Mysterious filaments of the Milky Way point to the central black hole

By | June 6, 2023

Milky Way filaments: smear of hundreds of mostly radial, thin, colored streaks of different lengths (shorter).
View larger. | A new view into the heart of our Milky Way galaxy, through the MeerKAT radio telescope in South Africa. The color coding shows the position angles of the Milky Way’s mysterious filaments, which can be seen expanding like the spokes of a wheel from the central supermassive black hole of our galaxy, Sagittarius A*. Image via MeerKAT/ Northwestern University.

Northwestern University published this story on June 2, 2023. EarthSky edits.

Mysterious filaments of the Milky Way

We cannot see the center of our galaxy, the Milky Way, in visible light. Located about 25,000 light-years from Earth, it is hidden from view by large clouds of gas and dust. But probes of the galaxy at other wavelengths have revealed a central supermassive black hole, called Sagittarius A* or Sag A* (pronounced Sag A-star). The black hole has about 4 million times the mass of our Sun.

And, in the early 1980s, astronomer Farhad Yusef-Zadeh of Northwestern University in Illinois used radio telescopes to discover giant one-dimensional filaments dangling vertically near Sag A*. And on Friday (June 2, 2023), Northwestern announced that Yusef-Zadeh has now seen something else. He’s spied on hundreds of filaments along the galactic plane threads, visible at radio wavelengths, measuring 5 to 10 light-years in length. These threads spread out like the spokes of a black hole wheel. Yusef-Zadeh commented:

I was truly amazed when I saw these.

The new population of filaments or threads is much shorter than the one first discovered by Yusef-Zadeh in the 1980s. Yusef-Zadeh and his collaborators believe the structures likely originated a few million years ago when outflow from our supermassive black hole interacted with surrounding materials. Their statement explained:

Although the two filament populations share several similarities, Yusef-Zadeh assumes they have different origins. While the vertical filaments criss-cross the galaxy, towering up to 150 light-years high, the horizontal filaments look more like the dots and dashes of Morse code, dotting only one side of Sagittarius A*.

The letters from the astrophysicist diary released the new study on June 2. The study is titled Galactic Center Filament Population: Position Angle Distribution Reveals Collimated Degree-Scale Outflow from Sgr A* Along the Galactic Plane.

It was a surprise

Yusef-Zadeh explained in a statement:

It was a surprise to suddenly find a new population of structures that appear to point in the direction of the black hole. I was truly amazed when I saw these. We had to work hard to establish that we weren’t kidding ourselves.

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.

Decades in the making

This study builds on four decades of research by Yusef-Zadeh. After first discovering the vertical filaments in 1984 with Mark Morris and Don Chance, Yusef-Zadeh along with Ian Heywood and their collaborators subsequently discovered two giant radio-emitting bubbles near Sagittarius A*. Then, in a series of publications in 2022, Yusef-Zadeh (in collaboration with Heywood, Richard Arent and Mark Wardle) revealed nearly 1,000 vertical filaments, which appeared in pairs and groups, often stacked evenly or side by side like strings on a harp.

Yusef-Zadeh attributes the flood of new discoveries to advanced radio astronomy technology, particularly the MeerKAT telescope at the South African Radio Astronomy Observatories (SARAO). To locate the filaments, Yusef-Zadeh’s team used a technique to remove background and smooth out the noise from MeerKAT images in order to isolate the filaments from surrounding structures. He commented:

The new MeerKAT observations were a game changer. 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.

Horizontal filaments vs vertical filaments

After studying the vertical filaments for decades, Yusef-Zadeh was shocked to discover their horizontal counterparts, which he says are around 6 million years old. He 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.

While both the vertical and horizontal populations comprise one-dimensional filaments that can be visualized with radio waves and appear to be related to activities in the galactic center, the similarities end there.

The vertical filaments are perpendicular to the galactic plane. The horizontal filaments are parallel to the plane but point radially towards the center of the galaxy where the black hole is located. The vertical filaments are magnetic and relativistic. The horizontal filaments appear to emit thermal radiation. The vertical filaments enclose particles moving at speeds close to the speed of light. The horizontal filaments appear to accelerate the thermal material into a molecular cloud.

There are several hundred vertical filaments and only a few hundred horizontal filaments.

And the vertical filaments, which measure up to 150 light-years in height, far outnumber the horizontal filaments, which measure only 5 to 10 light-years in length. Vertical filaments also adorn the space around the galaxy’s core; the horizontal filaments appear to spread out to one side only, pointing towards the black hole. Yusef Zadeh said:

One of the most important implications of the radial outflow we have detected is the orientation of the accretion disk and jet outflow from Sagittarius A* along the galactic plane.

Our work is never complete

The new discovery is full of unknowns, and Yusef-Zadeh’s work to unravel its mysteries has only just begun. For now, he can only consider a plausible explanation of the mechanisms and origins of new populations. He said:

We think they must have originated with some sort of runoff from an activity that occurred a few million years ago. It appears to be the result of an interaction of that outgoing material with nearby objects. Our work is never complete. We always need to make new observations and continually challenge our ideas and strengthen our analysis.

Bottom line: The mysterious filaments of the Milky Way — found by the MeerKAT telescope in South Africa — appear to point to the supermassive black hole at the heart of our galaxy.

Source: Galactic Center Filament Population: Position angle distribution reveals collimated degree-scale outflow from Sgr A* along the galactic plane

Through Northwestern University

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