Astronomers map lifetime of over 100,000 molecular clouds across 66 galaxies

An international team of astronomers has analyzed the data from the James Webb Space Telescope (JWST) and Atacama Large Millimeter/submillimeter Array (ALMA) to investigate giant molecular clouds in nearby galaxies. The new study, presented April 27 on the arXiv preprint server, unveils crucial information regarding the lifetime of more than 100,000 such clouds across 66 galaxies.

Huge reservoir of interstellar gas

Molecular clouds are huge complexes of interstellar gas and dust left over from the formation of galaxies, composed mostly of molecular hydrogen. Such clouds with masses greater than 100,000 solar masses are called giant molecular clouds (GMCs). In general, GMCs are 15–600 light-years in diameter and are the coldest and densest parts of the interstellar medium.

Observations show that GMCs are gas reservoirs where most star formation takes place. Therefore, studying their life cycle and characteristics is crucial to advancing our knowledge about the formation and evolution of galaxies.

Cloud atlas

That is why a group of astronomers led by Zein Bazzi of the University of Bonn in Germany decided to characterize the secular growth and evolutionary timescales of GMCs in 66 nearby star-forming galaxies.

For this purpose, they combed through a catalog of 108,466 GMCs identified with JWST as part of the Physics at High Angular resolution in Nearby GalaxieS (PHANGS), analyzed the data from ALMA measurements, and employed the giant molecular cloud mass function model (GMCMF).

“In this paper, we use the giant molecular cloud mass function model to estimate the formation time of individual clouds across different galactic environments. This model suggests that the formation and evolution of GMCs are mainly driven by multiple expanding Hii regions, supernovae, and radiative feedback,” the researchers explain.

The new findings

The study found that spiral massive clouds dominate the mass distribution in the centers, spiral arms, and disks of the galaxies in the sample, when comparing bars and interarms. Clouds with masses of less than 100,000 solar masses form in about 20 million years, while it takes more massive clouds up to 100 million years to form.

The results indicate that cloud formation varies systematically across galaxies. It turns out that cloud growth proceeds fastest in systems with higher molecular gas surface densities and more massive GMC populations. These findings suggest that dense, gas-rich conditions favor efficient secular cloud assembly.

Furthermore, the study found that regions with high molecular mass surface density and high star formation rate surface density accelerate cloud growth. Hence, GMCs in such regions have shorter self-growth timescales. Moreover, central regions of the investigated galaxies show the shortest self-growth timescales (typically about 16 million years), which is some 5–10 million years lower than in spiral arms, inter-arms, or disks.

According to the study, the GMC depletion time operates at scales of billions of years, while freefall times are within the range of 5–20 million years. Shear and orbital times turned out to be much longer—about 60–200 million years. These findings indicate that GMC evolution unfolds on timescales much shorter than those associated with galactic rotation or shear.

General conclusion

All in all, the results of the study suggest that the inspected GMCs are expected to convert only about 1% of their gas into stars over their lifetime.

“This is consistent with a picture in which cloud assembly, collapse, and destruction are jointly regulated by self-gravity, feedback, and large-scale galactic flows, leading to inherently inefficient star formation in nearby disk galaxies,” the authors of the paper conclude.

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