A quasar is a supermassive black hole at the center of a galaxy that devours matter and shines brighter than all the stars around it. Some of that energy goes into radio emission, which, it was thought, could heat or expel cold gas — the raw material for new stars. Astronomers set out to test how the strength of radio emission relates to the reserves of this gas.
They assembled a sample of quasars about nine billion light-years away, where the thermal glow of dust is visible — from which the total gas mass can be estimated. Then they split them into three groups: those with loud radio emission, those with weak but detectable emission, and those with no radio signal at all. The comparison revealed a surprise: quasars with weak radio emission have, on average, half as much dust and gas as the others, even though their black hole masses, luminosities, and star formation rates are indistinguishable.
Moreover, in the optical spectra of these 'quiet' quasars, fast flows of ionized gas streaming from the center are more common. This hints at a connection: moderate radio emission somehow accelerates gas and pushes it out of the galaxy. It turns out that for stopping star formation, what matters is not loudness but persistence and fine-tuning.
Why weak radio emission acts more strongly than loud emission remains a puzzle. Perhaps powerful jets punch through the gas without disturbing it, while quiet jets stir and heat everything around. To figure it out, we need higher-resolution observations — then we'll see exactly how weak radio waves interact with gas in galaxies.
Quasars are so bright that a single one can shine like hundreds of billions of stars combined, yet occupy a region the size of our solar system.
In Frederik Pohl's novel Gateway, heroes fly ships left by an ancient civilization that may have used the energy of active galactic nuclei, just such quasars.