Despite Its Swirly Nature, The Milky Way Is Not Well Mixed At All

Clouds and streams of cosmic pristine gas (magenta) accrete onto the Milky Way, but this gas does not efficiently mix in the Galactic disk, as highlighted for the Solar neighborhood (zoom-in). Image Credit: Dr Mark A. Garlick

Our galaxy, the Milky Way, is a large spiral galaxy. Its swirly nature might give the impression that it gets a good mixing over time, but it turns out that the interstellar medium – the gas found between stars – is a lot less homogenous than astronomers previously expected.  

The interstellar medium of a galaxy is affected by three main elements. First is the pristine gas that condensed from the big bang. This is mostly hydrogen, and it was the only component at the very beginning of the Milky Way. Such gas still flows in from intergalactic space.

Then there are the star-produced elements. Everything that is not hydrogen and helium (known as metals in astronomical jargon) enriches portions of hydrogen gas and is pushed around the galaxy. Finally, some of those metals condense into dust, the crucial elements where planets (and even us) come from.

The region around the Sun was assumed to have roughly the same level of enrichment of metals – the so-called metallicity – that we see in our star. However, new observations showed that this is not the case, and there is in fact a lot of variability in the gas found between stars in the Milky Way. The findings are reported in Nature.

“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 centre, where the stars are more numerous,” co-author Patrick Petitjean, from the Institut d’Astrophysique de Paris, Sorbonne University, said in a statement. “We wanted to observe this in detail using an Ultraviolet spectrograph on the Hubble Space Telescope.”

Studying the intergalactic medium, especially focusing on the solar neighborhood, is far from easy. The team used 25 bright stars and a long observation time to estimate the enrichment of that particular region. This approach doesn’t account for dust, so lead author Annalisa De Cia and her team had to develop a new observational technique.

“It 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, who works at the University of Geneva. “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.”

The findings show that there are pockets of interstellar medium that have a metallicity one-tenth of what we find in the Sun. The work suggests that models of our galaxies will have to be reviewed to be more faithful to reality.

“This discovery plays a key role in the design of theoretical models on the formation and evolution of galaxies,” added Jens-Kristian Krogager, also at the University of Geneva. “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.”


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