A composite image shows a dark matter disk in red.  From images in the Two Micron All Sky Survey.  Credit: Credit: J. Read & O. Agertz.

Missing Milky Way Dark Matter

8 Nov , 2010 by

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Although dark matter is inherently difficult to observe, an understanding of its properties (even if not its nature) allows astronomers to predict where its effects should be felt. The current understanding is that dark matter helped form the first galaxies by providing gravitational scaffolding in the early universe. These galaxies were small and collapsed to form the larger galaxies we see today. As galaxies grew large enough to shred incoming satellites and their dark matter, much of the dark matter should have been deposited in a flat structure in spiral galaxies which would allow such galaxies to form dark components similar to the disk and halo. However, a new study aimed at detecting the Milky Way’s dark disk have come up empty.


The study concentrated on detecting the dark matter by studying the luminous matter embedded in it in much the same way dark matter was originally discovered. By studying the kinematics of the matter, it would allow astronomers to determine the overall mass present that would dictate the movement. That observed mass could then be compared to the amount of mass predicted of both baryonic matter as well as the dark matter component.

The team, led by C. Moni Bidin used ~300 red giant stars in the Milky Way’s thick disk to map the mass distribution of the region. To eliminate any contamination from the thin disc component, the team limited their selections to stars over 2 kiloparsecs from the galactic midplane and velocities characteristic of such stars to avoid contamination from halo stars. Once stars were selected, the team analyzed the overall velocity of the stars as a function of distance from the galactic center which would give an understanding of the mass interior to their orbits.

Using estimations on the mass from the visible stars and the interstellar medium, the team compared this visible mass to the solution for mass from the observations of the kinematics to search for a discrepancy indicative of dark matter. When the comparison was made, the team discovered that, “[t]he agreement between the visible mass and our dynamical solution is striking, and there is no need to invoke any dark component.”

While this finding doesn’t rule out the presence of dark matter, it does place constraints on it distribution and, if confirmed in other galaxies, may challenge the understanding of how dark matter serves to form galaxies. If dark matter is still present, this study has demonstrated that it is more diffuse than previously recognized or perhaps the disc component is flatter than previously expected and limited to the thin disc. Further observations and modeling will undoubtedly be necessary.

Yet while the research may show a lack of our understanding of dark matter, the team also notes that it is even more devastating for dark matter’s largest rival. While dark matter may yet hide within the error bars in this study, the findings directly contradict the predictions of Modified Newtonian Dynamics (MOND). This hypothesis predicts the apparent gain of mass due to a scaling effect on gravity itself and would have required that the supposed mass at the scales observed be 60% higher than indicated by this study.

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Navneeth
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Navneeth
November 8, 2010 8:57 AM

Am I the only one not able to see the image?

Daniel Rey M.
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Daniel Rey M.
November 8, 2010 11:05 AM

There’s a typographical error on the very last phrase: “by 60% higher” should be “be 60% higher”.

Torbjorn Larsson OM
Member
Torbjorn Larsson OM
November 8, 2010 12:26 PM

this study has demonstrated that it is more diffuse than previously recognized or

Stars above! That is the 4th putatively related observation/prediction/article within the last week or so touching on sterile neutrinos:

1) MiniBoone may have seen sterile neutrinos.

2) Sterile neutrinos may predict matter/antimatter symmetry by CP violation (see link)

3) Sterile neutrinos are suggested to be “warm” (neither cold nor hot) dark matter candidates and predict dwarf galaxy diffusive DM.

4) … and now the Milky Way diffusive DM disk?

I hear it also predicts how socks go missing in wash machines.

(No, really, I’m cautiously optimistic, if one hypothesis is that predictive.)

Question
Member
Question
November 8, 2010 6:28 PM

dark matter theory: RIP?

Lawrence B. Crowell
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Lawrence B. Crowell
November 8, 2010 7:10 PM

It is MOND that goes RIP — good riddance as well.

LC

TLVL
Member
TLVL
November 8, 2010 7:34 PM

Doesn’t anyone consider STVG, or is that lumped together with MOND?

