Black Hole's Colossal Jets Pierce the Cosmic Void

        From - Sky & Telescope

        By  -     Monica Young

       Edited by - Amal Udawatta

 

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A pair of plasma jets powered by a supermassive black hole span far beyond their host galaxy — potentially affecting the cosmic web around it.

The supermassive black hole at the center of a massive galaxy has powered a giant pair of plasma jets, spanning 23 million light-years long from tip to tip. That’s almost 10 times the distance between our Milky Way and the Andromeda Galaxy.

Astronomers have nicknamed the record-breaking system Porphyrion, after the king of the giants in Greek mythology. And the giant is indeed the king of many: A new catalog of such radio-emitting giant jet pairs tallies more than 11,000 of them, each spanning more than 2.3 million light-years.

“Giant jets were known before we started the campaign, but we had no idea that there would turn out to be so many,” says Martin Hardcastle (University of Hertfordshire, UK), coauthor on two associated studies. Porphyrion’s discovery is published in Nature; the accompanying catalog will appear in Astronomy & Astrophysics.

The Long Escape

The recently cataloged giant jets as well as their smaller brethren come from supermassive black holes. These black holes rest at the center of galaxies, drawing in hot gas, or plasma, around themselves. As the plasma swirls in, some of it escapes in the form of a pair of narrow jets. Powered by the black hole itself, they shoot out along the black holes’ poles at nearly the speed of light.

But because that plasma is moving much faster than the gas around the galaxy, its very speed can be its end.

Just as wind moving over water generates waves — a phenomenon known as the Kelvin-Helmholtz instability — the denser jet material speeding alongside the sparser, slower gas outside it creates similar wave-like billows. Such waves are common in the universe, not only in clouds and oceans on Earth, but also in Saturn’s bands and the solar corona. Along a narrow jet extending hundreds of thousands of light-years, the phenomenon can slow the flow or even disrupt it entirely.

Yet when astronomers went looking for black hole-powered jets in a radio survey by the Low-Frequency Array (LOFAR), located primarily in the Netherlands, they found giant jets by the dozens, then by the thousands. “Giant” here has a technical definition: these jets’ tip-to-tip length as projected on the sky span 23 million light-years or more.

Rafaël Mostert (Leiden University) led a team in cataloging these jets, combining the power of citizen-science with machine learning to find more than 11,000 giant radio jets in LOFAR’s Two-meter Survey. Martijn Oei (also at Leiden) led the study on the largest of these, Porphyrion.

This picture, taken by the LOFAR radio telescope, shows the longest-known pair of black hole jets, spanning 23 million light-years. The galaxy hosting the supermassive black hole is a dot in the center of the image. (The largest blob-like structure near the center is a separate smaller jet system.)
LOFAR Collaboration / Martijn Oei (Caltech)

“Although [Porphyrion] is certainly an extreme and curious case, the reason why [its] jets are so long is related with the position of the galaxy in the cosmic web,” explains Manel Perucho (University of Valencia, Spain), who was not involved in the study.

We’re seeing the jet and its massive host galaxy as they were when the universe was half its current age and, generally, about 10 times denser than it is today. But the environment around this galaxy is unusually sparse. As the jets exit the galaxy, they penetrate millions of light-years into a cosmic void, where the galaxies and even the gas between galaxies is much more spread out. There, the Kelvin-Helmholtz effect is weaker, Perucho says:  “With these ingredients, a long and fantastic trip is ensured.”

The circumstances are unusual not only in the surroundings but also in the black hole itself. “My interpretation is that we need an unusually long-lived and stable accretion event around the central, supermassive black hole to allow it to be active for so long — about a billion years,” Hardcastle says. What’s more, the jets have to be pointing in the same direction for the duration, so that means the black hole has wobbled very little on its spin axis during that time.

Into the Web and the Void

An artist's illustration of the longest black hole jet system ever observed, nicknamed Porphyrion. Porphyrion dates back to a time when our universe was less than half its present age. During

Porphyrion may be exceptional in itself, but giant jets are not. The team estimates, based on the number of giants in the catalog, that such jets are in fact relatively common in the universe. And given their extreme length, they might even affect the universe’s large-scale structure, known as the cosmic web.

Gas and dark matter gather along cosmic web filaments, flowing into galaxies that grow along these filaments as well as at nodes where filaments cross. Giant jets might slow that flow, especially in the early universe when filaments were closer together.

The jet plasma is moving so much faster than the material it’s traveling through that it heats anything it comes across, Perucho explains. Heated gas stays spread out, so it temporarily resists the gravitational pulls of galaxies and the larger filaments they reside in. “Therefore, all the particles that were shocked by the jet will stay within the intergalactic space longer than they would if not shocked.” Oei’s team calculates that Porphyrion’s jets are powerful enough to both heat and expand cosmic web filaments.

The team even speculates the magnetic fields carried by Porphyrion and its ilk might contribute to the origin of magnetism across large swaths of the universe, known as cosmic magnetogenesis. Such questions await further study.