Galaxies as cosmic cauldrons & # 39;

The formation of stars in interstellar clouds of gas and dust, called molecular clouds, takes place very rapidly while remaining very inefficient. Most of the gas is scattered by stellar radiation, revealing that galaxies are highly dynamic systems, such as "cosmic cauldrons", made up of components that change constantly in appearance. Based on new observations of the NGC 300 spiral galaxy, a team of scientists led by Dr. Diederik Kruijssen, an astrophysicist from the University of Heidelberg, has successfully reconstructed the temporal evolution of molecular clouds for the first time. the process of forming stars that compose them. Their analysis shows that these clouds are ephemeral structures undergoing rapid life cycles, driven by the intense radiation of the nascent stars. The results are published in Nature.

The observed intensity of star formation in the NGC 300 spiral galaxy can be explained in two ways. Molecular clouds can last a very long time and eventually convert all their mass into stars. In this case, the positions of the young stars must generally correspond to those of the molecular clouds from which they were formed. Alternatively, stars can form very quickly in molecular clouds and disperse the gas with their intense radiation, leaving only a small fraction of the gas available for conversion to stars. In this case, the young stars and the molecular clouds should generally reside in different places.

To determine which of these molecular cloud life cycle models is correct, Dr. Kruijssen and his team combined two different sets of observations from the NGC 300 galaxy, located about 6 million years old. light of the Milky Way. The first observation is a map of the light emitted by carbon monoxide, which shows the location of molecular clouds. The second is a hot ionized hydrogen map that indicates the positions of the newly formed massive stars. These maps were obtained using the large Atacama millimeter (ALMA) measurement system from the European Southern Observatory (ESO) and the 2.2-meter telescope from Max Planck and ESO. The ALMA observations were conducted by Dr. Andreas Schruba, a scientist at the Max Planck Institute for Extraterrestrial Physics in Garching and one of the co-authors of the study. Scientists analyzed the data with the help of a new statistical method that determines the relationship between molecular gas and star formation in galaxies at different spatial scales. For the first time, this method accurately quantifies the positions of molecular clouds and young stars relative to each other.

According to scientists, the results leave no doubt: the positions of molecular clouds and young massive stars rarely coincide. This effect becomes stronger on small scales. Scientists conclude that stars form very rapidly, so that gaseous stars and young stars represent distinct phases in the life cycle of molecular clouds.

"Our results demonstrate that star training is happening very quickly and in an extremely inefficient way," says Dr. Kruijssen, head of the research group at the Institute for Astronomical Computing. "The molecular clouds in NGC 300 last about ten million years and take only 1.5 million years to be destroyed, long before the most massive stars have reached the end of their lives and explode in supernovae. " Dr. Mélanie Chevance, researcher in her team and also co-author of the article, adds: "The intense radiation emitted by the young stars disperses the original molecular cloud by heating it and dispersing it in form. of hot interstellar gas bubbles, only two to three percent of the mass of the molecular clouds are converted into stars. "

The team of researchers now wants to apply its new statistical method to observations of very distant galaxies to deduce how the formation of stars in molecular clouds has unfolded during the course of history of the Universe. "We will now examine the relationship between molecular clouds and young stars in galaxies of the cosmos, and in the near future this will enable us to understand galaxies as sets of components that undergo training-driven life cycles. of stars and together shape their host galaxies, "says Dr. Kruijssen.