Two large research programs in astronomy, called EMU and PEGASUS, have teamed up to solve one of the mysteries of our Milky Way: where is all the supernova remnants?

A supernova remnant is an expanding cloud of gas and dust that marks the last phase in a star’s life, after it exploded as a supernova. But the number of supernova remnants that we have detected so far with radio telescopes is too low. Models predict five times as many, so where are the missing ones?

We have combined observations from two of Australia’s world-leading radio telescopes, the ASKAP Radio Telescope and the Parkes Radio Telescope, Murriyang, to answer this question.

The new image reveals thin tendrils and lumpy clouds associated with the hydrogen gas that fills the space between the stars. We can see sites where new stars are forming, as well as remnants of supernovae. In this small patch alone, only about 1% of the entire Milky Way, we have discovered more than 20 new possible supernova remnants where only seven were previously known. These discoveries were led by PhD student Brianna Ball of the University of Alberta in Canada, in collaboration with her supervisor, Roland Kothes of the National Research Council of Canada, who prepared the image. These new discoveries suggest that we are close to accounting for the missing remains. So why can we see them now when we couldn’t before?

The power to join forces I lead the Evolutionary Map of the Universe or EMU program, an ambitious project with ASKAP to make the best radio atlas of the Southern Hemisphere. EMU will measure around 40 million new distant galaxies and supermassive black holes, to help us understand how galaxies have changed over the history of the universe. Early EMU data has already led to the discovery of strange radio circles (or “ORCs”), and revealed rare oddities like “Dancing Ghosts”. For any telescope, the resolution of its images depends on the size of its aperture. Interferometers like ASKAP simulate the aperture of a much larger telescope. With 36 relatively small dishes (each 12 m in diameter) but a distance of 6 km connecting the farthest of these, ASKAP mimics a single telescope with a 6 km wide dish.

That gives ASKAP good resolution, but it comes at the expense of lack of radio emission on the larger scales. In the comparison above, the ASKAP image alone looks too skeletal.

To retrieve that missing information, we turned to a complementary project called PEGASUS, led by Ettore Caretti of the Italian National Institute for Astrophysics. PEGASUS uses the 64 m diameter Parkes/Murriyang telescope, one of the world’s largest single-dish radio telescopes, to map the sky. Even with such a large dish, Parkes is quite limited in resolution. By combining the information from Parkes and ASKAP, each fills in the gaps of the other to give us the best fidelity picture of this region of our Milky Way galaxy. This combination reveals radio emission at all scales to help discover the missing supernova remnants.

Linking the EMU and PEGASUS data sets will allow us to reveal more hidden gems. In the next few years we will have an unprecedented view of almost the entire Milky Way, about a hundred times larger than this initial image, but with the same level of detail and sensitivity. We estimate that there may be as many as 1,500 or more new supernova remnants yet to be discovered. Solving the puzzle of these missing remnants will open new windows into the history of our Milky Way.