Torbjorn Larsson OM
Member
Torbjorn Larsson OM
November 8, 2010 11:16 PM
It’s more of a case of theories about its fine structure being flawed, And I believe we knew that, since (C)DM comes up seriously flawed precisely for galaxies in models, a physically realistic one is AFAIU not seen as of yet. They break down in the galaxy core, if nothing else. [Disclaimer: this is how I read one review on this, so it’s really armchair ‘understanding’ at best.] @ TLVL: Doesn’t anyone consider STVG, or is that lumped together with MOND? In this case I believe it is, it is effectively a realization of a hypothetical model-less MOND. Here is Starts With A Bang take: “In summary, MOND explains galactic rotation curves better than dark matter does, and… Read more »
Navneeth
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Navneeth
November 8, 2010 11:19 PM

Jon, sorry about that. It turned out to be an issue with AdBlock+.

Torbjorn Larsson OM
Member
Torbjorn Larsson OM
November 8, 2010 11:21 PM

Also, whether or not MOND goes RIP from pursuing this particular area of research, I note that at this stage it is ironically back to being the ad hoc hypothesis for galaxy rotation curves it started out as. (Even if there now are more basic models like STVG that could make the same predictions.)

That can’t be good.

lars
Member
lars
November 9, 2010 12:07 AM

While you’re throwing out MOND, you may as well sh*t-can Dark Matter and Dark Energy, as they are both completely moronic concepts and totally erroneous postulations.

Dilip G Banhatti
Member
November 9, 2010 3:10 AM
There has been ample solid evidence from many independent calculations for some years now that disk galaxy rotation curves are fully consistent with Newtonian gravity and dynamics without any need for dark matter or dark energy. I will be glad to provide details to anyone interested. Please see these links: [http://arxiv.org/ftp/arxiv/papers/0805/0805.4163.pdf] [http://arxiv.org/ftp/arxiv/papers/0806/0806.1131.pdf] The following review may also be useful: [http://www.ias.ac.in/currsci/apr252008/986.pdf] [http://arxiv.org/ftp/astro-ph/papers/0703/0703430.pdf] although the reviewer became aware of much sensible work when the review was already almost at proof stage, and could not include it in the review or modify the review at places to take account of it. To be very frank & forthright about this issue, people have sort of jumped on the dark matter bandwagon w/o… Read more »
Lawrence B. Crowell
Member
Lawrence B. Crowell
November 9, 2010 5:33 AM
Lars, The evidence for dark matter is huge, and same goes for dark energy. The excess mass is required to explain the velocity function with respect to galactic radius v(r) ~ constant, which is distinct from the Kepler law v(r) ~ 1/sqrt{r}. So there is plenty of evidence there must be some sort of physics at work here. Either there is some form of mass energy which accounts for this. Motion of a body inside a uniform distribution of matter has v(r) = const, which lead to the idea of DM halos. With respect to Dilip’s papers, the main question comes with whether there is sufficient luminous matter in the disk of a galaxy to get the result… Read more »
tsturm
Member
tsturm
November 9, 2010 8:05 AM

Dark Matter: Pictures or it didn’t happen.

The longer there is no observational evidence of its existence, the more we should look into what we got wrong about our basic understanding of gravity.

Lawrence B. Crowell
Member
Lawrence B. Crowell
November 9, 2010 9:19 AM

There is plenty of evidence. Check out the Bullet Galaxy cluster, where DM is deduced by Einstein lensing.

LC

Jean Tate
Member
November 9, 2010 12:32 PM

Several recent comments were held up in the Pending queue, for longer than normal; sorry about that.

TLVL
Member
TLVL
November 9, 2010 2:57 PM

@TORBJORN LARSSON OM
I was under the impression that Abell 520 couldn’t be explained until DM models were modified to make the particles weakly interacting.

Anyway, I think I will sit on the fence until some conclusive evidence is found (or not found)

Lawrence B. Crowell
Member
Lawrence B. Crowell
November 10, 2010 5:41 AM
I illustrate some aspects of weakly interacting matter particles (WIMPS) as DM in the blog entry on the Fermi Telescope data: http://www.universetoday.com/78050/78050 The WIMP element is not terribly important with Abell 520, except for the fact that the DM in the galactic coalescence does not coalesce. The critical element there is that Einstein lensing, such as the Bullet or Abell 520 and other cases, measures the existence of a large amount of mass that is not visible in the EM spectrum. This is not luminous matter in the standard sense. The C in the CDM refers to cold, where an application of Boltzmann statistics for stable distributions of halos indicates the stuff is cold. If the particle motions… Read more »
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