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An international team of astronomers has found that jets of 10 nearby active galactic nuclei (AGN) have changed their shape from parabolic to conical. The finding, reported in a paper published July 2 on the arXiv pre-print repository, could hold the key to improving our understanding about the formation, acceleration and collimation of relativistic jets in AGNs.

AGNs are small central regions in some galaxies emitting powerful, high-energy radiation as they accrete gas and dust. These nuclei can form jets, having mostly cylindrical, conical, or parabolic shapes, which are observed even on megaparsec scales.

The morphology of AGN jets is a subject of numerous studies. Astronomers are especially interested in jets with morphology transition, as some such features are known to showcase changes in their shape. In 2012, the first transition from a parabolic to a conical jet shape was observed in the Messier 87 galaxy.

Now, a group of astronomers led by Yuri Kovalev of the Lebedev Â鶹ÒùÔºical Institute in Moscow, Russia, reports the finding of more geometry transitions in AGNs. They found such activity in 10 nearby AGNs (with a redshift below 0.07) by analyzing the available data from the Monitoring Of Jets in Active galactic nuclei with VLBA Experiments (MOJAVE) program, the 2-cm Very Long Baseline Array (VLBA) Survey, and the National Radio Astronomy Observatory (NRAO).

"We have performed an automated search for jet shape breaks in a sample of 367 AGNs using VLBA data at 2 cm and 22 cm. This search has found ten nearby jets at redshifts z < 0.07 with a transition from a parabolic to conical shape," the paper reads.

As noted in the paper, a shape transition was identified in ten out of 367 analyzed AGN jets, namely: UGC 00773, NGC 1052, 3C 111, 3C 120, TXS 0815−094, Mrk 180, PKS 1514+00, NGC 6251, 3C 371, and BL Lac. Notably, for all these 10 AGNs, a sufficient linear resolution was achieved, which, according to the astronomers, suggests that geometry transitions may be common in the jets of active galactic nuclei.

"We conclude that the geometry transition may be a common effect in AGN jets. It can be observed only when sufficient linear resolution is obtained," the astronomers wrote.

The researchers added that many other jets from the studied sample could also exhibit such a transition, confirming that it is a common feature. However, more detailed observations are required to verify this. Therefore, they propose more observations of nearby AGNs using high frequency, very long baseline interferometry imaging.

The results also allowed the team to draw some conclusions regarding the possible mechanism behind the shape transitions in AGN jets.

"We suggest that the jet shape transition happens when the bulk plasma kinetic energy flux becomes equal to the Poynting energy flux, while the ambient medium pressure is assumed to be governed by Bondi accretion," the authors of the paper concluded.

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