Astronomers from the UNIGE have observed the composition of the gases in our galaxy and have shown that, contrary to the models established until now, they are not homogeneously mixed.
In order to better understand the history and evolution of the
Galaxies are made up of a collection of stars and are formed by the condensation of the gas of the intergalactic medium composed of mostly hydrogen and a bit of helium. This gas does not contain metals unlike the gas in galaxies – in astronomy, all chemical elements heavier than helium are collectively called “metals,” although they are atoms in gaseous form.
“Galaxies are fuelled by ‘virgin’ gas that falls in from the outside, which rejuvenates them and allows new stars to form,” explains Annalisa De Cia, a professor in the Department of Astronomy at the UNIGE Faculty of Science and first author of the study. At the same time, stars burn the hydrogen that constitutes them throughout their life and form other elements through nucleosynthesis.
When a star that has reached the end of its life explodes, it expels the metals it has produced, such as iron, zinc, carbon and silicon, feeding these elements into the gas of the galaxy. These atoms can then condense into dust, especially in the colder, denser parts of the galaxy.
“Initially, when the Milky Way was formed, more than 10 billion years ago, it had no metals. Then the stars gradually enriched the environment with the metals they produced,” continues the researcher. When the amount of metals in this gas reaches the level that is present in the Sun, astronomers speak of Solar metallicity.
A not so homogeneous environment
The environment that makes up the Milky Way thus brings together the metals produced by the stars, the dust particles that have formed from these metals, but also gases from outside the galaxy that regularly enter it.
“Until now, theoretical models considered that these three elements were homogeneously mixed and reached the Solar composition everywhere in our galaxy, with a slight increase in metallicity in the center, where the stars are more numerous,” explains Patrick Petitjean, a researcher at the Institut d’Astrophysique de Paris, Sorbonne University. “We wanted to observe this in detail using an Ultraviolet spectrograph on the