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Saturday, March 9, 2013

Monster bubbles evidence of dark matter?


Monster outflows of charged particles stretching above and below the centre of the Milky Way, could contain tantalising evidence of mysterious dark matter, say scientists.

Although dark matter constitutes 84 per cent of all matter in the universe, it has never been seen.

Its existence has only been inferred from its gravitational effects on visible matter, radiation, and the large scale structure of the universe.

Writing on the pre-press website ArXiv.org , Dr Dan Hooper from the Fermi National Accelerator Laboratory, and Dr Tracy Slatyer from Princeton University, say dark matter could be causing some of the features seen in Fermi bubbles.

These bubbles are gamma ray outflows travelling at 1000 kilometres per second, and consist of highly charged particles forming two lobes extending 50,000 light-years above and below the galactic centre.

Hooper and Slatyer found some regions of the bubbles contained gamma rays generated by cosmic ray electrons interacting with interstellar radiation.

These cosmic rays are thought to be produced by the super massive black hole at the centre of the galaxy, and by stellar material during the birth and death of giant stars orbiting near the galactic centre.

But Hooper and Slatyer say this theory does not account for the remaining emission signals.

"Annihilating dark matter can provide a simple explanation for this emission," the authors write.

"The signal is broadly consistent with that predicted from dark matter particles."

Hooper and Slatyer say dark matter particles with a mass of about 10 giga-electron volts could form subatomic particles called tau leptons with some going on to produce electrons, muons and neutrinos, as well as generating some gamma-rays.

"Alternatively, the annihilations of 40 to 50 giga-electron volt dark matter particles to quarks can also provide a reasonable fit," write the authors.

Hooper and Slatyer reached their conclusions after considering other possible sources for the signals they were seeing.

These included a type of star called a millisecond pulsar, which are fast spinning neutron stars, the cores of stars much more massive than the Sun, that have exploded as supernovae at the end of their lives.

"Millisecond pulsars, have been proposed as a source of the emission observed from the galactic centre," write the authors.

But this theory would require a large number of pulsars in regions not known to contain large stellar population, they say.

Dark matter or millisecond pulsar

Dr Roland Crocker of the Australian National University's Mount Stromlo Observatory, says dark matter annihilations would occur as dark matter particles collide.

Dark matter collisions that are big enough to annihilate dark matter particles would produce normal matter, which could be detected through radiation, says Crocker, who has worked with Slatyer on previous studies, but was not involved in this study.

"They might annihilate directly into photons producing high energy gamma-rays, or alternatively into charged particles," he says.

"This is a process you'd expect to be happening more often in the centre of the galaxy because the density of the dark matter particles is larger there."

"If you could confidently say that this was the signature of dark matter particles then it would tell you the mass scale of dark matter and more."

But, he says, the same emissions could be produced by millisecond pulsars.

"Millisecond pulsars have gamma ray spectra similar to what they found," says Crocker.

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