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Surprise – The Milky Way Galaxy is Not as Homogeneous as Previously Thought

Posted by Guy Pirro   09/17/2021 05:24PM

Surprise – The Milky Way Galaxy is Not as Homogeneous as Previously Thought

Artist impression of the Milky Way Galaxy -- Clouds and streams of cosmic pristine gas accrete onto the Milky Way, but this gas does not efficiently mix in the Galactic disk. Astronomers from the University of Geneva in Switzerland have observed the composition of these gases in our galaxy and have shown that, contrary to the models established until now, they are not as homogeneously mixed as previously thought.  (Image Credit: NASA, JPL)

 


Surprise – The Milky Way Galaxy is Not as Homogeneous as Previously Thought

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. This includes 1) the initial gas coming from outside our galaxy, 2) the gas between the stars inside our galaxy, often enriched with chemical elements, and 3) 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. A team of astronomers from the University of Geneva in Switzerland have observed the composition of these gases in our galaxy and have shown that, contrary to the models established until now, they are not as homogeneously mixed as previously thought. This has an impact on the current understanding of the evolution of galaxies and, as a result, our understanding of the Milky Way’s evolution will need to be revisited.

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 University of Geneva.  At the same time, stars burn hydrogen 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.” 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, and 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 individual colors or frequencies, a bit like in a 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.”


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 University of Geneva 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.

 

 

For more information:

 

https://www.unige.ch/communication/communiques/en/2021/surprise-la-voie-lactee-nest-pas-homogene/

 

https://astromart.com/news/show/there-is-a-reason-why-we-are-warped

 

https://astromart.com/news/show/the-milky-way-was-t-boned-by-a-dwarf-galaxy-3-billion-years-ago

 

https://astromart.com/news/show/the-universe-may-not-be-isotropic-the-same-in-all-directions-as-previously-thought

 

https://astromart.com/news/show/so-how-much-does-our-milky-way-galaxy-weigh

 

https://astromart.com/news/show/the-milky-way-galaxy-once-died-but-then-came-back-to-life

 

https://astromart.com/news/show/theres-something-strange-in-the-neighborhood

 

https://astromart.com/news/show/astronomers-create-first-large-scale-map-of-stellar-ages-in-the-milky-way

 

https://astromart.com/news/show/largest-astronomical-image-of-all-time-the-milky-way-in-46-billion-pixels

 

 

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