Observations from the GMRT, MeerKAT and the Very Large Array have revealed four pairs of radio structures arranged symmetrically around a central galaxy.
A 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.
Radio galaxies are a form of active galactic nucleus with a central supermassive black hole that launches powerful jets of particles, which are visible in radio waves. The most powerful types are known as FR II. In these galactic structures- which are some of the largest objects in the known universe- the jets impact surrounding gas and produce bright hotspots.
In some FR II galaxies, the jets appear to activate more than once over time, with each burst of activity leaving distinct pairs of hotspots. The age of the pairs can be distinguished by their distance from the galaxy’s core, with older pairs travelling out over time. Galaxies with two known jet episodes are called double-double radio galaxies, with less than 200 examples identified so far. Triple-double radio galaxies, where the jets burst out three times, have only been identified seven times. This study has identified what is believed to be the first example of a radio galaxy with four recorded bursts.
The team studied observations from multiple telescopes to properly gauge age and distance
Using data from the GMRT, MeerKAT and the Very Large Array, the team were able to identify four distinct pairs of hotspots moving outwards from the core, designated N1-N4 and S1-S4.
After calculating each hotspot’s physical distance from the galaxy’s core, the team were able to estimate the ‘kinematic ages’ of each hotspot pair, assuming a typical jet speed. Researchers were also able to determine the ‘spectral age’ of the pair by examining how the radio ‘colour’ had changed over time.
The four hotspot pairs show spectral ages ranging from 4.5 to 20.5 million years
Independent tests were also carried out to rule out alternative explanations, such as these being ‘knots’ in a single continuous jet rather than separate bursts of activity. If the knots were part of a single instance of jet activity, they would have nearly identical ages- but the team found that the ages of the knots followed the pattern expected for sequential jet episodes. Age clearly increased with distance from the core, with the radio emission following suit.
It was also noted that the observation quality of the southern side was poorer, meaning that estimations and trends were harder to specify compared to the northern jets.
The first known Quadruple-double radio galaxy
As described on arXiv, the team then ran a statistical symmetry test, which found that the jets activity being four separate episodes rather than a single outburst with random knots was around 3 million times more likely: “We estimated that the probability that these are 3 inner pairs of hotspots is ∼3 × 106 times more than that of the knots-in-the-jet model,” the team writes in the paper.
“… we conclude that J023721.13−010528.5 exhibits four distinct episodes of jet activity, i.e., a quadruple-double radio galaxy (QDRG),” they add.
The team also observed that the axes of successive hotspot pairs are rotating counterclockwise by increasing amounts, with a shift of −7° for N2-S2, −16° for N3-S3 and −24° for N4-S4, suggesting that the central black hole’s spin axis is slowly “wobbling” over time.
This “progressive rotation,” researchers say, is consistent with the galaxy’s jets also showing an S-shaped morphology. Stretching roughly 3.9 million light-years across (23 quadrillion miles), this is the second-largest S-shaped radio source known. Researchers note that giant, S-shaped radio galaxies could be the most promising hunting ground for finding more of these rare, multi-episode systems.
