'Space tornadoes' discovered at the center of our galaxy
Deep in the galaxy’s central molecular zone (CMZ), surrounding the supermassive black hole at the Milky Way’s center, clouds of dust and gas swirl amid energetic shock waves.
Now, a collaboration of international astronomers – using the Atacama Large Millimeter/submillimeter Array (ALMA) – has greatly sharpened our view of these processes by a factor of 100.
The team has uncovered an unexpected class of long, narrow filaments within this turbulent region, giving fresh insight into the cyclical formation and destruction of material in the CMZ.
Tracking shock-generated turbulence
Although researchers have long recognized the CMZ as a zone filled with ever-churning dust and gases, the driver behind these dynamic flows of matter had remained murky.
Molecules in molecular clouds serve as markers of different phenomena, with silicon monoxide (SiO) standing out as a tracer of shock waves.
By capitalizing on ALMA’s high sensitivity to identify multiple molecular emission lines, a team led by Kai Yang of Shanghai Jiao Tong University has managed to map these slender filaments at an unprecedentedly fine scale.
The findings reveal a striking interaction between shock-generated turbulence and the newly observed structures.
Observing a turbulent environment
Because the CMZ lies so close to our galaxy’s central black hole, it endures intense gravitational forces and frequent disturbances, making the environment especially dynamic.
The narrow filaments now identified confirm that shock waves play a considerable role, stirring and redistributing material.
“When we checked the ALMA images showing the outflows, we noticed these long and narrow filaments spatially offset from any star-forming regions,” Kai Yang explained.
“Unlike any objects we know, these filaments really surprised us. Since then, we have been pondering what they are.”
These “slim filaments” emerged unexpectedly in the SiO emission lines – together with lines from eight other molecules – and present velocities along our line of sight that do not align with other, previously known filaments or outflows.
On top of that, they display no correlation with dust emission and apparently lack hydrostatic equilibrium.
Space tornadoes and violent gas flow
“Our research contributes to the fascinating Galactic Center landscape by uncovering these slim filaments as an important part of material circulation,” said Xing Lu, a research professor at the Shanghai Astronomical Observatory and a corresponding author on the paper.
One interpretation is that these filaments act like channels for gas flow, and facilitate the integration or removal of shock-released molecules from the surrounding environment.
“We can envision these as space tornados: they are violent streams of gas, they dissipate quickly, and they distribute materials into the environment efficiently,” Lu added.
Determining exactly what triggers these filaments remains open to investigation, yet their presence points to a strong link with ongoing shock processes, according to the research team.
Based on several pieces of evidence – the detection of the SiO 5-4 rotational transition in ALMA data, the presence of CH3OH masers, and the abundances of complex organic molecules – the researchers inferred that these structures are shock-driven.
Each shock event appears to release SiO and other organic compounds, injecting them into the interstellar medium.
How space tornadoes emerge
Yichen Zhang is a professor at Shanghai Jiao Tong University and a corresponding author of the paper.
“ALMA’s high angular resolution and extraordinary sensitivity were essential to detect these molecular line emissions associated with the slim filaments, and to confirm that there is no association between these structures with dust emissions,” said Zhang.
“Our discovery marks a significant advancement, by detecting these filaments on a much finer 0.01 parsec scale to mark the working surface of these shocks.”
Understanding how these structures emerge offers a deeper look at CMZ dynamics. First, the hypothesized shock waves create filaments that emit certain molecular lines, distributing molecules such as CH3OH, CH3CN, and HC3N into the local environment.
Next, the filaments decay, releasing newly shock-released materials across broader regions.
Finally, the molecules, once again free in the gas phase, eventually re-freeze onto dust grains, preserving an equilibrium between depletion and replenishment in the CMZ.
If these slim filaments are as common as the sample suggests, this cyclical process might be significant for the broader region’s matter budget.
Vital conduits for recycling matter
“SiO is currently the only molecule that exclusively traces shocks, and the SiO 5-4 rotational transition is only detectable in shocked regions that have both relatively high densities and high temperatures,” Yang continued.
“This makes it a particularly valuable tool for tracing shock-induced processes in the dense regions of the CMZ.”
Such revelations cast the CMZ in a new light, highlighting shock waves as a central driver in reconfiguring dust, gas, and molecules.
By fueling cyclical changes in chemical composition, these “space tornadoes” presumably leave their imprint on the formation and dissolution of clouds that might spawn new stars – or even feed material into the supermassive black hole.
With future ALMA surveys of multiple SiO transitions and wide-ranging observations over the CMZ, astronomers aim to corroborate the filaments’ shock origin and map their potential distribution across this turbulent galactic region.
These efforts, combined with advanced simulations, may confirm or refine the emerging hypothesis that “slim filaments” serve as vital conduits for recycling matter in and around the Milky Way’s center, thus shaping the environment in which stars and black holes interact.
The study is published in the journal Astronomy & Astrophysics.
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