What happens when you mix clouds of gas and dust with energetic shock waves at the core of the Milky Way Galaxy? Space tornadoes. At least, that’s how researchers using the Atacama Large Millimeter Array (ALMA) in Chile to study the galaxy’s heart described what they found.
An international team of astronomers, led by Kai Yang of the Shanghai Jiao Tong University, focused the array on a region called the Central Molecular Zone (CMZ). That region of the Milky Way takes up several hundred parsecs of space surrounding Sagittarius A*. It’s got an irregular shape with a diameter of up to 1900 light-years.
The CMZ is a pretty extreme environment, rife with shock waves. It does have some star-forming activity, which produces small-scale outflows. The region receives material from the rest of the Galaxy as gas gets funneled in via the central bar. Gas in the CMZ is much denser, hotter, and more turbulent than molecular clouds in the rest of the disk. There are stronger magnetic fields there, as well as increased cosmic ray counts. Various regions in the zone exhibit high levels of emissions from interstellar molecules, including silicon oxide (SiO). Those emissions show up in millimeter-wavelength spectral images and look like “slim filaments”. They’re unrelated to any outflows in the region.
Tracing Activity in the CMZ
Yang’s team used ALMA to map the spectral lines of Si0 and other molecules that make up the filaments. They found some unexpected features. “When we checked the ALMA images showing the outflows, we noticed these long and narrow filaments spatially offset from any star-forming regions. Unlike any objects we know, these filaments really surprised us. Since then, we have been pondering what they are,” Yang said.
Slim filaments in the CMZ. Panel a is a MeerKAT view in 1.28 GHz radio emission of the Sgr A region. Blue boxes mark zoom-in regions where slim filaments are detected. Courtesy: Yang, et al. A more detailed description is in the paper cited below.
Those “slim filaments” are in the emission lines of SiO and eight other molecules and are not structures we see in visible light. Since they’re not necessarily connected to star-forming activity, and they don’t seem to be associated with emissions from heated dust, the team is looking at the dynamics of the region to explain their cause.
“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,” Yichen Zhang, a professor at Shanghai Jiao Tong University said. “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.”
Call Them Tornadoes
Let’s look at several key features of these filaments. Rotational transitions of SiO 5-4 are clearly seen in ALMA observations. Basically, the molecule transitions from a higher energy state to a lower one. That changes its rotational energy and that can be seen in the spectral images. The team also noted the presence of CH3OH masers, which are structures that produce microwave emissions. The filaments are also rich in complex organic molecules.
Each of the spectral filaments shows a unique morphology (or shape). They also have different velocity structures and molecular abundances. In a paper describing the team’s work, the authors state, “We speculate that these slim filaments represent a distinct class from the dense gas filaments typically observed in nearby molecular clouds, and they may result from interactions between shocks and molecular clouds.”
“We can envision these as space tornadoes: they are violent streams of gas, they dissipate shortly, and they distribute materials into the environment efficiently,” said Xing Lu, a research professor at Shanghai Astronomical Observatory and co-author on a paper about the filaments.
What’s Next?
The team’s breakthrough discovery offers a window into the processes occurring in the CMZ and their cyclical nature. To recap what they know so far, shocks create slim filaments, which release silicon SiO and the organic molecules CH3OH, CH3CN, and HC3N into the region. They freeze onto dust grains. After a time, the filaments dissipate. That refuels the widespread shock-released material in the CMZ and balances molecular depletion and eventual replenishment.
Future observations at other wavelengths should help astronomers understand the evolutionary nature of the filaments and the events that spur their formation and dissipation.
For More Information
Astronomers Discover “Space Tornadoes” around the Milky Way’s Core
ALMA Observations of Massive Clouds in the Central Molecular Zone: Slim Filaments Tracing Parsec-scale Shocks
3-D CMZ I: Central Molecular Zone Overview