The most active and greedy black holes in the universe are often found with two jets of matter coming from their center. These jets are accelerating with astonishing speed through space in opposite directions, and they are usually aligned along the axis of the spinning black hole. But not always.
Some of these supermassive black galaxy cores, called active galactic nuclei, have jets bent at mysteriously odd angles. New research from astronomers at the University of Wisconsin-Madison, published recently in The Astronomical Journal, shows that these jets are likely bent by a combination of their galaxies moving at tremendous speed and by the drag on the jets when they pass through clouds of intergalactic gas.
“These active galactic nuclei are a subset of black holes that — even for black holes — gobble up an enormous amount of matter very quickly,” says Melissa Morris, a UW-Madison graduate student in astronomy and lead author of the new study. “They’re powered up so quickly that a ton of energy is released into the area around the black hole. That’s what’s causing these wild AGN jets.
Understanding the environment that shapes the direction of the jets helps astronomers understand how galaxies evolve, but how matter is thrown away from a black hole remains an open question. Supermassive black holes are found at the center of almost all large galaxies.
But whatever the formation of the jets, astronomers find them useful. Incredibly hot plasma in the jets produces radio waves that astronomers can observe extending far beyond their galaxies, pointing to the centers of the galaxies that produce them like helpful arrows on an otherwise blank map.
“It’s not always easy to see things in space. Sometimes you have to get creative,” says Morris. “The fact that we can ‘see’ these jets – we can pick up the radio waves they emit – means that we can also see them interacting with things that exist outside the galaxy.”
This makes the cause of the bent jets particularly interesting, and why UW-Madison researchers – including astronomy professors Eric Wilcots and Sebastian Heinz and scientist Eric Hooper – have sought to find out if something in the environment galaxies pushed the jets askew.
“The fact that we can see them bend probably means we can infer something about their environment,” says Morris. “But can we be sure?
Using extensive 3D maps of the universe, the Sloan Digital Sky Survey and the FIRST Radio Survey, Morris has collected a list of 160 galaxies with powerful radio-emitting jets that are also bent. From other star catalogs, she constructed a pool of galaxies with unfolded jets for comparison.
For each galaxy, astronomers used a technique called the buddy-of-buddy algorithm to identify each nearby galaxy within a certain search radius, then each neighbor of each of those galaxies within the radius, repeating the hunt until there are no more neighbours. in the chain.
A nearby cluster of galaxies is more likely to be thick with gas and dust, the kind of dense environment astronomers expected could push an AGN jet slightly off axis. Indeed, AGNs with unfolded jets were more likely to be found alone or associated with just one other galaxy. AGNs with bent jets were more likely to be found mixed with groups of three or more galaxies.
An AGN zooming through space at high speed is also likely to have jets that appear bent, as the ends flow out and appear to trail behind the moving galaxy core. So astronomers also compared the relative brightness of the galaxies they studied. Brighter galaxies are generally more massive. The largest galaxy in a cluster is the anchor of the group, sitting at the center of the interactions of the members of the cluster. Darker and lighter galaxies would be pulled faster into their vicinity.
“The brightest galaxy in a cluster is sort of the boss, with the other galaxies in greater motion around it,” Morris says. “And we saw that over 90% of our inflexible AGNs in groups were the brightest in the cluster. They probably aren’t moving fast enough to make their jets look bent.
It’s likely a combination of gas-laden surrounding space and high velocity that puts a bend in an AGN’s jets.
“It’s a balancing act,” says Morris. “If you have a super dense medium to go through, it wouldn’t take a ton of speed to do curved jets. And galaxies moving at high speed would not need so much gas to have an AGN with a bent jet.
Researchers continue to unravel the relative influence of different conditions.
“We don’t see a single galaxy changing over time. It takes billions and billions of years,” says Morris. “So the better we can determine what made certain types of galaxies different from others, the better we can understand how they evolve and interact.”
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