Astronomers have observed a rare case of a galaxy changing shape.


A few decades ago, an object located about 630 million light-years away named PBC J2333.9-2343 was classified as a giant radio galaxy. It projected large, radio-emitting structures perpendicular to our line of sight, formed by colossal jets that once erupted from the galactic center.

However, more recent observations reveal that the galaxy’s core has turned on again and is now pointing its jet directly at us.

It’s nothing to worry about; in fact, it is quite common. So common that we actually have a name for it; a blazer. With its new classification, the blazar PBC J2333.9-2343 can give us a deeper understanding of how galaxies can transform, even on human time scales.


Galaxies come in a variety of shapes and sizes, but they also have different activity levels based on the activity of the supermassive black holes in their cores. The Milky Way, for example, is a relatively peaceful galaxy; our supermassive black hole is fairly inactive and only collects a small amount of material.

A supermassive black hole positively sucking up dust and gas from its surrounding space looks very different. That material forms a torus and a disc that circles the black hole; the extreme gravitational and frictional forces at play cause the disc to blaze with light across the electromagnetic spectrum.

From the inner edge of the disc, material falls onto the black hole, which water swirling down a drain. But all that material does not end up beyond the event horizon. Some of it is channeled away and accelerated along magnetic field lines outside the black hole. When it reaches the poles, this material is ejected into space at tremendous speed and forms beams of plasma which erupts into space at a significant fraction of the speed of light in a vacuum.

An image of PBC J2333.9-2343 obtained with Pan-STARRS. (Institute for Astronomy at the University of Hawaii)

When the black hole finishes its meal and goes quiet again, what remains of these jets can continue to travel through space and spread out into lobes that continue to emit radio waves. These are known as giant radio galaxies, and they can be colossal. PBC J2333.9-2343 has such radio lobes, evidence of past black hole activity, spanning a total distance of 3.9 million light years.

But the galaxy has shown strange behavior at different wavelengths, leading a team of astronomers led by astrophysicist Lorena Hernández-García of the Millennium Institute of Astrophysics in Chile to believe that PBC J2333.9-2343 may now be a blazar. The published a paper presenting their arguments in 2017and now they have obtained observational evidence to match.

“We started studying this galaxy because it showed special properties,” Hernández-García explains. “Our hypothesis was that the relativistic jet from its supermassive black hole had changed its direction, and to confirm that idea we had to perform many observations.”

The research team conducted an extremely thorough survey, collecting observations in radio, infrared, optical, ultraviolet, X-ray and gamma ray wavelengths. They then compared their observational data against a large database of blazar and non-blazar galaxies.

The results showed that J2333.9-2343’s properties are more consistent with blazar galaxies, suggesting that the galaxy has somehow been dramatically reoriented by up to 90 degrees so that its black hole is pointing one of its jets in our direction.

“The fact that we see that the core is no longer feeding the lobes means that they are very old,” says Hernández-García. “They are relics of past activity, while the structures closer to the core represent younger and active jets.”

How the black hole could have changed position so dramatically is still unknown. There is a distinct lack of activity detected between the lobes and the galaxy, suggesting that the black hole was knocked sideways during a major event, such as a collision and merger with another galaxy.

This, in turn, could mean that we are looking for the first time at what the researchers call a “very exceptional case of jet reorientation”, transforming J2333.9-2343 and leading to its reclassification from a giant radio galaxy to a blazar. .

The research has been published in Monthly Notices of the Royal Astronomical Society.

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