According to a team of astronomers led by Dr. Keith Bannister of CSIRO Astronomy and Space Science Division, invisible noodle- or hazelnut-shaped plasma structures could be floating around in our Milky Way Galaxy.
This wide-field image shows the patch of sky around the distant quasar PKS 1939-315 (green cross). Image credit: Centre de Données astronomiques de Strasbourg / SIMBAD.
“These structures could radically change ideas about the interstellar gas, which is the Galaxy’s star recycling depot, housing material from old stars that will be refashioned into new ones,” said Dr. Bannister, who is the first author of a study published today in the journal Science.
Dr. Bannister and his colleagues described observations of one of these structures that have allowed them to make the first estimate of their shape.
“Lumps in the interstellar gas work like lenses, focusing and defocusing the radio waves, making them appear to strengthen and weaken over a period of days, weeks or months,” Dr Bannister explained.
Pointing the Australia Telescope Compact Array at a quasar called PKS 1939–315, Dr Bannister and co-authors saw a lensing event that went on for a year.
“The lens is about the size of the Earth’s orbit around the Sun and lies around 3,000 light-years away,” the astronomers said.
They have shown this lens could not be a solid lump or shaped like a bent sheet.
“We could be looking at a flat sheet, edge on. Or we might be looking down the barrel of a hollow cylinder like a noodle, or at a spherical shell like a hazelnut,” said co-author Dr. Cormac Reynolds, of the International Center for Radio Astronomy Research – Curtin University. “Getting more observations will definitely sort out the geometry.”
While the lensing event went on, the scientists observed it with other radio and optical telescopes. The optical light from PKS 1939-315 didn’t vary while the radio lensing was taking place.
“This is important because it means earlier optical surveys that looked for dark lumps in space couldn’t have found the one we have detected,” Dr. Bannister said.
So what can these lenses be? One suggestion is clouds of gas that stay pulled together by the force of their own gravity.
That model, worked through in detail, implies the clouds must make up a substantial fraction of the mass of our Galaxy.
“Nobody knows how the invisible lenses could form. But these structures are real, and our observations are a big step forward in determining their size and shape,” Dr. Bannister said.
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Keith W. Bannister et al. 2016. Real-time detection of an extreme scattering event: Constraints on Galactic plasma lenses. Science, vol. 351, no. 6271, pp. 354-356; doi: 10.1126/science.aac7673
