Solved: Mystery of Gamma Ray Distribution in the Milky Way

Article written: 9 Jul , 2009
Updated: 24 Dec , 2015
by

A team of astrophysicists has solved the mystery of the distribution of gamma rays in our Milky Way galaxy.  While some researchers thought the distribution suggested a form of undetectable “dark matter”, the team from the University of California, San Diego, proposed an explanation based on standard physical models of the galaxy.

In two separate scientific papers, the most recent of which appears in the July 10 issue of the journal Physical Review Letters, the astrophysicists show that this distribution of gamma rays can be explained by the way “antimatter positrons” from the radioactive decay of elements, created by massive star explosions in the galaxy, propagate through the galaxy. That means, the scientists said, the observed distribution of gamma rays is not evidence for dark matter.

“There is no great mystery,” said Richard Lingenfelter, a research scientist at UC San Diego’s Center for Astrophysics and Space Sciences who conducted the studies with Richard Rothschild, a research scientist also at UCSD, and James Higdon, a physics professor at the Claremont Colleges. “The observed distribution of gamma rays is in fact quite consistent with the standard picture.”

Over the past five years, gamma ray measurements from the European satellite INTEGRAL have perplexed astronomers, leading some to argue that a “great mystery” existed because the distribution of these gamma rays across different parts of the Milky Way galaxy was not as expected.

To explain the source of this mystery, some astronomers had hypothesized the existence of various forms of dark matter, which astronomers suspect exists—from the unusual gravitational effects on visible matter such as stars and galaxies—but have not yet found.

What is known for certain is that our galaxy—and others—are filled with tiny subatomic particles known as positrons, the antimatter counterpart of typical, everyday electrons. When an electron and positron encounter each other in space, the two particles annihilate and their energy is released as gamma rays. That is, the electron and positron disappear and two or three gamma rays appear.

”These positrons are born at nearly the speed of light, and travel thousands of light years before they slow down enough in dense clouds of gas to have a chance of joining with an electron to annihilate in a dance of death,” explains Higdon. “Their slowing down occurs from the drag of other particles during their journey through space. Their journey is also impeded by the many fluctuations in the galactic magnetic field that scatter them back and forth as they move along. All of this must be taken into account in calculating the average distance the positrons would travel from their birthplaces in supernova explosions.”

”Some positrons head towards the center of the Galaxy, some towards the outer reaches of the Milky Way known as the galactic halo, and some are caught in the spiral arms,” said Rothschild. “While calculating this in detail is still far beyond the fastest supercomputers, we were able to use what we know about how electrons travel throughout the solar system and what can be inferred about their travel elsewhere to estimate how their anti-matter counterparts permeate the galaxy.”

The scientists calculated that most of the gamma rays should be concentrated in the inner regions of the galaxy, just as was observed by the satellite data, the team reported in a paper published last month in the Astrophysical Journal.

“The observed distribution of gamma rays is consistent with the standard picture where the source of positrons is the radioactive decay of isotopes of nickel, titanium and aluminum produced in supernova explosions of stars more massive than the Sun,” said Rothschild.

In their companion paper in this week’s issue of Physical Review Letters, the scientists point out that a basic assumption of one of the more exotic explanations for the purported mystery—dark matter decays or annihilations—is flawed, because it assumes that the positrons annihilate very close to the exploding stars from which they originated.

“We clearly demonstrated this was not the case, and that the distribution of the gamma rays observed by the gamma ray satellite was not a detection or indication of a ‘dark matter signal’,” said Lingenfelter.

Source: UC San Diego


14 Responses

  1. Jon Hanford says

    Any links to preprints of the original papers?

  2. Torbjorn Larsson OM says

    Found them in arxiv: here and here.

    Modulo the astronomy and other details 😉 that I don’t get, it looks superficially elegant for this layman. From broad spectral features -> mostly positronium vs direct annihilation, and add thermalisation in molecular clouds -> positrons must travel. From current understanding of particle propagation in plasmas and the thermalisation -> positrons can travel. And then they get that the remainder escaped particle rates matches the halo annihilation rates.

  3. Nexus says

    Uh, correct me if I’m wrong, but wouldn’t an “antimatter positron” be a regular electron? Double negatives…

  4. Jon Hanford says

    @ Torbjorn Larsson OM , thanks for the links to the papers you provided. I’ll be sure to scrutinize and add them to my folder on this purported ‘positron excess’.

  5. Sili says

    I’m impressed that there are radioactive elements enough to account for this.

    Much though I love Dark Matter, these ‘mundane’ explanations make me very happy.

    (And, hey, it gives should help us estimate the crosssection of DM, right?)

  6. Anaconda says

    ” I must investigate.”

    That’s all anybody can ask for.

    The post states: “That means, the scientists said, the observed distribution of gamma rays is not evidence for dark matter.”

    But is the conclusion of the paper’s authors the only possible answer?

    Gamma rays are also created (or emitted from electrons) as a result of electrons and ions (charged particles) being accelerated by application of an intense electric field.

