Alien Structures or Shock Waves? Strange Filaments Discovered in the Milky Way Baffle Astronomers

ALMA has revealed evidence of a new type of slim, filamentary structure that drives cycles of depletion and replenishment.

At the heart of our galaxy, in the turbulent region surrounding the supermassive black hole, dust and gases swirl constantly, driven by energetic shock waves rippling through space. Using the Atacama Large Millimeter/submillimeter Array (ALMA), an international team of astronomers has improved our view of this chaotic region by a factor of 100, uncovering a surprising new filamentary structure.

The galaxy’s Central Molecular Zone (CMZ) has long been recognized as a region abundant with dust and gas molecules undergoing continuous cycles of formation and destruction. However, the exact mechanism behind this activity has remained unclear. Scientists often use molecules as markers to study various processes occurring within molecular clouds. Among these markers, silicon monoxide (SiO) is especially valuable for identifying shock waves.

Using ALMA’s high resolution and sensitivity to map distinct spectral lines within the molecular clouds at the center of the Milky Way, an international team of astronomers led by Kai Yang of the Shanghai Jiao Tong University has delineated a new type of long, narrow filamentary structure at a significantly finer scale. The dynamic interaction between this turbulent environment and the slim filaments produced as shocks ripple through provides a more complete view of the cyclical processes within the CMZ.

A Mysterious Phenomenon

“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 summarized.

These “slim filaments” were an unexpected, serendipitous find in the emission lines of SiO and eight other molecules. Their line-of-sight velocities are coherent and are inconsistent with outflows. Thus, they do not fit the profile of other, previously discovered types of dense gas filaments; furthermore, the filaments show no association with dust emission, and they do not appear to be in hydrostatic equilibrium.

“Our research contributes to the fascinating Galactic Center landscape by uncovering these slim filaments as an important part of material circulation,” Xing Lu, a research professor at Shanghai Astronomical Observatory and a corresponding author of the research paper, summarizes, “We can envision these as space tornados: they are violent streams of gas, they dissipate shortly, and they distribute materials into the environment efficiently.”

It remains unknown how these slim filaments initially arise, but shock processes emerge as a likely explanation, Yang’s team reports. This inference is based on several key observations: the rotational transition of SiO 5-4 clearly seen in ALMA observations, the presence of CH3OH masers, and the relative abundances of complex organic molecules in these slim filaments.

The Power of ALMA

“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 and a corresponding author of the research paper, emphasized. “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.”

This breakthrough offers a more detailed view of the dynamic processes occurring in the CMZ, and suggests a cyclical process of material circulation. First, shocks act as a mechanism to create these slim filaments, releasing SiO and several complex organic molecules such as CH3OH, CH3CN, and HC3N into the gas phase and into the interstellar medium. Then, the slim filaments dissipate to refuel the widespread shock-released material in the CMZ. Finally, the molecules freeze back into dust grains, resulting in a balance between depletion and replenishment. Assuming that the slim filaments exist throughout the CMZ as abundantly as in this sample, there would be a cyclical balance between depletion and replenishment.

“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. This makes it a particularly valuable tool for tracing shock-induced processes in the dense regions of the CMZ,” Yang said. It is hoped that future ALMA observations covering multiple SiO transitions and census observations spanning the CMZ, combined with numerical simulations, will confirm the origin of the slim filaments as well as the possibility of the cyclic processes within this extraordinary region of the Milky Way.

Reference: “ALMA observations of massive clouds in the central molecular zone: slim filaments tracing parsec-scale shocks” by Kai Yang, Xing Lu, Yichen Zhang, Xunchuan Liu, Adam Ginsburg, Hauyu Baobab Liu, Yu Cheng, Siyi Feng, Tie Liu, Qizhou Zhang, Elisabeth A. C. Mills, Daniel L. Walker, Shu-ichiro Inutsuka, Cara Battersby, Steven N. Longmore, Xindi Tang, Jens Kauffmann, Qilao Gu, Shanghuo Li, Qiuyi Luo, J. M. Diederik Kruijssen, Thushara Pillai, Hai-Hua Qiao, Keping Qiu and Zhiqiang Shen, 4 February 2025, Astronomy & Astrophysics.
DOI: 10.1051/0004-6361/202453191

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