An investigation into the mystery filaments hanging in space around the heart of the Milky Way has turned up an entirely new population of them, aligned along the galactic plane and pointing in the direction of the galactic center.
The magnetized strands are likely the remnants of an outflow from the supermassive black hole Sagittarius A* interacting with the surrounding gas a few million years ago, says astrophysicist Farhad Yusef-Zadeh of Northwestern University.
Although Sgr A* is pretty quiet now, these remnants suggest that our galaxy's center has been active recently – on cosmic timescales, that is. And their discovery also means that our galaxy's center, as wild and wooly as we already knew it to be, has more fascinating secrets lurking within.
"It was a surprise to suddenly find a new population of structures that seem to be pointing in the direction of the black hole," Yusef-Zadeh says.
"I was actually stunned when I saw these. We had to do a lot of work to establish that we weren't fooling ourselves. And we found that these filaments are not random but appear to be tied to the outflow of our black hole. By studying them, we could learn more about the black hole's spin and accretion disk orientation. It is satisfying when one finds order in the middle of a chaotic field of the nucleus of our galaxy."
Filaments floating about the galactic center aren't a novel finding. In fact, Yusuf-Zadeh with two of his colleagues discovered them in the 1980s – around 1,000 long, vertical, magnetic structures up to around 150 light-years in length, and hanging in surprisingly orderly arrangements, like harp strings. These could be the result of winds gusting from an active supermassive black hole, or turbulence in the intergalactic medium, stirred 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 up the data, removing the background to make the vertical filaments more visible, when something else emerged.
That something was a new population of galactic 'harp strings'.
"We have always been thinking about vertical filaments and their origin," Yusuf-Zadeh says. "I'm used to them being vertical. I never considered there might be others along the plane."
And the discovery is something else entirely compared to the vertical ones. The new structures are more like dashes than lines: short, hundreds of structures around 5 to 10 light-years in length, and laying horizontally along the galactic plane, rather than vertically like the other filaments.
Although all of the structures are magnetized, the vertical ones accelerate particles to near light speed, while the newly discovered horizontal ones appear to emit thermal radiation.
They're also radially arranged just on one side of the galactic center, pointing back towards Sgr A*, compared to the parallel arrangement of the vertical ones, arrayed all around the galactic center. This radial arrangement also seems linked to the orientation of Sgr A*. It seems to point, not just at the black hole, but at a radial outflow driven by the astrophysical jets that erupt from around a black hole when it is actively accreting material.
"One of the most important implications of radial outflow that we have detected is the orientation of the accretion disk and the jet-driven outflow from Sagittarius A* along the galactic plane," Yusuf-Zadeh says.
"Active", for a supermassive black hole, means that material is falling, or accreting, onto it from a huge disk of material that swirls around it like water around a drain. But not all the material ends up beyond the black hole's event horizon. Some of it is diverted and accelerated along magnetic field lines to the black hole's poles, where it is launched into space at high speeds in the form of an astrophysical jet.
There are other signs Sgr A* fired up its jets in the relatively recent past, such as giant bubbles that extend vast distances above and below the galactic plane. The radial dashes, according to Yusuf-Zadeh and his colleagues, could be the result of ram-pressure produced by a jet-driven outflow from Sgr A*. And analysis of their extent and position suggest that this took place around 6 million years ago.
"We think they must have originated with some kind of outflow from an activity that happened a few million years ago," Yusef-Zadeh says. "It seems to be the result of an interaction of that outflowing material with objects near it."
Given new structures seem to be emerging as we build and refine the technology to detect them, we're far from knowing the complete history and dynamics of the center our our Milky Way.
"Our work is never complete," Yusif-Zadeh adds. "We always need to make new observations and continually challenge our ideas and tighten up our analysis."
The research has been published in The Astrophysical Journal Letters.