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All Massive Galaxies Emit Radio Waves, Study Finds

Image: Event Horizon Telescope Collaboration

Ah, black holes—those distressing voids in space known for gobbling up everything they touch. Those side effects of the theory of relativity are now believed to be present at the core of every galaxy in the universe, and there’s still a lot we don’t know about them.

One of those outstanding mysteries is radio wave emission. Since the 1940s, radio telescopes on Earth and, later, from space have been picking up on subtle radio signals coming from some—but not all—observable galaxies. The reason that some galaxies emit very strong radio signals while others emit next to none, and that this discrepancy has no correlation with the amount of light emitted by these different types of galaxies, likely has something to do with those deep dark engines at their centers.

A study published recently in the Publications of the Astronomical Society of Australia used very sensitive radio telescope observations to come to the conclusion that all black holes do, in fact, emit radio waves. 

“If some galaxies were not emitting radio waves, we would expect random noise to produce a mix of positive and negative signals.” Michael Brown, the study’s lead author, wrote in an article for The Conversation. “Getting a positive signal every time suggested all massive galaxies are radio sources. But digging into the noise left us unsure, until now.”

The birth of a radio wave: Before matter swirls into the depths of a black hole and disappears forever, the incredible gravitational pull speeds it up to close to the speed of light. Particles that collide at that speed can release a ton of energy, and send part of the material destined for the black hole spinning outward into space. We can pick up these emissions with sensitive radio telescopes as radio waves.

Some galaxies, like Messier 87, emit very strong radio signals, easily distinguishable from the noisy background of the universe. Until recently, though, about two thirds of the large galaxies probed didn’t emit a signal that was detectable by existing radio sensors.

The research team set out to understand A) whether all black holes emit radio waves, and B) what the reason for the difference might be.

Diving in: To answer these questions, very sensitive radio observations were required. The researchers used the ASKAP radio telescope and the Murchison Widefield Array in Australia, both new radio telescopes in the past decade or so, to gather data. Using these new tools, the researchers gathered data on 40 different massive galaxies, and found a distinct radio signal from each.

What does it mean? The finding that all massive galaxies emit radio waves is a step forward toward understanding the nature of black holes, and hints that it isn’t necessary for a black hole to be actively gaining mass in order to emit radio waves.

There are lots of unanswered questions, though. For instance, it’s unclear whether the black holes at the centers of these galaxies are all being fed (i.e., accruing new material). It’s also unclear why some galaxies emit radio waves at such high rates while others of similar size and luminosity are so quiet.

“We don’t know the answer yet, but there are some clues,” Brown wrote. “Our work and a recent study with Lofar find that, on average, the galaxies that rotate the least are the strongest radio wave emitters. Some of the exceptions to this trend are curious, with evidence of mergers with other galaxies.”

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