Astrophysical Journal – Kinematical and Chemical Vertical Structure of the Galactic Thick Disk. I. Thick Disk Kinematics
The variation of the kinematical properties of the Galactic thick disk with Galactic height Z is studied by means of 412 red giants observed in the direction of the south Galactic pole up to 4.5 kpc from the plane. We confirm the non-null mean radial motion toward the Galactic anticenter found by other authors, but we find that it changes sign at |Z| = 3 kpc, and the proposed inward motion of the local standard of rest alone cannot explain these observations. The rotational velocity decreases with |Z| by –30 km s–1 kpc–1, but the data are better represented by a power law with index 1.25, similar to that proposed from the analysis of Sloan Digital Sky Survey data. All the velocity dispersions increase with |Z|, but the vertical gradients are small. The dispersions grow proportionally, with no significant variation of the anisotropy. The ratio σU/σW = 2 suggests that the thick disk could have formed from a low-latitude merging event. The vertex deviation increases with Galactic height, reaching ~20° at |Z| = 3.5 kpc. The tilt angle also increases, and the orientation of the ellipsoid in the radial-vertical plane is constantly intermediate between the alignment with the cylindrical and the spherical coordinate systems. The tilt angle at |Z| = 2 kpc coincides with the expectations of MOdified Newtonian Dynamics, but an extension of the calculations to higher |Z| is required to perform a conclusive test. Finally, between 2.5 and 3.5 kpc we detect deviations from the linear trend of many kinematical quantities, suggesting that some kinematical substructure could be present.
If the ESO analysis is correct, it could just mean that dark matter behaves very differently — or is distributed very differently in space — than has been thought. “It would mean that dark matter would need to be distributed on a wider scale within the inner parts of a galaxy,” Clowe said, “which is [mathematically confirmed] if you make the dark matter particles less massive than the currently favored models.”
John Moffat, a physicist at the Perimeter Institute for Advanced Study in Canada, has proposed a sub-theory of MOND called MOG, or “modified gravity.” He claims MOG explains the peculiar motion of galaxies, as well as galaxy clusters and cluster collisions, without invoking dark matter at any scale.
“I take Einstein’s gravity and I add to this three fields,” Moffat explained. One of the fields has a mass, and this introduces variations in the force law at different distance scales. However, in order to have a mass, the field must have a particle associated with it, which Moffat calls the phion. And, like dark matter particles, the phion’s existence has not been verified.
People have not yet studied MOG so much as they have studied MOND, so we do not yet know. There are many interesting questions here —
1. What ARE the FTL implications if MOG happens to be true?
2. Are there problems with stability and well-definedness with MOG which may justify
some technical changes in the theory — or which introduce novel stochastic factors?
3. Does his phi field really have exactly the form given? Does it require modifications such as those I argue we need for the “Higgs field” in electroweak theory? Or does it also couple with neutrinos or such? (Phi itself could be considered
a KIND of dark matter, a very different kind from what reactive thinkers have assumed.)
And, of course, there are the questions of whether the new experimental result will hold up when new surveys of stars or galaxies are done. (Yet it is still possible that the dark matter hypothesis is consistent with data on one scale but not another; it would be a typical mental aberration for folks to assume that consistency on one scale “shows that the earlier result” (about another scale) “is wrong.” Also, as Moni-Bidin has noted, “we might find that the dark matter is there, but is simply shaped like a giant cigar in space… though I can’t quite imagine why that would be as yet.”
UNLIKE MOffat’s theory, these cigar alternatives are not so well motivated right
now… but we will see.
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Brian Wang is a Futurist Thought Leader and a popular Science blogger with 1 million readers per month. His blog Nextbigfuture.com is ranked #1 Science News Blog. It covers many disruptive technology and trends including Space, Robotics, Artificial Intelligence, Medicine, Anti-aging Biotechnology, and Nanotechnology.
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