This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Astronomers have identified what may be the first example of a new class of radio galaxy, one whose central black hole appears to have launched powerful jets on four separate occasions. High-resolution observations with the upgraded Giant Metrewave Radio Telescope (GMRT), combined with data from MeerKAT and the Very Large Array, reveal four pairs of radio structures arranged symmetrically around the central galaxy.
A paper outlining this discovery was posted to the arXiv preprint server on Sept. 8. Radio galaxies are a type of active galactic nucleus in which a central supermassive black hole launches powerful jets of particles, visible in radio waves. The most powerful class, known as FR II, produces bright "hotspots" where these jets slam into surrounding gas, forming structures that can span millions of light-years, making them some of the largest objects in the universe.
In a small subset of these FR II galaxies, the central black hole appears to switch its jets on more than once over time. Each burst of activity leaves behind its own pair of hotspots, with older episodes sitting farther from the galaxy's core because they've had more time to travel outward and newer episodes forming closer in. Galaxies with two such episodes are known as double-double radio galaxies.
About 192 known examples of this subclass exist. Triple-double radio galaxies show three separate episodes. They are rarer, with only seven known or candidate examples.
Until now, no galaxy had ever been found with four confirmed episodes of jet activity. In this study, the team led by Pavan Vijay Khadekar of the Indian Institute of Science Education and Research, Pune, studied a radio source known as J023721.13−010528.5, hosted by a massive elliptical galaxy at redshift 0.372. By combining radio observations from three powerful telescopes across multiple radio frequencies, the team identified four distinct pairs of hotspots labeled N1–N4 and S1–S4, moving outward from the core.
They calculated each hotspot's physical distance from the core and, assuming a typical jet speed, estimated the "kinematic ages" of each hotspot pair. By calculating how much the radio "color" had aged and steepened over time, researchers also derived the "spectral ages" of these pairs. To rule out an alternative explanation—that these might be random "knots" in one continuously active jet rather than truly separate historical episodes—they also ran independent tests.
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