Sciences. – Milky Way evolution simulations must be corrected

Madrid, 8 (European Press)

The composition of gases in the Milky Way is not homogeneously mixed, a finding reported in Nature that has a strong impact on the current understanding of galactic evolution.

Thus, astronomers from the University of Geneva (UNIGE) point out that simulations of the evolution of the Milky Way must be modified.

To better understand the history and evolution of the Milky Way, astronomers study the composition of the gases and minerals that make up an important part of our galaxy. Three main elements stand out: primary gas from outside our galaxy, interstellar gas within our galaxy – enriched with chemical elements – and dust from the condensation of minerals in this gas.

So far, theoretical models have assumed that these three elements mixed homogeneously throughout the Milky Way and reached a level of chemical enrichment similar to that found in the Sun’s atmosphere, the so-called solar mineralogy. However, scientists have now discovered that this is not the case.

Galaxies consist of a group of stars and are formed by condensation of gas in the intergalactic medium, consisting mainly of hydrogen and a little helium. This gas does not contain metals, unlike gas in galaxies – in astronomy, all chemical elements heavier than helium collectively are called “metals”, although they are atoms in a gaseous form.

“Galaxies feed on ‘virgin’ gas falling from the outside, which regenerates them and allows new stars to form,” explains Annalisa de Seia, a professor in the Department of Astronomy at UNIGE’s School of Science and first author of the book. studying. At the same time, stars burn their hydrogen components throughout their lives and form other elements through nucleosynthesis.

When a star that has reached the end of its life explodes, it expels the minerals it produced, such as iron, zinc, carbon, and silicon, and introduces these elements into galactic gas. These atoms can then condense into dust, especially in the coldest and densest parts of the galaxy.

The researcher continues: “In the beginning, when the Milky Way was formed, more than 10,000 million years ago, it had no minerals. Later, the stars enriched the environment with the minerals they produce.” When the amount of metals in this gas reaches the level found in the Sun, astronomers talk about heliometallism.

Thus, the environment that makes up the Milky Way collects minerals produced by stars, dust particles formed from these minerals, as well as gases from outside the galaxy that regularly enter it.

“So far, theoretical models have considered that these three elements were homogeneously mixed and reached the solar composition in all parts of our galaxy, with a slight increase in metallicity in the center, where the stars are more numerous – explains Patrick Pettijian, a researcher at the Paris Institute of Astrophysics at the Sorbonne University in Paris. France – We wanted to observe this in detail using the ultraviolet spectrophotometer on the Hubble Space Telescope.

Spectroscopy allows starlight to be separated into their individual colors or frequencies, such as a prism or a rainbow. In this decaying light, astronomers are especially interested in absorption lines: “When we observe a star, the minerals that make up the gas between the star and us absorb a very small part of the light in a characteristic way and at a certain frequency., which allows us not only to determine its presence, but also to determine What is the mineral and how abundant is it,” he continues.

For 25 hours, the team of scientists observed the atmospheres of 25 stars using the Hubble and Very Large Telescope (VLT) in Chile. The problem is that powder cannot be counted with these spectrometers, even if it contains minerals. For this reason, the Annalisa De Cia team has developed a new monitoring technology.

“It is about taking into account the overall composition of gas and dust by observing several elements at the same time such as iron, zinc, titanium, silicon and oxygen – explains the researcher in Geneva – then we can trace the amount of metals present in the dust. And adding it to that which has already been determined by previous observations to get Total “.

Thanks to this dual observation technique, astronomers have discovered that the environment of the Milky Way is not only heterogeneous, but some of the studied regions are only up to 10% of the sun’s metallicity.

“This discovery plays a key role in the design of theoretical models about the formation and evolution of galaxies,” says Jens-Kristian Krogager, researcher in the Department of Astronomy at UNIGE. From now on, we will have to improve the simulations. Increase the accuracy, to be able to include these mineral changes in places different from the Milky Way.”

The researchers add that these findings have a powerful impact on our understanding of the evolution of galaxies and our evolution in particular. In fact, minerals play an essential role in the formation of stars, cosmic dust, particles, and planets. Now we know that today new stars and planets can be formed from gases of very different compositions.