Astronomers discover a radio galaxy with evidence of four separate jet outbursts

Astronomers have identified a potential new class of radio galaxy featuring four separate jet outbursts. Utilizing high-resolution data from the upgraded Giant Metrewave Radio Telescope (GMRT), MeerKAT, and the Very Large Array, researchers discovered four symmetrical pairs of radio structures encircling a central galaxy. Published on September 8, the study published on arXiv outlines this groundbreaking discovery.

Radio galaxies, a type of active galactic nucleus, boast central supermassive black holes that launch powerful jets of particles visible in radio waves. The most luminous variety, FR II galaxies, create extensive structures where these jets collide with surrounding gas. In some FR II galaxies, the central black hole appears to activate its jets multiple times, leaving behind distinct pairs of hotspots.

Typically, double-double radio galaxies exhibit two such episodes, while triple-double radio galaxies show three. However, until now, no radio galaxy had been detected with four confirmed jet activity episodes.

The team led by Pavan Vijay Khadekar from the Indian Institute of Science Education and Research, Pune, analyzed a radio source known as J023721.13−010528.5, residing in a massive elliptical galaxy at a redshift of 0.372. By merging radio observations from three powerful telescopes across multiple frequencies, the researchers identified four distinct pairs of hotspots, labeled N1–N4 and S1–S4, moving outward from the core.

By calculating the physical distances of each hotspot from the core and estimating their kinematic ages based on typical jet speeds, the researchers inferred the spectral ages of these pairs. The spectral ages ranged from 4.5 to 20.5 million years, increasing with distance from the core, indicating sequential jet episodes rather than random knots.

The team further confirmed this hypothesis by running statistical symmetry tests, which found the four separate episodes explanation to be approximately 3 million times more probable than the random knots model. This suggests that the radio emission evolved progressively older from the inner structures toward the outer ones. Notably, the axes of successive hotspot pairs rotated counterclockwise by increasing angles, suggesting the central black hole's spin axis is gradually wobbling.

This progressive rotation aligns with the galaxy's observed S-shaped morphology. The radio source stretches across roughly 3.9 million light-years, making it the second-largest S-shaped radio galaxy known.

The researchers conclude that J023721.13−010528.5 demonstrates four distinct episodes of jet activity, labeling it a quadruple-double radio galaxy (QDRG). They further propose that giant, S-shaped radio galaxies may serve as prime targets for uncovering more of these rare multi-episode systems.