The search for mysterious streaks hanging in the sky around the heart of the Milky Way has found new people, connecting the edge of the galactic plane and pointing to the intergalactic space.
The magnetic strings are the remnants of the Sagittarius A* black hole that interacted with the surrounding atmosphere millions of years ago, says astronomer Farhad Yusef-Zadeh of Northwestern University.
Although Sgr A* is quiet now, these remnants suggest that the center of our galaxy has been active in the recent past – in astronomical times, that is. And their discovery also means that the center of our galaxy, which is as wild and woolly as we know it, has some fascinating secrets within it.
“It was surprising to suddenly find new people who seem to be pointing towards the black hole,” says Yusef-Zadeh.
“I was very surprised when I saw this. We had to do a lot of work to make sure that we are not fooling ourselves. And we found that these galaxies are not random but seem to be tied to the outflow of our black hole. By studying them, we can learn more about the rotation of the black hole and the accretion disk orientation. It becomes satisfying when one finds order in the midst of the chaos at the center of our galaxy.”
Floating filaments in the center of the galaxy are nothing new. In fact, Yusuf-Zadeh and two colleagues found it in the 1980s – about 1,000 long, vertical, magnetic up to a height of 150 light years, and hanging in a strange order, like strings of harps. This could be the result of wind blowing from a supermassive black hole, or interstellar turbulence, driven by the motion of galaxies.
The new population was discovered in data collected by the MeerKAT radio telescope in South Africa. Yusuf-Zadeh and his team were cleaning the data, removing the background so that the vertical filaments could be seen, when something popped up.
Something was the new ‘harp strings’ category.
“We’ve always thought about vertical threads and their origins,” says Yusuf-Zadeh. “I’m used to standing. I never thought there would be others on the plane.”
And discovery is another thing similar to verticals. These new structures are more like lines than lines: short, hundreds of objects around 5 to 10 light-years in length, and spread horizontally along the plane of the galaxy, rather than straight like other filaments.
Although both are magnetic, vertical ones accelerate particles near the speed of light, while newly discovered horizontal ones appear to emit thermal radiation.
They are also arranged around one side of the galaxy’s center, pointing back towards Sgr A*, compared to the corresponding vertical arrangement, rotated around the galaxy. This radial configuration also appears to be consistent with the shape of Sgr A*. It seems to point, not at the black hole, but at the radial outflow driven by the astrophysical jets that erupt from around the black hole as it accrets.
“One of the most important aspects of the radial outflow that we have detected is the accretion disk and the outflow of the jet from Sagittarius A* along the edge of the galactic plane,” says Yusuf-Zadeh.
“Active”, in the case of a supermassive black hole, means that matter is falling, or accreting, from a massive disk of matter that is spinning around like water around a tunnel. But not all matter can survive beyond the core of a black hole. Some of it is deflected and propelled along magnetic lines toward the poles of the black hole, where it is ejected into space at the same speed as a space plane.
There are also other signs that Sgr A* has recently fired its jets, such as large bubbles that extend far from the top and bottom of the jet. The radial dashes, according to Yusuf-Zadeh and colleagues, could be the result of the punching speed created by the jet outflow from Sgr A*. And the analysis of their size and position shows that this happened 6 million years ago.
“We think they came from things that happened several million years ago,” says Yusef-Zadeh. “It appears to be the result of the interaction of the outgoing material with the material around it.”
Since new structures seem to be growing as we build and refine the technology to detect them, we are not far from knowing the entire history of the center of our Milky Way.
“Our work is not over,” adds Yusif-Zadeh. “We must always look for new things and challenge our assumptions and harden our analysis.”
Research has been published in Astrophysical Journal Letters.
