Astrophysicists solve mystery in Milky Way galaxy

July 08, 2009

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 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.
The scientists were supported in their studies by grants from the National Aeronautics and Space Administration.

University of California - San Diego

Related Dark Matter Articles from Brightsurf:

Dark matter from the depths of the universe
Cataclysmic astrophysical events such as black hole mergers could release energy in unexpected forms.

Seeing dark matter in a new light
A small team of astronomers have found a new way to 'see' the elusive dark matter haloes that surround galaxies, with a new technique 10 times more precise than the previous-best method.

Holding up a mirror to a dark matter discrepancy
The universe's funhouse mirrors are revealing a difference between how dark matter behaves in theory and how it appears to act in reality.

Zooming in on dark matter
Cosmologists have zoomed in on the smallest clumps of dark matter in a virtual universe - which could help us to find the real thing in space.

Looking for dark matter with the universe's coldest material
A study in PRL reports on how researchers at ICFO have built a spinor BEC comagnetometer, an instrument for studying the axion, a hypothetical particle that may explain the mystery of dark matter.

Looking for dark matter
Dark matter is thought to exist as 'clumps' of tiny particles that pass through the earth, temporarily perturbing some fundamental constants.

New technique looks for dark matter traces in dark places
A new study by scientists at Lawrence Berkeley National Laboratory, UC Berkeley, and the University of Michigan -- published today in the journal Science - concludes that a possible dark matter-related explanation for a mysterious light signature in space is largely ruled out.

Researchers look for dark matter close to home
Eighty-five percent of the universe is composed of dark matter, but we don't know what, exactly, it is.

Galaxy formation simulated without dark matter
For the first time, researchers from the universities of Bonn and Strasbourg have simulated the formation of galaxies in a universe without dark matter.

Taking the temperature of dark matter
Warm, cold, just right? Physicists at UC Davis are using gravitational lensing to take the temperature of dark matter, the mysterious substance that makes up about a quarter of our universe.

Read More: Dark Matter News and Dark Matter Current Events is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to