Who's Lurking in the Radio Glow of Galaxies? ⚡ экспресс
Astronomers using the VLA in high-resolution mode discovered 115 compact radio sources in galaxies NGC 5474, NGC 4631, and M51. Among them are eight star-forming regions, four candidates for anomalous microwave emission sources, and the supernova remnant SN 2011dh. Nine objects have X-ray counterparts, most of which are background active galactic nuclei (supermassive black holes) projected onto the disk of the studied systems. Such “impostors” can contaminate samples of X-ray binaries, skewing statistics; this highlights the need for ultra-sensitive future radio telescopes like the SKA and ngVLA.
Radio astronomers aimed the VLA telescope at three spiral galaxies – NGC 5474, NGC 4631, and M51 – and saw a scene resembling a city at night: 115 bright points, like streetlights and lit windows. The telescope worked in maximum resolution mode, capturing radio waves to which space is almost transparent.
The big surprise came when comparing with X-ray images. Nine sources coincided with X-ray dots. They were expected to be binary systems with black holes within galaxies, but almost all turned out to be distant quasars – their light piercing the disk of a neighboring galaxy like a searchlight through frosted glass. Worse, some of these "searchlights" had been wrongly labeled as local objects for decades. Without accounting for such impostors, astronomers risk overestimating the number of their own black holes many times over. That's why new giant radio telescopes are learning to separate native lights from background high beams.
🎯 The radio echo of supernova SN 2011dh, captured in this survey, resembles a slowly dissipating patch of fog that reveals an unexpected detail: before the explosion, the star shed a record amount of mass, hastening its end.