Suspended
What is the intent of your comment? We all know that it is possible to search for papers among the tens of thousands published every year. Is this just an ad hominem attack implying that I'm lazy? Or perhaps you intend to imply that dark matter distributions are so well known from globular cluster motions that you need not bother to offer specific reference?Originally Posted by StupendousMan
If you know something or if you can cite some references, why not offer that instead of chiding me?
Administrator
TooMany,
Please take it down a notch. Do not read more into StupendousMan's comments than are explicitedly stated. Do not assume it is an attack if he is suggesting you search the literature.
And, in the future, if you think someone is being rude to you, the correct course of action is to Report their post, not respond in kind.
Established Member
Radially Extended Kinematics in the S0 Galaxy NGC 2768 from Planetary Nebulae, Globular Clusters and Starlight would be of interest to readers who want to know how one can determine the mass distribution in the outer reaches of a galaxy.
Suspended
Did you see any conclusion about mass distribution in that paper aside from an estimate that the bulge contains 70% of the mass? Dark matter is not even mentioned in the paper. I wonder why?
Order of Kilopi
Because they are determining mass distribution, by using radial velocities? After all, according to current mainstream theory, it doesn’t matter whether the mass is dark or regular matter, so why mention what isn’t needed?
Suspended
The paper is not very transparent, however this contradicts your assumption that DM is implicitly included:
Those two components are the bulge and the disk which are visible. They do not include a DM halo as an element in the kinematic analysis.Further support for the distinct nature of the two galaxy components come from our kinematic analysis.
I'd still be interested if StupendousMan can provide some references that demonstrate that the mass distribution of disk galaxy halos have been determined by the motion of their Globular Clusters.
Established Member
Spirals are tough to use globular clusters to separate halo shapes for, because their cluster systems are so sparse compared to ellipticals. There are some pretty impressive results for ellipticals which look at the total mass profile, comparing with various kinds of modelled or predicted dark-matter halos. Some recent examples are
NGC 1399 (468 cluster velocities)
NGC 4636 (289 clusters)
NGC 5128 (Centaurus A; 563 clusters)
NGC 4472 (263 clusters).
M87 has a very rich globular-cluster system , but there is some ambiguity between the potential wells of the galaxy and the surrounding cluster.
For spirals, the best samples I can find outside the Milky Way and M31 are for M104 (34 clusters), later extended to 104, which give a halo mass estimate but not much beyond that.
Order of Kilopi
LOL, the two components have dark matter. That's a given, since this is a mainstream paper. And, again, radial velocities depend only on mass and distribution, whether that mass is generate by normal or dark matter, isn't an issue. It's not a requirement that it's mentioned in every paper or in a way that specifically states "dark matter". These are papers for and by professionals. You needed to learn the vernacular if you want to understand.
They don't? Then you can explain how they separate the velocities due to the dark matter from the velocities from regular matter, out of each of the spectra. That is, after all, what a kinematical analysis is, the study of the spectra to determine the velocities of the objects . Your explanation will be interesting.
And, as for your contention that they don't mention dark matter, there is this:
So, let's see. 50% of the stellar mass, but only 10% of the total mass produces half the total light within one Re(oh, and the half the total light is the definition of Re). Which means everything outside one Re has 90% of the total mass, the other half of stellar mass and the other half of the total light. And that 50% of light outside one Re, which is produced by 10% of the matter, fades off into darkness. That's leaves ~80% of the matter unseen outside of one Re. See, they do mention dark matter, it's just not conveniently labeled as such.
Suspended
Your arrogant assessment of what I know or don't know reminds me of a know-it-all teenager.
Cut the personal attacks and stick to the facts. It's a mantra. Repeat seven times to memorize.
You don't see the point. They can only directly measure mass distributions and velocities of the luminous matter. They do not know the distribution and especially not the angular momentum of the dark matter, so it seems a bit silly to draw conclusions about all of the mass. Anyhow the paper conveys little or nothing relevant to this thread. We all agree that if the current theory of gravity is correct, there is lots of matter around that has not been directly detected. The issues are "what the heck is it" and how is it distributed?
Suspended
Thanks for the pertinent studies. Just from reading the abstracts, I don't see any claims to have measured dark matter distribution. A couple state that the dynamics are consistent with an NFW distribution.
This one NGC 4472 (263 clusters) states:
Presumably this is the same as the visual ellipsoid. Does this imply that the dark matter distribution could be consistent with the distribution of the luminous matter?The GC system as a whole is shown to be consistent with an almost perfectly isotropic velocity ellipsoid.
Established Member
I would say that it is little rather than nothing. As StupendousMan posted
It is about the first step in finding out that there is matter in a galaxy other than visible matter (dark matter).
There is "lots" of matter that has not been detected yet. For example, the WMAP data shows that ~4% of the universe is normal matter. But astronomers have not found all of that 4% (I believe the % detected is about 2%).
The distribution of dark matter has been quite well mapped using tools in various places such as gravitational lensing.
