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.
as written by the authors arXiv:2601.17951
The energy released by active galactic nuclei (AGNs) is considered to have a profound impact on the cold gas properties of their host galaxies, potentially heating or removing the gas and further suppressing star formation. To understand the feedback from AGN radio activity, we investigate its impacts on the cold gas reservoirs in AGNs with different radio activity levels. We construct a quasar sample with a mean $$z\sim1.5$$ and a mean $$L_{\rm bol}\sim10^{45.8}\ \rm erg\ s^{-1}$$, all with Herschel detections to enable estimates of the total gas mass through the galactic dust continuum emission. The sample is then cross-matched with radio catalogs and divided into radio loud (RL) quasars, radio-detected radio quiet (RQ) quasars and radio-undetected quasars based on their radio loudness. Through spectral energy distribution (SED) fitting, we find the radio-detected RQ quasars exhibit evidence of gas deficiency with host galaxies possessing $$\sim 0.3$$ dex lower dust and gas masses compared to the other two groups, despite being matched in $$M_{\rm BH}$$, $$L_{\rm bol}$$, $$M_{*}$$ and SFR. Furthermore, evidence from optical spectra shows that both the fraction and velocity of outflows are higher in the radio-detected RQ group, suggesting a connection between the ionized gas outflows and the moderate radio activity. These results suggest that the AGN feedback could be more efficient in AGNs with weak/moderate radio emission than in those without radio detection or those with strong radio emission. Further high-resolution observations are needed to understand the interaction between the interstellar medium and the weak/moderate AGN radio activity.
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.