The classification of a galaxy is typically determined by its morphological features, such as its shape, size, and structure. These features can be used to categorize galaxies into different types, such as elliptical, spiral, and irregular. The direction of a galaxy’s jet, on the other hand, is typically determined by the orientation of its central supermassive black hole and its accretion disk. The jet itself is a narrow stream of material that is emitted from the black hole at relativistic speeds.
A group of international astronomers has discovered a galaxy that has been classified differently due to unusual activity within its core. PBC J2333.9-2343, the galaxy, was previously classified as a radio galaxy, but new research has revealed otherwise. The paper was published in the Royal Astronomical Society’s Monthly Notices.
PBC J2333.9-2343, which is 656 844 372 light years away, has now been classified as a giant radio galaxy with a blazar in its core; a blazar is an active galactic nucleus (AGN) with a relativistic jet (a jet moving close to the speed of light) directed towards an observer. Blazars are extremely high-energy objects that are thought to be among the most powerful phenomena in the Universe. According to the research, in PBC J2333.9-2343, the jet changed its direction by up to 90 degrees, going from being in the plane of the sky, perpendicular to our line of sight, to pointing directly at us.
We began investigating this galaxy because it displayed unusual properties. Our hypothesis was that the relativistic jet of its supermassive black hole had changed direction, and we needed to conduct a large number of observations to confirm that idea.
Dr Lorena Hernández-Garca
A blazar jet is composed of elemental charged particles such as electrons or protons that travel at near-light speeds. These spin in circles around a strong magnetic field, emitting radiation across the entire electromagnetic spectrum. The jet in PBC J2333.9-2343 is thought to originate from or near the supermassive black hole at its center.
With the jet pointing in our direction, the emission is greatly amplified and can easily outstrip that of the rest of the galaxy. This causes high-intensity flares that are stronger than those from other radio galaxies, changing its classification.
The orientation of the jets to us determines how a galaxy is classified. When two jets point towards the plane of the sky, they are classified as a radio galaxy, but if one of the jets points towards us, then the AGN of the galaxy is known as a blazar. With jets in the plane of the sky and one directed at us, PBC J2333.9-2343 has been reclassified as a radio galaxy with a blazar at its centre.
Changes in the direction of jets have been described in the past, for example with X-shaped radio galaxies. This is the first time that such a phenomenon has been observed where it does not suggest the presence of two different phases of jet activity from its morphology observed at radio frequencies — the direction change appears to have taken place in the same nuclear outburst originating from the AGN.
Astronomers had to observe this mysterious galaxy across a wide range of the electromagnetic spectrum to learn more about it. Radio, optical, infrared, x-ray, ultraviolet, and gamma-ray telescopes were used to observe PBC J2333.9-2343. Data was obtained from the Max Planck Institute for Radio Astronomy’s German 100m-Radio Telescope Effelsberg, the Yale University 1.3m-SMARTS optical telescope, and the Penn State Neil Gehrels Swift Observatory.
The researchers then compared the properties of PBC J2333.9-2343 to data from the ALeRCE (Automatic Learning for the Rapid Classification of Events) project in Chile, as well as data from the Zwicky Transient Facility (ZTF) and the Asteroid Terrestrial-impact Last Alert System (ATLAS).
The team concluded from the observational data that this galaxy has a bright blazar in the center and two lobes in the outer areas of the jet. The observed lobes are related to old jets and are no longer fed by nucleus emission, implying that they are relics of past radioactivity. As seen in typical radio galaxies, the AGN no longer drives the lobes.
The team is still unsure what caused the jets’ abrupt change of course. They speculate that it could have been a merging event with another galaxy or any other relatively large object, or it could have been a strong burst of activity in the galactic nucleus following a dormant period.
Dr Lorena Hernández-Garca, the paper’s lead author and a researcher at the Millenium Institute of Astrophysics, says “We began investigating this galaxy because it displayed unusual properties. Our hypothesis was that the relativistic jet of its supermassive black hole had changed direction, and we needed to conduct a large number of observations to confirm that idea.”
She continues, “The fact that the nucleus is no longer feeding the lobes indicates that they are very old. They are remnants of previous activity, whereas the structures closer to the nucleus represent younger and more active jets.”
