Astrophysicists solve mystery in Milky Way galaxy

Jul 09, 2009
This is the European satellite INTEGRAL. Credit: ESA

A team of astrophysicists has solved a mystery that led some scientists to speculate that the distribution of certain gamma rays in our Milky Way galaxy was evidence of a form of undetectable "dark matter" believed to make up much of the mass of the universe.

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. Thus, 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 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 on visible matter such as stars and galaxies—but have not yet found.

This is the European satellite INTEGRAL. Credit: ESA

What is known for certain is that our galaxy—and others—are filled with tiny subatomic particles known as positrons, the 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— 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 observed by the gamma ray satellite was not a detection or indication of a 'dark matter signal'," said Lingenfelter.

Source: University of California - San Diego (news : web)

Explore further: Thermonuclear X-ray bursts on neutron stars set speed record

add to favorites email to friend print save as pdf

Related Stories

Proposed Particle Help Explains Odd Galactic Photons

Jul 25, 2008

In 2002, a satellite called INTEGRAL was launched by the European Space Agency with an instrument on board to detect and measure gamma rays from space. Four years later, it yielded some intriguing data: An unusually high ...

Is Dark Matter a Source of High Energy Gamma Rays?

Dec 08, 2006

“We know there is much more matter in the universe than what we see. For instance, the rotation velocity of observed spiral galaxies is much faster than the visible mass could explain,” says Joachim Ripken, ...

High Energy Mystery lurks at the Galactic Centre

Sep 22, 2004

A mystery lurking at the centre of our own Milky Way galaxy - an object radiating high-energy gamma rays - has been detected by an international team of astronomers. Their research, published today (September 22nd) in the ...

Integral: Three Years Of Insight Into The Violent Cosmos

Oct 24, 2005

Observing the cosmos, full of violent phenomena and extreme energy, has been the task of ESA's Integral gamma-ray observatory since its launch on 17 October 2002. Three years later, the mission is going very well and has ...

Recommended for you

How can we find tiny particles in exoplanet atmospheres?

Aug 29, 2014

It may seem like magic, but astronomers have worked out a scheme that will allow them to detect and measure particles ten times smaller than the width of a human hair, even at many light-years distance.  ...

Spitzer telescope witnesses asteroid smashup

Aug 28, 2014

(Phys.org) —NASA's Spitzer Space Telescope has spotted an eruption of dust around a young star, possibly the result of a smashup between large asteroids. This type of collision can eventually lead to the ...

Witnessing the early growth of a giant

Aug 27, 2014

Astronomers have uncovered for the first time the earliest stages of a massive galaxy forming in the young Universe. The discovery was made possible through combining observations from the NASA/ESA Hubble ...

User comments : 7

Adjust slider to filter visible comments by rank

Display comments: newest first

Tissa_Perera
1.4 / 5 (9) Jul 09, 2009
Where have all the matter gone?
Long time passing
Where have all the energy gone?
Long time ago

The answer my friends
is flowing in the web

See http://cosmicdarkmatter.com/
When will they ever learn?
When will they everrrrr learn?
out7x
2.5 / 5 (6) Jul 10, 2009
Excellent article. More evidence dark matter is a fudge factor due to lack of understanding of gravity and supernovae.
omatumr
1 / 5 (5) Jul 11, 2009
TRUTH LEAKS THROUGH CONSENSUS CRACKS

Excellent article. More evidence dark matter is a fudge factor due to lack of understanding of gravity and supernovae.


I agree, and I applaud the authors - Richard Lingenfelter, Richard Rothschild and James Higdon - and their academic institutions - University of California at San Diego and Claremont Colleges.

It is refreshing to see that the "great mystery" created by mystical models of the cosmos has been solved by ordinary material doing ordinary things.

With kind regards,
Oliver K. Manuel
http://www.omatumr.com
yyz
5 / 5 (3) Jul 13, 2009
Oliver K. Manuel writes "TRUTH LEAKS THROUGH CONSENSUS CRACKS". Since when has the observed positron excesses being produced by Dark Matter annihilation been considered the 'consensus' in the astrophysics community? I've read dozens of papers on this subject, both for and against DM annihilation being the source of the observed 'positron excess'. AFAIK, there was no consensus in the astronomy community as to the source of the 'excess'. This finding certainly does not rule out the existence of DM, just that no local annihilation signal exists.
omatumr
1 / 5 (3) Jul 18, 2009
DARK MATTER MAY EXIST; NO SIGNAL EXISTS

"This finding certainly does not rule out the existence of DM, just that no local annihilation signal exists."

Yyz is exactly right,
Oliver K. Manuel
omatumr
1 / 5 (5) Jul 30, 2009
THE MYSTERY OF BETELGEUSE

A recent news story on mass loss from the super-giant star Betelgeuse also involves a mystery energy source:

http://www.physor...732.html

Is it too much to ask astronomers and astrophysicists to study the rest masses of all 3,000 known nuclei and find for themselves that repulsive interactions between neutrons is the energy source that powers the stars and the cosmos:

http://tinyurl.com/2otxps

That would clear the field of these daily reports of cosmic observations driven by yet another mysterious source of energy.

With kind regards,
Oliver K. Manuel
http://www.omatumr.com
lengould100
not rated yet Aug 05, 2009
Such a shape and position corresponds to the expected distribution of dark matter in the centre of our galaxy, so it was suggested that dark matter was annihilating or decaying into pairs of electrons and positrons, which then annihilated to produce the gamma rays.
So the dark matter at the centre of the galazy is required in order to explain the reduced rate of dispersion of the spiral arms of large galaxies? Perhaps one could also explain the present dispersion by proposing a creation event by an omnipotent being at a far more recent date than 13 billion years past. See "Calculating God" by Robert Sawyer. (joking of course).

http://www.amazon...12580354