At the center of our galaxy, in the turbulent region that surrounds a supermassive black hole, dust and gas are constantly spinning, driven by high-energy shock waves that ripple in space. Using the Atacama Large Millimeter/Submillimeter Array (ALMA), an international team of astronomers has improved our observations of this chaotic region by a factor of 100 and discovered a surprisingly new filamentous structure.

The central molecular region (CMZ) of the galaxy has long been thought to be a region rich in dust and gas molecules, which is constantly experiencing cycles of formation and destruction. However, the exact mechanism behind this activity remains unclear. Scientists often use molecules as markers to study the various processes that occur within molecular clouds. Among these markers, silica (SiO) is particularly important for identifying shock waves.

An international team of astronomers led by Kai Yang of Shanghai Jiao Tong University used the high resolution and sensitivity of ALMA to map clear spectral lines within the molecular cloud at the center of the Milky Way and depict a new type of elongated filamentous structure at a finer scale. This dynamic interaction between the turbulent environment and the filament-like structures generated when the shock wave passes through provides a more complete perspective on the periodic processes within the CMZ.

The ALMA antenna is pointed at the Milky Way over the Atacama Desert. Image credit: NSF/ AUI/ NSF NRAO/ B.Foott

"When we looked at the ALMA image showing the effluent, we noticed that these elongated filaments were spatially offset from any star-forming regions. Unlike any object we know, these filaments really surprised us. Since then, we've been thinking about what exactly they are," Yang concluded.

These "elongated filamentous structures" were accidentally discovered by chance in the emission lines of silica and eight other molecules. Their gaze velocity is consistent with the outflow velocity. As a result, they do not correspond to other previously discovered types of dense gas filamentous structures; In addition, these elongated filamentous structures are independent of dust emission and do not appear to be in hydrostatic equilibrium.

CMZ 中的細絲。面板 a：MeerKAT 對 Sgr A 區域的 1.28 GHz 射電發射。紅色方框標記了 20 公里/秒雲層和 50 公里/秒雲層。面板 b-c：ALMA 低解析度（~1.9 英寸）觀測得到的 20 公里/秒雲層和 50 公里/秒雲層中 SiO 5-4 的積分強度圖。藍色方框標記了檢測到細絲的放大區域。虛線環表示我們 ALMA 高解析度（~0.23 英寸）觀測的 50% 主光束。面板 d-g：我們 ALMA 高解析度觀測得到的細絲狀 SiO 5-4 發射，對於 20 公里/秒雲層和 50 公里/秒雲層，其積分速度範圍分別為 [-20， 40] 和 [25， 75] 公里/秒。粉色虛線表示已識別的細絲。黑色輪廓線表示 ALMA 1.3 毫米連續譜發射，輻射能量水準為 [5， 25， 45] × 40 µJy beam−1。圖片來源：Yang 等人。

Xing Lu, a researcher at the Shanghai Astronomical Observatory and corresponding author of the research paper, concluded: "Our study reveals that these slender filaments are an important part of the material cycle, contributing to the fascinating landscape of the Milky Way's center. We can think of them as space tornadoes: they are violent air currents that quickly dissipate and efficiently distribute matter into the environment. ”

How these slender filamentous structures were originally formed is unclear, but Prof. Yang's team reported that the shockwave process appears to be a possible explanation. This inference is based on several key observations: the clearly visible SiO 3-0 rotational transition in ALMA observations, the presence of CH0OH veins, and the relative abundance of complex organic molecules in these delicate filamentous structures.

Yichen Zhang, a professor at Shanghai Jiao Tong University and corresponding author of the research paper, emphasized: "The high angular resolution and extraordinary sensitivity of ALMA are essential to detect the molecular line emission associated with these elongated filamentous structures, and to confirm that there is no correlation between these structures and dust emissions. Our discovery marks a significant advance in detecting these elongated filamentous structures on a finer 01.0 parsec scale, thus marking the working surface of these shock waves. ”

This breakthrough provides a more detailed perspective on the dynamic processes taking place in the CMZ and reveals the cyclical processes of the material cycle. First, shock waves act as a mechanism that forms these delicate filaments that release SiO, as well as several complex organic molecules such as CH3OH, CH0CN, and HC0N, into the gas phase and interstellar medium. These slender filaments then dissipate to replenish the energy of the release of matter from the shock waves that are ubiquitous in the CMZ. Finally, these molecules freeze into dust particles, resulting in a balance between depletion and refill. Assuming that these thin filaments are as abundant throughout the CMZ as they are in this sample, there will be a periodic equilibrium between loss and replenishment.

"SiO is currently the only molecule that can specifically track shock waves, and the SiO 4-0 rotational transition can only be detected in regions affected by shock waves with relatively high density and temperature," said Prof. Yang. This makes it a particularly valuable tool for tracking the process of shock wave induction within the dense region of the CMZ. It is hoped that in the future, ALMA will be able to confirm the origin of these filamentous structures and the possibility of periodic processes in this particular region of the Milky Way through observations covering multiple SiO transitions and census observations across the CMZ, combined with numerical simulations.

編譯自/ScitechDaily