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 Milky Way,
astronomers are studying the composition of the gases and metals that make
up an important part of our galaxy. Three main elements stand out: the
initial gas coming from outside our galaxy, the gas between the stars inside
our galaxy – enriched with chemical elements – and the dust created by the
condensation of the metals present in this gas. Until now, theoretical
models assumed that these three elements were homogeneously mixed throughout
the Milky Way and reached a level of chemical enrichment similar to the
Sun’s atmosphere, called the Solar metallicity.
Today, a team of astronomers from the University of Geneva (UNIGE)
demonstrates that these gases are not mixed as much as previously thought,
which has a strong impact on the current understanding of the evolution of
galaxies. As a result, simulations of the Milky Way’s evolution will have to
be modified. These results can be read in the journal Nature.
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 Hubble Space Telescope.”
Spectroscopy allows the light from stars to be separated in its individual
colors or frequencies, a bit like a with prism or in a rainbow. In this
decomposed light, astronomers are particularly interested in absorption
lines: “When we observe a star, the metals that make up the gas between the
star and ourselves absorb a very small part of the light in a characteristic
way, at a specific frequency, which allows us not only to identify their
presence, but also to say which metal it is, and how abundant it is,” he
continues.
A new method developed to observe the total metallicity
For 25 hours, the team of scientists observed the atmosphere of 25 stars
using Hubble and the Very Large Telescope (VLT) in Chile. The problem? The
dust cannot be counted with these spectrographs, even though it contains
metals. Annalisa De Cia’s team has therefore developed 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,” explains the Geneva researcher. “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.”
Thanks to this dual observation technique, the astronomers have found that
not only is the Milky Way’s environment not homogeneous, but that some of
the areas studied reach only 10% of the Solar metallicity. “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.”
These results have a strong impact on our understanding of the evolution of
galaxies and of our own in particular. Indeed, metals play a fundamental
role in the formation of stars, cosmic dust, molecules, and planets. And we
now know that new stars and planets could be formed today from gases with
very different compositions.
Reference:
De Cia, A., Jenkins, E.B., Fox, A.J. et al. Large metallicity variations in
the Galactic interstellar medium. Nature 597, 206–208 (2021).
DOI: 10.1038/s41586-021-03780-0
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