    This process of generating gamma rays has been confirmed in plasma physics laboratories.

    Shouldn’t one consider possible avenues of gamma ray generation that have been confirmed by experiment in the laboratory, even if it is a plasma physics laboratory that applies pinciples and processes of electromagnetism, such as the acceleration of electrons via double layers.

    Or as NASA puts it for young people: How Does the Universe Make Gamma-Rays?
    *Particle-Particle Collisions
    *Matter-Antimatter Annihilation
    *Radioactive Decay
    *Acceleration of Charged Particles

    Yes, that’s right, NASA lists one of the ways to generate gamma rays is by “Acceleration of Charged Particles”, as in plasma.

    And how does one do that both in the laboratory and space? The formation of ‘double layers’ per Wikipedia:

    http://en.wikipedia.org/wiki/Double_layer_%28plasma%29

    Yes, let’s investigate…all the possibilities.

  7. Nereid says

    Do you have any references for this, Anaconda?

    I’m quite curious, esp. wrt how intense (the electric fields were) , what the gamma SED is, and what role DLs played …

    Gamma rays are also created (or emitted from electrons) as a result of electrons and ions (charged particles) being accelerated by application of an intense electric field.

    This process of generating gamma rays has been confirmed in plasma physics laboratories.

    Shouldn’t one consider possible avenues of gamma ray generation that have been confirmed by experiment in the laboratory, even if it is a plasma physics laboratory that applies pinciples and processes of electromagnetism, such as the acceleration of electrons via double layers.

    Do you agree that the observed SED is almost certainly the most powerful discriminator wrt underlying physical mechanisms (for astronomical objects)?

  8. Anaconda says

    Hannes Alfvén, in a NASA sponsored conference on double layers in astrophysics in 1986 (NASA CP 2469) said:

    “Double layers in space should be classified as a new type of celestial object (one example is the double radio sources). It is tentatively suggested that x-ray and gamma ray bursts may be due to exploding double layers. In solar flares, DL’s with voltages of 109 V or even more may occur, and in galactic phenomena, we may have voltages that are several orders of magnitude larger. Examples are given of possible galactic DL voltage differences of 10’2 V. This means that by a straightforward extrapolation of what we know from our cosmic neighborhood, we can derive acceleration mechanisms which brings us up in the energy region of cosmic radiation.”

    The NASA link:

    http://imagine.gsfc.nasa.gov/docs/science/know_l1/gamma_generation.html

    Yes, Virginia, electrical processes can generate gamma rays.

  9. Nereid says

    @Anaconda: Torbjorn Larsson OM, in his July 9th, 2009 at 7:35 pm comment, provided links to the two papers the UT story refers to.

    Did you read them?

    Somehow I don’t think you did.

    Why?

    Because they are about “the observed 511 keV annihilation flux distribution”.

    Do you know what “511 keV annihilation flux” refers to? (HINT: gammas produced when electrons and positrons annihilate).

    ” I must investigate.”

    That’s all anybody can ask for.

    The post states: “That means, the scientists said, the observed distribution of gamma rays is not evidence for dark matter.”

    But is the conclusion of the paper’s authors the only possible answer?

    You then went on to talk about how DLs (etc) may be able to create gammas (among other things).

    But surely whether DLs can create gammas is pretty much irrelevant wrt this research, isn’t it?

    I mean, you aren’t proposing that the observed 511 keV annihilation flux comes from anything other than electron-positron annihilation, are you?

  10. Jon Hanford says

    @ Anaconda, what exactly is the origin of the 511 keV emission line that is main topic of the two quoted papers? Peer-reviewed, journal published links to complete papers would be most informative to the astrophysics and particle physics community. Please enlighten me 🙂

  11. Anaconda says

    @ Jon Hanford:

    I’ll have to dig around.

    A note, Hanford has asked the toughest questions, reasonable questions.

    I have no problem with that…other than not being able to answer them 🙂

  12. Jon Hanford says

    @ Anaconda, thanks for your reply to my question I posted on July 15. I do appreciate your response and effort to provide additional info. One observation, though. Above you were quoted as saying “Shouldn’t one consider possible avenues of gamma ray generation that have been confirmed by experiment in the laboratory, even if it is a plasma physics laboratory that applies pinciples and processes of electromagnetism, such as the acceleration of electrons via double layers.” The 511 keV line from electron-positron annihilation has also been observed in several physics labs around the world (see Annihilation radiation Wiki page here: http://en.wikipedia.org/wiki/Annihilation_radiation). The same with particle-particle collisions and of course radioactive decay. So it would seem to me that all four possible sources of gamma-rays have been investigated in physics labs around the world. I was just curious as to how a DL could reproduce the observed 511 keV gamma-ray emission line. This is a specific emission line observed in the gamma-ray spectrum. The gamma-ray SED would be most informative regarding this query. Thanks in advance for any info you can locate wrt this question 🙂

  13. Jon Hanford says

    Sorry, the Annihilation radiation link I provided above doesn’t work. Try: http://en.wikipedia.org/wiki/Annihilation_radiation

Comments are closed.