Lead author Dr Keith Bannister said that the finding, detected using the Australian Commonwealth Scientific and Industrial Research Organisation’s (CSIRO) new compact telescope array, “could radically change ideas about this interstellar gas, which is the Galaxy’s star recycling depot, housing material from old stars that will be refashioned into new ones.”
The technique allowed the team to monitor “lensing” — also known as extreme scattering events (ESEs) — a phenomenon which was first identified thirty years ago as the cause of fluctuations in the radio waves being picked up from a distant pulsar. Astronomers established that this was caused by the galaxy’s invisible atmosphere, described as a thin gas of electrically charged particles that fills the space between the stars.
The fluctuating radio waves were the result of lumps in the interstellar gas which, Dr Bannister explains, “work like lenses, focusing and defocusing the radio waves, making them appear to strengthen and weaken over a period of days, weeks or months.” The lenses are thought to lie approximately 3000 light-years away and be about the size of the Earth’s orbit around the Sun.
Lensing events are notoriously hard to find, but the team’s new survey technique allowed them to identify ESEs in real time and model the radio data obtained from the telescope array to analyse the refractive index of the interstellar plasma.
The team’s findings for the first time give an indication of the shape of the lenses, demonstrating that they cannot possibly be either a solid lump or shaped like a bent sheet. CSIRO’s Dr Cormac Reynolds said that “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,” and noted that further observations would be required to firmly establish the geometry of the phenomena.
The team pointed CSIRO’s Compact Array at a quasar called PKS 1939-315 and saw a lensing event that went on for a year. During this period, they observed it with other radio and optical telescopes. Although characteristic radio fluctuations were present, the optical light from the quasar didn’t vary while the radio lensing was taking place. This shows that earlier optical surveys that looked for dark lumps as a means of detecting ESEs would not have spotted this one.
There’s still much to be learned about galactic lenses. It’s not clear what they are exactly, although one model suggests that they could be cold clouds of gas that are drawn together by the force of their own gravity. If that’s the case, then such clouds must, it follows, make up a significant fraction of our galaxy’s mass.
Nor is it clear how they form but, Dr Bannister says, “these structures are real, and our observations are a big step forward in determining their size and shape.”