#53
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Beyond IDCS J1426.5+3508
On 10/2/12 3:16 PM, Nicolaas Vroom wrote:
On Thursday, September 20, 2012 8:30:10 AM UTC+2, Richard D. Saam wrote: A more appropriate study may be: Implicit Priors in Galaxy Cluster Mass and Scaling Relation Determinations http://arxiv.org/abs/1106.4052 Adam Mantz (NASA/GSFC), Steven W. Allen (KIPAC, Stanford/SLAC) Table C1 page 11 shows 4 lines with of galaxy cluster data with almost identical kT values. De values for z, E(z), M2500 and kT are shown: 16) 0.295 1.163 2.67 8.03 22) 0.352 1.201 4.02 8.05 23) 0.355 1.203 6.02 8.08 37) 0.686 1.462 3.07 8.08 The lines 22 and 23 are almost identical but the M2500 values are rather different. What is the explanation ? The explanation is not readily apparent. Also: The kT values in the the keV range represent the inter galactic gas temperature as related to gravitational energy. A graphical plot of kT vs z indicates no correlation Also A graphical plot of M2500 vs z indicates no correlation but A graphical plot of M2500 vs kT indicates the slope 1.9 correlation. and A graphical plot of E(z)*M2500 vs kT indicates the slope 1.9 correlation. What is the explanation ? We need a better idea of the galactic distribution with time to be provided in part by http://www.darkenergysurvey.org/ "an extremely sensitive 570-Megapixel digital camera, DECam, mounted on the Blanco 4-meter telescope at Cerro Tololo Inter-American Observatory high in the Chilean Andes" for detecting redshifted long-wavelength red and infrared light and coupled with X-ray gamma-ray surveys. |
#54
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Beyond IDCS J1426.5+3508
In article , Nicolaas Vroom
writes: On Wednesday, October 3, 2012 9:58:43 AM UTC+2, Phillip Helbig wrote: Why do you expect the density to be as you described it? I expect that a galaxy cluster more or less is the same as a dwarf eliptical galaxy like: NGC 147 Why? Also, for comparison one needs the mass, which is usually less precisely known than the light. That means the highest density is in the center and slowly decreases. OK, the NFW drops off more quickly. They use the equation rho c = 3*H^2/8*pi*G to study the behaviour of a single galaxy cluster while at the same type that same equation is used to study the whole Universe. The critical density, of course, is important for the entire universe. However, in order for something to collapse and form structure, then, at least to first order, it has to locally exceed the critical density, so it is not surprising to see this crop up in the context of smaller structures as well. |
#55
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Beyond IDCS J1426.5+3508
In article , "Richard D. Saam"
writes: Thus the critical density is, by definition, 3H^2/(8\pi G) since the value of the density is always the critical density in the Einstein-de Sitter universe. So H(z) is proportional to the square root of the density. (If lamnda and/or k are not zero, then the expression for H(z) is of course more complicated.) In a purely mathematical sense critical density rho_c is proportional to H^2/G and critical density rho_c is proportional to H^2 if G is a constant. but alternatively critical density rho_c is proportional to H if H/G is a constant. As previously posted by Jonathan Thornburg, lunar ranging in our local bound system indicates (d^2G/dt^2)/G = (4 ± 5) * 10^{-15}/year^2 but this does not negate a delta G in galactic cluster systems in near equilibrium with expanding critical density rho_c. There are also constraints on the variability of G over cosmological distances. Not as strict as the local constraints, but strict enough to rule out it being important for explaining something---at best, one could hope to marginally DETECT a variation. |
#56
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Beyond IDCS J1426.5+3508
On Friday, October 5, 2012 9:17:32 PM UTC+2, Phillip Helbig---undress to reply wrote:
The critical density, of course, is important for the entire universe. However, in order for something to collapse and form structure, then, at least to first order, it has to locally exceed the critical density, so it is not surprising to see this crop up in the context of smaller structures as well. The problem is that the density of the Universe is smaller than the density of a galaxy cluster which inturn is smaller than the density of a single large galaxy. For the critical density I expect the same relation. If that is true it means that you can not compare one with the other. This picture is even more complex if you assume that 80% of all the matter in the Universe is non-baryonic. Table 4 in http://iopscience.iop.org/0004-637X/...1038/fulltext/ shows the amount of gas in a galaxy cluster is roughly 10% (20%) of the total mass. Is my assumption understanding correct that the total mass of a galaxy cluster including all baryonic and non baryonic mass is a factor 5 higher than the total mass ? Nicolaas Vroom |
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