It shows the limits on the cross section of dark matter annihilation into b-quark pairs derived from gamma-ray observations of satellite galaxies of the Milky Way. These so-called dwarf galaxies are the most dark matter dominated objects known, which makes them a convenient place to search for dark matter. For example, WIMP dark matter annihilating into charged standard model particles would lead to an extended gamma-ray emission that could be spotted by the Fermi space telescope. Such emission coming from dwarf galaxies would be a smoking-gun signature of dark matter annihilation, given the relatively low level of dirty astrophysical backgrounds there (unlike in the center of our galaxy). Fermi has been looking for such signals, and a year ago they already published limits on the cross-section of dark matter annihilation into different final states. At the time, they also found a ~2 sigma excess that was intriguing, especially in conjunction with the observed gamma-ray excess from the center of our galaxy. Now Fermi is coming back with an updated analysis using more data and better calibration. The excess is largely gone and, for the bb final state, the new limits (blue) are 5 times stronger than the previous ones (black). For the theoretically favored WIMP annihilation cross section (horizontal dashed line), dark matter particle annihilating into b-quarks is excluded if its mass is below ~100 GeV. The new limits are in tension with the dark matter interpretation of the galactic center excess (various colorful rings, depending who you like). Of course, astrophysics is not an exact science, and by exploring numerous uncertainties one can soften the tension. What is more certain is that a smoking-gun signature of dark matter annihilation in dwarf galaxies is unlikely to be delivered in the foreseeable future.
12 comments:
There have been other claims of astrophysical evidence for a particular dark matter candidate, e.g the claim from the 3.5keV line? From my naive non-astronomer perspective I would think that a dwarf galaxy dominated by dark matter would be an especially interesting place to look for that signal. Are such studies out there?
Also on the astrophysical front, the just published CIBER results appear to suggest that as many stars exist in intergalactic space as in the interior of galaxies. These intergalactic stars were not previously thought to exist in such vast numbers and are unobservable individually but can be inferred by collective EM emissions.
So: billions of MACHOs and hundreds of billions of unbound rogue planetary-mass objects, and now countless intergalactic stars.
Anybody ready to rethink the dark matter enigma?
Alex, for the 3.5 keV line you need a different instrument, but such a search was done, see http://arxiv.org/abs/1408.3531
No signal was found, which puts some tension on the other detection claims but does not rule them out conclusively.
Robert,
Finding stars in intergalactic space is important, but it won't help us understand galaxy rotation curves, which were the original motivation for hypothesizing the existence of dark matter.
Indeed, my recollection is that there have been some problems even with missing baryonic matter, and finding all of these stars between galaxies certainly helps with that. But it won't explain galaxy rotation curves, and hence won't resolve the dark matter question.
Alex,
Two of the three populations I mentioned are clearly galactic.
Neither were widely expected; there may be well be other galactic populations of dark objects that have yet to be found.
The CIBER populations are, in the words of the authors, "outside of galaxies (with boundaries as traditionally defined)", but that qualifier should not be ignored. Galactic halos may be much more extended than is conventionally assumed.
A little rethinking of the dark matter problem, in the absence of 30 years of bias, might be a good idea.
I'm not gonna give the secret away that WIMPs do not exist because they would not allow a Galaxy filled up with WIMP seekers. I just say: Go on, search and you will find.
It is unfortunate that so many of the physicists who look so hard for the dark matter have such a weak command of the astronomical data that indicate its existence. I've lost count of the number of physics colloquia I've sat in where rotation curves are invoked with me being the only person in the room to have published papers on them. There are good, widely known arguments that there should be WIMP dark matter. There is also considerable, if less well known evidence to contradict this interpretation (e.g., section 4 of http://relativity.livingreviews.org/Articles/lrr-2012-10/fulltext.html). All we really know is that current theory can't explain the data. If we find WIMPs experimentally, great - Nobel prizes all around. But if we don't, how do we know when we've been barking up the wrong tree?
Robert,
Dark matter is a reasoned hypothesis that answers many different questions:
(1) The shape of galaxy rotation curves (Vera Rubin)
(2) The rotation of galaxies around each other (Zwicky)
(3) The cause of the initial clumpiness in the universe, as seen in the CMB.
(4) Gravitational lensing of objects with no know massive objects between us and the object.
You need to understand the full picture in order to understand why most physicists are looking for dark matter. If we were to assign all of dark matter's mass/energy density to normal matter, then it would change the predictions for helium fraction and Deuterium fraction in the universe, such that the predictions and measurements would not line up (without perhaps making up a new term that is even less physical motivated than dark matter.)
Jester, what are (are where?) the constraints on WIMP masses from above? How high they could go while staying consistent with Standard model?
The upper limit on WIMP masses is around 10 TeV. Heavier dark matter is possible but it could not be produced thermally.
@Jester: I expect to see commentary relatively soon from those groups associated with the now ruled out detections. If my memory serves, some of those were made with pretty emphatic announcements of statistical significance.
Hi,
Looks like a tough day for the GeV excess. Could you write an entry explaining these latest results for some of us non-experts?
http://arxiv.org/abs/1411.7623
http://arxiv.org/abs/1411.7410
http://www.insu.cnrs.fr/node/5108
Merci,
Fiora
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