Astronomers identify nature of “sleeping” active galaxies

Compact active radio galaxies with two-way plasma jets emanating from their centers have been found to be sinks for massive stars. Thanks to accumulated observational data, scientists were able to confirm old hypotheses that these unusual sources are a completely new type of object.

In the 1970s, when researchers examined the most compact galaxies less than a kiloparsec (3.2 thousand light-years) in diameter, it turned out that radio galaxies were asymmetric. They have a single jet coming from the bright center. This discovery formed the basis for the study of the active nuclei of jet galaxies. But scientists soon identified a subclass of unusual objects, compact symmetric objects (CSOs), small active galaxies with two jets. And finally the researchers managed to explain the nature of these radio sources.

Larger galaxies such as Cygnus A and NGC 1052 also have bidirectional jets. Therefore, small SWRs were assumed to be the cores of galaxies with large jets, the jets gradually growing larger. There was also a version that the growth of jets stopped the surrounding dense space environment.

However, astronomer Anthony Readhead, who found the first evidence of SWR symmetry in the 1980s, studied the spectra of three such objects and hypothesized that it was an entirely new class of radio sources. Two versions of their nature emerged: either ancient, slowly rotating objects, or very short-lived. He soon hypothesized that SWRs were “fed” by tidal collapse events, that is, debris from a star that came too close to a supermassive black hole.

Unfortunately, testing these hypotheses required a set of observational data that were not available at the time. In 2020, Anthony Readhead, now a professor of astronomy at Caltech University (USA), gathered his colleagues with a proposal to return to the question and test hypotheses.

For two years, the group examined all published scientific papers on the subject to find candidates for compact symmetric objects. They selected 79 real CSRs from 3,125 sources, and 15 of them were verified for the first time. The researchers also identified 167 high-quality CSR candidates that are currently being monitored. And more than a thousand “second-rate” candidates that the group plans to check later. In the coming years, scientists hope to expand the list of approved CSRs at least three times.

The reason for this decrease from three thousand candidates to 79 KSS is simple. Candidates recorded these objects according to two parameters: the presence of bidirectional jet signatures and their size less than one kiloparsec. The authors of the new study put forward two additional requirements noted during the initial description of these objects: low variability (high stability) and low jet speed. The scientists described the results of this phase of work in the first of three articles published in the journal. Astrophysical Journal.

In the second publication, the authors took a closer look at the selected CSRs. A few years ago scientists showed that there are two classes of SWRs that differ in the way they emit: the brightest radiation in the core (SWR 1s) and the emission peak at the end of the jet, apparently where it collides. with intergalactic matter cluster (CSR) 2s). It turns out that only one-fifth of compact symmetrical objects account for 1s CSR. Scholars focused on KSS 2s.

After comparing their properties (size, redshift and other parameters), the authors of the new paper concluded that more than 99 percent of these objects are absolutely not capable of evolving into large radio sources. Their size does not exceed 1.6 thousand light-years (500 parsecs), and the duration of existence of such jets is limited to five thousand Earth years, which is extremely short by cosmic standards. For comparison: Cygnus A’s jets reach a length of 230 thousand light-years and last tens of millions of years.

“These KSSs are not young at all. You wouldn’t call a 12-year-old dog a youngster, even though it lives far fewer years than an adult. “These are different types of dogs that live and die over thousands of years rather than millions of years, as is usually the case near galaxies with larger jets,” Readhead said. “They are kind of objects,” he explained.

Finally, in the third paper, scientists showed that the nature of such radio galaxies can be explained by catastrophic events. By comparing different VSWRs, the researchers were able to reconstruct the evolution of the jets and divide the VSWR 2s into three evolutionary subclasses. The authors explained the extraordinary “survival” of the jets by the possible size of the destroyed stars. Although they explained in the broadcast that other jet formation scenarios cannot be ruled out at this stage. For example, instability in the accretion disk around a black hole.

“We think that when a star breaks apart, all the energy goes into jets along the black hole’s spin axis. At first we can’t see the giant black hole, but when it swallows the star, boom! “The black hole is taking in fuel and we can see it,” Readhead added.

Compact symmetric objects are of interest not only as an independent type of radio galaxy, but also as “laboratories” for studying the behavior of active galactic nuclei, supermassive black holes, their accretion disks and mechanisms of emergence of relativistic jets. New sky surveys with more sensitive instruments will allow finding fainter SWRs.