Molecular clouds, the cradle of star formation, are well known in the Milky Way. But are they the same in distant galaxies that form more stars ? Until now it was very difficult to isolate clouds in distant galaxies, for lack of spatial resolution. Astronomers then came up with the idea of benefiting from a natural telescope - the gravitational lens phenomenon - coupled with the use of ALMA (Atacama Large Millimeter / submillimiter Array), an interferometer of 50 millimeter-scale radio antennas that reconstruct the entire image of a galaxy instantly. Thanks to the alignment of a massive object between the observer and the distant object, the gravitational lenses produce a magnifying effect, and considerably enlarge the size of the distant object studied. This resolution, further improved thanks to the ALMA interferometer (resolution of 0.2 ’’) made it possible to characterize the clouds individually in a distant galaxy, nicknamed the Cosmic Snake, located at 8 billion light-years (see Figure 1).
These observations revealed that the molecular clouds of distant galaxies had a mass, density and turbulence 10 to 100 times higher than the clouds of nearby galaxies. Yet this galaxy is "normal" for its epoch, and is not subject to a starburst. The international team has also discovered that the level of star formation efficiency of the Cosmic Snake’s molecular clouds is particularly high, favored by the large internal cloud turbulence. In nearby galaxies, a cloud forms about 5% of its mass in stars. In distant galaxies, this number jumps to 30%.
Figure 2 compares the cloud surface densities in near galaxies, normal or starbursts, with those of distant galaxies, which are much larger.
Reference
- Molecular clouds in the Cosmic Snake, a normal star-forming galaxy 8 billion years ago, Dessauges-Zavadsky, M., Richard, J., Combes F. et al 2019, Nature Astronomy