There are hundreds of mysterious filaments at the center of the Milky Way

By | June 6, 2023

Several million years ago, the core of our galaxy experienced a powerful event. A huge bilobed bubble exploded and blasted through the interstellar medium in two directions. Whatever it was, it released massive amounts of energy from the central supermassive black hole, Sagittarius A* (Sgr A* for short).

These bubbles contain strange one-dimensional vertical filaments that emit radio signals of their own. In 1984, astronomer Farhad Yusef-Zadeh of Northwestern University and his collaborators were the first to observe these thin contrails. Now they know more than a thousand. They emit radio waves in the microwave portion of the electromagnetic spectrum.

MeerKAT image of the galactic center with color-coded position angles of the vertical filaments.  Courtesy of Farhad Yusef-Zadeh/Northwestern University
MeerKAT image of the galactic center with color-coded position angles of the vertical filaments. Courtesy of Farhad Yusef-Zadeh/Northwestern University

Wait, there’s more!

Recently, the team found a smaller population of filaments near Sgr A*. However, these are not verticals. Instead, they lie along the galactic plane and radiate out like the spokes of a wheel. It was a surprise to suddenly find a new population of structures that appear to point in the direction of the black hole, Yusef-Zadeh said. We have always thought about vertical filaments and their origin. I’m used to being vertical. I never thought there might be more along the plane.

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He and the team were amazed when they saw these radially elongated structures. “We had to work very hard to establish that we weren’t kidding each other,” he said. “And we found that these filaments are not random, but appear to be related to the outflow from our black hole. By studying them, we could learn more about the spin of black holes and the orientation of the accretion disk. It is satisfying when order is found in the midst of a chaotic field of our galaxy’s core.

Yusef-Zadeh pointed out that something clearly occurred in the nucleus to create the short radial filaments. We think they must have originated with some sort of runoff from an activity that happened a few million years ago, he said. It appears to be the result of an interaction of that outgoing material with nearby objects.

MeerKAT image of the galactic center with color-coded position angles of the short radial filaments.  Courtesy of: Farhad Yusef-Zadeh/Northwestern University
MeerKAT image of the galactic center with color-coded position angles of the short radial filaments. Courtesy of: Farhad Yusef-Zadeh/Northwestern University

Pin the stories of the filaments

Like other supermassive black holes, Sgr A* is known to occasionally gobble up material. Then, an energetic, highly magnetized explosion of energy and plasma “explodes”. This seems to be the most likely explanation for the appearance of the bubbles and strings. It is possible that such a jet collided with clouds of gas and dust in the immediate vicinity. It may have led to twisted magnetic structures that form the filaments.

Another idea is that the molecular gas was stretched by the expansion of material near the core. This somehow creates the radial filaments. Such an action would also explain the asymmetrical appearance of the Sgr E complex. It lies on one side of the black hole at the intersection of the central molecular zone and a broad central dust swath. Finally, it’s also possible that the gravitational potential of our galaxy’s central bar is pulling the Sgr E cloud along. This could play a role in the formation of those filaments.

A schematic diagram of the outflow from Sagittarius A* showing the direction of the radial filaments.  Courtesy of: Farhad Yusef-Zadeh/Northwestern University
A schematic diagram of the outflow from Sagittarius A* showing the direction of the radial filaments. Courtesy of: Farhad Yusef-Zadeh/Northwestern University

Vertical ones present a different challenge to understand. They tower up to 150 light-years above the plane of the galaxy. Yusef-Zadeh and his team suggest a couple of ideas for their existence. The first is cosmic ray pressure forcing the filaments into a vertical orientation. That pressure is the result of the explosive event a few million years ago that created the bubbles.

Some verticals could be the result of the interaction of a large-scale wind in the region and obstacles embedded in the flow. This combination creates filaments by wrapping the wind’s magnetic field around obstacles. Clearly, there is a lot of observational and analytical work to be done to really understand what is going to create both sets of strands.

Different types of filaments, Milky Way style

Their origins likely explain the radically different orientations and characteristics of each filament population. The vertical filaments are perpendicular to the plane of the galaxy and extend up to about 150 light-years in length, while the horizontal ones are parallel to the plane and are only 5 to 10 light-years long. The horizontals point radially towards the center of the galaxy where Sgr A* is located. The vertical ones are magnetic and relativistic (meaning that their particles move close to the speed of light). The horizontal filaments appear to emit thermal (hot) radiation and appear to accelerate the heated material into a molecular cloud. There are only a few hundred of these horizontal filaments seen so far.

Vertical filaments appear around the core of the galaxy while horizontal filaments extend only to one side. According to Yusef-Zadeh, radial outflows are somewhat asymmetric. This could provide some clues about the black hole region itself. “One of the most important implications of the radial outflow that we have detected is the orientation of the accretion disk and jet-driven outflow from Sagittarius A* along the galactic plane,” he said.

Looking at the filaments over time

While the radial filaments found by Yusef-Zadeh and his team are a more recent discovery, the vertical ones are old friends. There are almost a thousand of them grouped in pairs and groups. Yusef-Zadeh says they are about 6 million years old.

He has been studying strange structures in the galactic core for decades. In 1984, he worked with astronomers Mark Morris and Don Chance to make a Very Large Array radio map of the galactic center in the 20cm wavelength band when they first discovered the vertical structures. The emissions they detected came from gas that does not emit heat (i.e. cold) arranged along magnetic structures.

A video on the discovery of the radio bubble coming from the center of the Milky Way. Courtesy: Northwestern University.

Later, in 2019, Yusef-Zadeh was part of a team led by astronomer Ian Haywood (University of Oxford) that used the MeerKAT radio observatory facility in South Africa to discover two giant radio-emitting bubbles near Sgr A *. That huge burst of energetic activity described earlier created those bubbles several million years ago.

At the time, the team immediately suspected that Sgr A* was the source of the hourglass-shaped structure. The center of our galaxy is relatively calm compared to other galaxies with very active central black holes, Heywood said. Even so, the Milky Way’s central black hole can become unusually active from time to time, flaring up as it periodically gobbles up massive clumps of dust and gas. It is possible that such a feeding frenzy triggered powerful explosions that inflated this previously unseen feature.

MeerKAT analyzes bubbles

The discovery of radio bubbles in the galactic core a few years ago highlighted the great capabilities of the MeerKAT radio telescope. It is an array of 64 radio antennas sensitive to microwave emissions, ranging from wavelengths between 3 and 30 centimetres. Yusef-Zadeh credits the facility with helping his team spot the filaments against a busy background of other emitters. They used a technique to remove background and smooth out noise from MeerKAT images in order to isolate the filaments from surrounding structures.

MeerKAT’s new observations were a game changer, he said. The advancement of technology and the time devoted to observation have provided us with new information. It is really a technical achievement of radio astronomers.

For more information

Mysterious Lines Revealed in the Milky Ways Center
Galactic Center Filament Population: Position Angle Distribution Reveals Collimated Degree-Scale Outflow from Sgr A* Along the Galactic Plane
Gigantic balloon-like structures discovered at the center of the Milky Way
MeerKAT radio telescope

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