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Production sites of stars are rare

(24 July 2019 - NAOJ) Astronomers using the Nobeyama Radio Obeservatory (NRO) 45-m telescope found that high-density gas, the material for stars, accounts for only 3% of the total mass of gas distributed in the Milky Way.

This result provides key information for understanding the unexpectedly low production rate of stars.

production 1

Distribution of gas clouds obtained from the FUGIN project. The high-density gas (right) is detected only in small parts of the low-density gas (left). (courtesy: NAOJ)

Stars are born in gas clouds. The high-density gas pockets form in the extended, low-density gas clouds, and stars form in the very dense gas cores which evolve within the high-density gas. However, observations of distant galaxies detected 1000 times fewer stars than the production value expected from the total amount of low-density gas. To interpret the discrepancy, observations which detect both of the high-density and low-density gas with high-spatial resolution and wide area coverage were needed. However, such observations are difficult, because the high-density gas structures are dozens of times smaller than the low-density gas structures.

The Milky Way survey project “FUGIN” conducted using the NRO 45-m telescope and the multi-beam receiver FOREST overcame these difficulties. Kazufumi Torii, a project assistant professor at NAOJ, and his team analyzed the big data obtained in the FUGIN project, and measured the accurate masses of the low-density and high-density gas for a large span of 20,000 light-years along the Milky Way. They revealed for the first time that the high-density gas accounts for only 3% of the total gas.

These results imply the production rate of high-density gas in the low-density gas clouds is small, creating only a small number of opportunities to form stars. The researcher team will continue working on the FUGIN data to investigate the cause of inefficient formation of the high-density gas.

Publication

These observation results were published as K. Torii et al. “FOREST Unbiased Galactic plane Imaging survey with the Nobeyama 45 m telescope (FUGIN). V. Dense gas mass fraction of molecular gas in the Galactic plane” as an on-line paper of the Publications of the Astronomical Society of Japan on April 17, 2019 , and will be published as a special issue.