Star clusters are formed by the condensation of molecular clouds, the dense, cold gas found in all galaxies. The physical properties of these clouds in our galaxy and in nearby galaxies have long been known. But are they identical in distant galaxies, more than 8 billion light-years away? Thanks to unprecedented resolution in a distant galaxy, an international team led by the University of Geneva (UNIGE), with the participation of French researchers, has detected molecular clouds in a future Milky Way for the first time. These observations, published in the journal Nature Astronomy, demonstrate that these clouds have a higher mass, density and internal turbulence than in nearby galaxies, and produce far more stars. Astronomers attribute these differences to the ambient interstellar conditions in distant galaxies, which are too extreme for the survival of the molecular clouds typical of nearby galaxies.
Molecular clouds, the cradle of star formation, are well known in the Milky Way. But are they the same in distant galaxies, which form more stars? Until now, it was very difficult to isolate clouds in distant galaxies, due to a lack of spatial resolution. Astronomers came up with the idea of taking advantage of a natural telescope - the gravitational lensing phenomenon - coupled with the use of ALMA (Atacama Large Millimeter/submillimiter Array), an interferometer with 50 millimeter radio antennas that instantly reconstruct the entire image of a galaxy. Thanks to the alignment of a massive object between the observer and the distant object, the gravitational lenses produce a magnifying glass effect, considerably enlarging the distant object under study. This resolution, further enhanced by the ALMA interferometer (resolution 0.2''), has enabled us to characterize individual clouds in a distant galaxy, nicknamed the Cosmic Serpent, located 8 billion light-years away (see figure 1 below).
Figure 2 (below) compares the surface densities of clouds in nearby normal and starburst galaxies with those in distant galaxies, which turn out to be much larger.