Established Member
The velocity ellipsoid is the distribution of (vector) velocities, in principle at each point in the system. We can directly measure only one component of the velocities (radial, along the line of sight), so one has to infer the other components from the angular and radial distributions of velocities. sometimes assisted by stablity considerations. For example, if the clusters are typically on very elongated orbits, the ones seen near the core of the galaxy will include some which are moving very fast along the line of sight, while those seen far from the core will be seen at apogalacticon and move very slowly. If the orbits are close to circular, the velocity/radius relation will be much more constant. This degeneracy is one of the things best broken by observing Milky Way clusters (unlike most other systematics of galaxy structure).
So the NGC 4472 study concludes that their measures are well fit if the clusters are equally likely to be moving in any direction at each point within the galaxy and its halo.
Order of Kilopi
I recently learned that there is a greater proportion of DM for dwarf galaxies, apparently because dwarfs lose stars due to immigration to larger galaxies. This greater amount of DM for the dwarfs would be more evidence against a planetary-like body hypothesis for DM, right?
Administrator
It's not so much that they lose stars, but rather that they lose the gas that would later become stars. Rogue planets, primordial black holes, brown dwarfs, are not excluded by this. I think the 130 GeV signal is pretty likely to make WIMPs the one true candidate very soon, and the Macho and Pulsar Timing Array studies will pretty much rule out massive objects as dark matter... but that is all in the future.
Forming opinions as we speak
Order of Kilopi
Yes, gas loss would make more sense, though the article (general readership) I read only mentioned stars. Thanks for the clarification.
Looks like they are closing in on the nature of DM, maybe. The lack of gamma rays in spheroidal dwarf galaxies strongly suggests that the WIMPS are not big WIMPS, as much as I like oxymorons. Instead, they must be the smaller class of WIMPS... wimpinos. *wink*
Administrator
I'd say the word "strongly" is misplaced above. I think it merely shows the cross-section for the interaction is smaller than hoped for by observationalists.
Forming opinions as we speak
Order of Kilopi
Are you suggesting I should have been wimpier in my assesment? [ok, that was worse than usual] I take that the smaller cross-section claim, due to the lack of the gamma ray emissions, reduces the WIMP sizes significantly. Admittedly, I know next to nothing of WIMPS, so I am really giving you more of a average Joe take on what little I'm reading. Perhaps, however, most WIMP size estimates are in the smaller size range. Is this what you are saying?
Administrator
Cross-section is a measure of how likely they are to interact with each other if they are passing each other. It isn't really physical size or mass of the particles that count, just reactiveness.
Forming opinions as we speak
Order of Kilopi
I was wondering what George could possibly mean by
the "size" of WIMPs. "Cross section" is undoubtedly it.
-- Jeff, in Minneapolis
http://www.FreeMars.org/jeff/
"I find astronomy very interesting, but I wouldn't if I thought we
were just going to sit here and look." -- "Van Rijn"
"The other planets? Well, they just happen to be there, but the
point of rockets is to explore them!" -- Kai Yeves
Order of Kilopi
Yep. Your explanation of the cross section is very important, but it is messing up my punmanship.
Suspended
I see. Wow, apogalacticon is a interesting word. It's sounds like something made up for Star Trek.
Does that mean orbits are virialized? Or is it more correct to say randomly virialized in this case? Is the term velocity ellipsoid then unrelated to the shape of the galaxy which is also an ellipsoid? Does that term rather refer to the elliptical shape of bound-object trajectories?
Suspended
Doesn't the finding that dwarf galaxies with high dark matter content do not emit gamma rays contradict the idea that "the 130 GeV signal is pretty likely to make WIMPs the one true candidate".
How would the Pulsar Timing Array studies rule out massive objects as DM?
Administrator
First, no one says that the Dwarf galaxies don't emit the 130 GeV gammas, only that if they do, it is not an intense enough number of them for us to unambiguously detect so far. The concentration of DM in the densest part of the Dwarf Galaxies is still unknown, but only the most hopeful of studies thought we'd be seeing gammas from them with Fermi-LAT.
As to your second question, there was a recent paper mentioned in Fun Papers which explained how the Pulsar Timing Array could detect PBHs of various sizes, and basically it boils down to needing some PBHs to be passing either near us, or near a pulsar, and the resulting change in the nearby gravitational field would be measurable.
Forming opinions as we speak
Established Member
A paper on astro-ph this morning provides a new approach to estimating the dark matter content in our galaxy.
"Constraints on the Shape of the Milky Way Dark Matter Halo from Jeans Equations Applied to SDSS Data"
http://arxiv.org/abs/1209.2708
Established Member
So, how unambiguously is the observable visible light accounted for?
Assuming dark matter is not strongly clumped... you´d expect diffuse light concentrated where dark matter is concentrated.
In these regions we also see light from point sources which goes astray in our optics, light emitted by ordinary gas atoms, light reflected by dust, light Rayleigh scattered by ordinary gas...
If the diffuse light of sky had modest but nonnegligible contribution from wide spectrum scattering of photons by dark matter - what are the observational upper bounds for such phenomena?
Administrator
This was a nice paper... It gave a some idea of the shape of the halo, but points out that in five or ten years when we have Gaia and LSST data, we'll be able to use this method to much more avail, and have a solid sense of the DM distribution in our galaxy.
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