Dark Matter: Dearth Evaded

While I’m catching up on developments over the last week or so I thought I’d post an update on a story I blogged about a few weeks ago. This concerns the the topic of dark matter in the Solar Neighbourhood and in particular a paper on the arXiv by Moni Bidin et al. with the following abstract:

We measured the surface mass density of the Galactic disk at the solar position, up to 4 kpc from the plane, by means of the kinematics of ~400 thick disk stars. The results match the expectations for the visible mass only, and no dark matter is detected in the volume under analysis. The current models of dark matter halo are excluded with a significance higher than 5sigma, unless a highly prolate halo is assumed, very atypical in cold dark matter simulations. The resulting lack of dark matter at the solar position challenges the current models.

In my earlier post I remarked that this  study   makes a number of questionable assumptions about the shape of the Milky Way halo – they take it to be smooth and spherical – and the distribution of velocities within it is taken to have a very simple form.

Well, only last week a rebuttal paper by Bovy & Tremaine appeared on the arXiv. Here is its abstract:

An analysis of the kinematics of 412 stars at 1-4 kpc from the Galactic mid-plane by Moni Bidin et al. (2012) has claimed to derive a local density of dark matter that is an order of magnitude below standard expectations. We show that this result is incorrect and that it arises from the invalid assumption that the mean azimuthal velocity of the stellar tracers is independent of Galactocentric radius at all heights; the correct assumption—that is, the one supported by data—is that the circular speed is independent of radius in the mid-plane. We demonstrate that the assumption of constant mean azimuthal velocity is physically implausible by showing that it requires the circular velocity to drop more steeply than allowed by any plausible mass model, with or without dark matter, at large heights above the mid-plane. Using the correct approximation that the circular velocity curve is flat in the mid-plane, we find that the data imply a local dark-matter density of 0.008 +/- 0.002 Msun/pc^3= 0.3 +/- 0.1 Gev/cm^3, fully consistent with standard estimates of this quantity. This is the most robust direct measurement of the local dark-matter density to date.

So it seems reports of the dearth were greatly exaggerated..

Having read the paper I think this is a pretty solid refutation, and if you don’t want to take my word for it I’ll also add that Scott Tremaine is one of the undisputed world experts in the field of Galactic Dynamics. It will be interesting to see how Moni Bidin et al. respond.

This little episode raises the question that, if there was a problem with the assumed velocity distribution in the original paper (as many of us suspected), why wasn’t this spotted by the referee?

Of course to a scientist there’s nothing unusual about scientific results being subjected to independent scrutiny and analysis. That’s how science advances. There is a danger in all this, however, with regard to the public perception of science. The original claim – which will probably turn out to be wrong – was accompanied by a fanfare of publicity. The later analysis arrives at a much less spectacular conclusion,  so will probably attract much less attention. In the long run, though, it probably isn’t important if this is regarded as a disappointingly boring outcome. I hope what really matters for scientific progress is people doing things properly. Even if it  don’t make the headlines, good science will win out in the end. Maybe.

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13 Responses to “Dark Matter: Dearth Evaded”

  1. “I’ll also add that Scott Tremaine is one of the undisputed world experts in the field of Galactic Dynamics”

    Indeed; the standard work is Galactic Dynamics by Binney and Tremaine. Interestingly, Binney is one of the few old-school chaps interested in MOND (though I believe this interest arose after having written the book). His page http://www-thphys.physics.ox.ac.uk/people/jamesbinney/ has a link to a presentation on the case for MOND.

    • telescoper Says:

      James Binney is indeed interested in MOND, but more importantly I also believe he is interested in doing things properly…

  2. “The original claim – which will probably turn out to be wrong – was accompanied by a fanfare of publicity. The later analysis arrives at a much less spectacular conclusion, so will probably attract much less attention.”

    Apparently the press release mis-quoted Kroupa. When, at the discussion on this at Cosmic Variance, I commented that, if true, it should be corrected as it was presented as a direct quotation, a few comments later was one to the effect that the press release on the web page had been changed and indeed it was. See http://blogs.discovermagazine.com/cosmicvariance/2012/05/09/dark-matter-vs-modified-gravity-a-trialogue/ and there comments 47 and 60.

  3. [...] the claim closely. They find it wanting. This was pointed out here in a comment by Ben; Jester and Peter Coles also have useful blog posts up about [...]

  4. You might have a point about the public perception thing. I’m a layman, here by chance, and I can tell you that MOND gets more traction than any already rejected idea in any branch of science I’m aware of. I find that interesting, but maybe I shouldn’t.

  5. [...] the claim closely. They find it wanting. This was pointed out here in a comment by Ben; Jester and Peter Coles also have useful blog posts up about [...]

  6. Bryn Jones Says:

    I used to teach how the mass density of the Galactic disc is determined from the Jeans Equations using observations of stars towards the Galactic poles. I wish I’d tried my own reanalysis of the original Moni Bidin et al. result now.

  7. [...] The second paper is not yet accepted by a journal but the authors are very well-respected and my favourite astroblogger thinks it’s right. I’d probably bet on that at this stage. At the very least it seems the first result was not [...]

  8. [...] The second paper is not yet accepted by a journal but the authors are very well-respected and my favourite astroblogger thinks it’s right. I’d probably bet on that at this stage. At the very least it seems the first result was not [...]

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