On a clear night, the Milky Way dominates the sky above the Australian outback, arching overhead in a way never seen from the Northern Hemisphere. As wondrous as this looks, our eyes see only a tiny part of the spectrum of light emitted from the galactic plane, so astronomers have pieced together an image of how the same region would look if our eyes could see at the frequencies of FM radio, and it's spectacular.
Most telescopes capture only tiny areas of the sky at a time, so a composite of thousands of square degrees would be an immense undertaking. As its name suggests, the Murchison Widefield Array (MWA) is distinguished by its capacity to capture 1,000 square degrees at a time. Even so, the image above was not created just so astronomers would have a breathtaking poster for their walls; in making it they learned a lot more about the disk of our galaxy, including the discovery of 27 remnants of ancient supernovas.
“This new view captures low-frequency radio emission from our galaxy, looking both in fine detail and at larger structures,” said Dr Natasha Hurley-Walker of the Curtin University node of the International Centre for Radio Astronomy Research (ICRAR), in a statement. The MWA is wide not only in the area of the sky it can view, but the frequencies it can capture, ranging from 72-231 Megahertz. Much of the world astronomers' view of the sky at these radio frequencies are drowned out by the latest hits. Murchison, 800 Kilometers (500 miles) from Perth is a rare place that remains “radio dark” leading it to be chosen to host the low-frequency component of the Square Kilometer Array, which will be the world's largest telescope when completed.
Despite the MWA's epically wide field, Hurley-Walker still needed to piece together thousands of images. This involved collecting radio waves at five frequencies and translating each into a color visible to human eyes, she explained to IFLScience. This allows us to understand the astrophysics in a way a single frequency never could.
“Regions around bright stars look blue, while red areas show older populations of electrons: the power of a color view allows us to discriminate,” she said.
In the course of the survey, Hurley-Walker and colleagues identified 27 supernova remnants, marked by the combination of a pulsar and an expanding shell of gas thrown off in the explosion, expanding the catalog by 9 percent.
The MWA's wide field means it includes supernova remnants in places far away from the major star-forming fields where astronomers would never have thought to look. Hurley-Walker told IFLScience it's hard to understand how the progenitor stars got there. Although various forces can cause stellar migration, the life-cycle of the giant stars that become supernovas are too short to allow much time for such journeys.
One of the discoveries lying far from the galactic plane exploded just 9,000 years ago, and would have been easily bright enough to be seen by inhabitants of the southern hemisphere. Hurley-Walker consulted with Dr Duane Hamacher of the University of Melbourne, who has been acting as a bridge between modern science and Indigenous Australian knowledge of the skies.
Hamacher said stories of the sudden appearance of new stars survive in some Aboriginal traditions, although so far he has not been able to match any to this event. In the light of evidence that Indigenous Australia oral traditions describe ancient sea level rises, Hamacher thinks it is possible tales of this event could have lasted.
“Now that we know when and where this supernova appeared in the sky, we can collaborate with Indigenous elders to see if any of their traditions describe this cosmic event. If any exist, it would be extremely exciting,” he said.