The Milky Way Hasn’t Been Evenly Mixed

Gas from the intergalactic medium constantly rains down on galaxies, fueling continued star formation. New research has shown that this gas is not evenly mixed, and stars are not equal across the galaxy. This result means that solar systems are not the same within the Milky Way.

Galaxies are born with a reservoir of gas which they use to manufacture stars. That gas is pristine, made almost entirely of hydrogen and helium. It doesn’t contain any heavier elements, which astronomers call “metals”.

But gas from outside the galaxy is always raining in, providing fresh material. That gas is equally pristine.

“Galaxies are fueled by ‘virgin’ gas that falls in from the outside, which rejuvenates them and allows new stars to form”, explained Annalisa De Cia, a professor in the Department of Astronomy at the UNIGE Faculty of Science and first author of a new study, recently published in Nature, examining the role that the new gas plays in star formation.

Stellar evolution pollutes the composition of the initial reservoir of gas. Stars fuse hydrogen and helium into heavier elements – the metals – like oxygen, carbon, silicon, and more. When they die, either through a drawn-out and dramatic unfolding of a planetary nebula or a spectacular supernova blast, they eject these metals into the surrounding galaxy. Those metals then go on to get mixed freely.

“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”, said De Cia.

Theoretical models had assumed that every source of gas – the initial reservoir, the metal-contaminated enrichment from stars, and the pristine gas falling in – all mixed equally.

“Until now, theoretical models considered that these three elements were homogeneously mixed and reached the Solar composition ([he ratios of elements found in the Sun] everywhere in our galaxy, with a slight increase in metallicity in the centre, where the stars are more numerous”, explained 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 Hubble Space Telescope.”

De Cia’s team developed a new technique for measuring the total amount of metals, including the metal content of dust grains, which is notoriously difficult to measure. They used this technique to study 25 stars in detail. “[The new method] involves taking into account the total composition of the gas and dust by simultaneously observing several elements such as iron, zinc, titanium, silicon and oxygen”, explained De Cia. “Then we can trace the quantity of metals present in the dust and add it to that already quantified by the previous observations to get the total.”

They found that the abundance of metals varied wildly across the galaxy, with some regions having only 10% of the metal content of the Sun. This means that the gas in our galaxy is not uniformly mixed and that, among other consequences, the formation of planets is not uniform throughout the Milky Way.

“This discovery plays a key role in the design of theoretical models on the formation and evolution of galaxies”, says Jens-Kristian Krogager, researcher at the UNIGE’s Department of Astronomy. “From now on, we will have to refine the simulations by increasing the resolution, so that we can include these changes in metallicity at different locations in the Milky Way.”