For the first Astronomy Twitter Journal Club meeting we discussed a paper by Dan Hooper and Tim Linden – Gamma Rays From The Galactic Center and the WMAP Haze.
The paper suggests that the gamma ray excess found in the Fermi Gamma-ray Space Telescope data, and the WMAP Haze, can be explained if we assume dark matter particles of mass range 7.3 to 9.2 GeV are annihilating democratically into leptons (i.e. all three leptons are produced). The leptons are then interacting with the Galactic magnetic field and producing synchrotron radiation. The democratic annihilation allows for a smaller, more realistic, Galactic magnetic field in the WMAP Haze region.
Supersymmetry predicts that four neutralinos exist and that each of them is identical to its antiparticle – hence they would self-annihilate. The lightest neutralino is an excellent candidate for dark matter.
The paper describes how the authors modelled such a scenario and compared their results to the WMAP data. The authors conclude:
“We find that a sizable flux of hard synchrotron emission is predicted in this scenario, and that this can easily account for the observed intensity, spectrum, and morphology of the WMAP Haze”.
The paper also notes that the particle mass range of 7.3 to 9.2 GeV is significant:
“This mass range is very similar to that required to accommodate the signals reported by CoGeNT and DAMA/LIBRA”.
The DAMA/LIBRA signals refer to a phenomenon reported by an experiment in Italy. The researches state that their dark matter detection experiment shows an annual detection spike around May – and this is what you would expect if the Earth was annually plunging into a cloud of dark matter. The researchers say that they have been seeing this spike for 13 years. The results have remained controversial but now CoGeNT may also have detected a spike that occurs around May.
For more information, and to watch the great “Dark Matters” PHD Comics video, please see the pre-meeting introductory post.
The discussion started with some criticisms of the paper:
“Can I start with one: line 3 of abstract: ‘If interpreted as products of annihilating dark matter’. If? Occam’s razor?” – @Matt_Burleigh
“I think it’s saying ‘Dark matter is probably weird. We’ve seen some weird stuff. Therefore it might be dark matter’” – @jaclong
Excess Gamma Radiation and the WMAP Haze
Independent lines of evidence pointing towards one conclusion makes theories convincing. And as @partialobs put it, one of the main “hooks” of the paper is the excess gamma radiation found in the Fermi data. However, as the meeting progressed we discovered that this excess was found by the authors in public Fermi data and is not necessarily supported by the Fermi team:
“Fermi team does not believe claimed Fermi result (anomalous spectrum of gamma-rays toward Gal Ctr).” – @partialobs
“Hooper & Goodenough are not Fermi team members. Data public, analysis their own, Fermi team disputes.” – @partialobs
And, if there is a gamma excess, shouldn’t we see it in the Andromeda Galaxy (M31)?
“Not sure there is much g from M31 http://www.chromoscope.net/astrojc/?l=121.1741&b=-21.5735&w=0.00&o=g,w,m,v,a,f,r&z=5 “ – @astronomyblog
There are also alternatives to what is causing the WMAP Haze:
Planck data should tell us if the WMAP Haze is actually there:
“It is higher res than WMAP. It should do a much better job at teasing out the foregrounds as it has more freq coverage” – @astronomyblog
“I suggest we have the same discussion after data of AMS-02 and Planck are published.” – @AdrianusV
The doubts expressed can be summed up by the following:
“My main question would be: Is there really anything there (in Fermi; in WMAP) that needs explaining?” – @partialobs
Final thoughts were gathered by asking the following question:
“So what are your overall thoughts on the paper? Does it convince you that dark matter is the best explanation for the observations?”
And the responses suggest “no – we are not convinced”.
“Lots of assumptions to get to one conclusion. Occam.” – @kashfarooq
“Neither expert nor astronomer, but seemed to be lots of assumptions in the paper; didn’t fill me w/confidence” – @saintaardvark
This is a great discussion, but no, no-one’s convinced me this is the likely explanation. Prob made me more skeptical” – @Matt_Burleigh
“There are too many uncertainties in observations and too many assumptions for me” – @astronomyblog
In fact, the discussion changed some minds:
“I believed the result a lot more an hour ago” – @astronomyjc
There was not much support for the paper. It was an interesting discussion and we think the participants enjoyed it and learnt a lot. We eagerly await more information about the “WMAP Haze” from Planck.
Unfortunately, a dark matter and/or supersymmetry expert was not in attendance so a number of questions remained unanswered.
Even though we were negative towards the ideas suggested in the paper, we’d like to end this post with a great positive quote and a flippant one. You can decide which one is which. 😉