Radio signals reveal secrets of massive galaxies

Black holes – the cosmic giants known to power some of the brightest sources of radio waves in the universe – were the focus of a study led by Associate Professor Michael Brown, from the School of Physics and Astronomy from Monash University.

Researchers explored the mysteries of radio waves emitted by the most massive black holes using Australia’s cutting-edge ASKAP (Australian Square Kilometer Array Pathfinder) system.

In their quest to answer the question of whether radio waves are systematically emitted by the most massive black holes, astronomers measured radio waves from the largest galaxies in the nearby universe. The full survey leveraged the ASKAP Continuum Rapid Survey (RACS). Associate Professor Brown said ASKAP was capable of surveying large areas of the sky and was more sensitive than previous comparable radio surveys.

The study, titled “Radio Continuum of the Most Massive Early-Type Galaxies Detected with ASKAP RACS,” has been accepted for publication in the Publications of the Astronomical Society of Australia and is currently available on the preprint server arXiv.

While recognizing that new star formation in galaxies can also produce radio waves, the research team focused on galaxies with minimal or no star formation. Of the 587 nearby galaxies examined, the 40 largest galaxies examined all emit radio waves.

“While it is possible that low-intensity star formation is hidden in these galaxies, black holes seem to be the most likely cause of what we are observing,” Associate Professor Brown said.

The study also revealed variations in the emission of radio waves among the largest galaxies, with some proving significantly more powerful than others. For example, the galaxy ESO 137-G 6 had a radio luminosity approximately 10,000 times greater than that of the galaxy NGC 6876.

Work on this study began under the challenges of COVID lockdowns in Melbourne, with undergraduate student Teagan Clarke undertaking preliminary work as part of Monash’s Physics and Astronomy Research Project Unit.

“We were able to really dig into this new data to start to uncover the differences in how these galaxies glow in radio waves,” Teagan said.

“This could tell us about their central black holes and how they power these massive galaxies.”

“Why different galaxies emit many more radio waves than others is a bit of a puzzle,” Associate Professor Brown said.

“However, we find that galaxies that are strong sources of radio waves appear to spin more slowly than comparable galaxies that are weak sources of radio waves. Getting to the bottom of this is going to be difficult work for me and my students .”