An international research team led by Dr. Suinan Zhang from the Shanghai Astronomical Observatory of the Chinese Academy of Sciences has uncovered the unique mechanisms governing star formation in the Central Molecular Zone (CMZ) of the Milky Way, using observations from the Atacama Large Millimeter/submillimeter Array (ALMA). The findings, published on March 13 in The Astrophysical Journal Letters, reveal that turbulent conditions in the Galactic Center suppress the birth of massive stars through a fragmented, hierarchical star formation process.

The CMZ, a region that hosts tens of millions of solar masses of gas in the center of the Galaxy, produces far fewer stars than expected. Previous studies attributed this anomaly to the chaotic environment: intense turbulence, strong magnetic fields, and energetic feedback from the supermassive black hole Sagittarius A* and frequent supernova explosions. However, a comprehensive exploration of the underproduction of stars is challenging due to limited observational resolution.

Leveraging ALMA’s unparalleled resolution and sensitivity, the team identified a few “stellar embryo clusters” within the CMZ. The stellar embryos are on average a few hundred astronomical units (AU)-sized objects harboring nascent stars. The stellar embryo clusters may evolve into massive star clusters over millions of years. An interesting finding is that, despite a similar overall star formation efficiency to active star-forming regions like Sagittarius B2, these CMZ young clusters exhibit a striking scarcity of young massive stars. Moreover, unlike the smooth, centralized distributions of young clusters in the Galaxy’s disk, the CMZ young clusters display a fragmented, hierarchical structure (see Figure 1).

Figure 1: One of the stellar embryo clusters revealed by high-resolution ALMA observations.

The researchers propose that extreme turbulence in the Galactic Center shreds gas into smaller parcels, forcing stars to form in fragmented, nested groups. This hierarchical process limits gas accretion, stifling the growth of massive stars. “Our findings suggest that these young CMZ clusters are not simply inefficient at forming stars overall - they preferentially produce low-mass stars over massive ones,” explained Dr. Zhang. “The CMZ has a distinct personality.”

Earlier studies by the team detected hundreds of dense cores in the CMZ that are associated with gas outflows tracing the accretion of newly born stars. “These outflows hinted at hidden stellar embryos, inspiring us to request higher-resolution observations to resolve individual forming stars,” added Dr. Xing Lu, coauthor and principal investigator of the ALMA project.

The discovery not only deciphers how extreme environments regulate star formation in the Galactic Center but also offers insights into nuclear regions of other galaxies and star formation in the early universe. “We are now pushing forward dynamical analyses of these young clusters to unravel the CMZ’s star formation mysteries,” said Dr. Zhang. “Understanding the CMZ is key to unlocking larger astrophysical puzzles.”

Figure 2: A sketch showing the hierarchical structure of a young cluster.

The paper is available at https://iopscience.iop.org/article/10.3847/2041-8213/adb30b