The galaxies of the young universe reveal themselves in the infrared

Matthew Bethermin, 35, an assistant astronomer at the Astrophysics Laboratory in Marseille, has worked on the infrared background emitted by galaxies. He now studies more specifically those that produced lots of stars 10 billion years ago. Some, surprisingly dusty, were inaccessible to optical and ultraviolet telescopes. Their detection by infrared helps us better understand the role of dark matter in their formation.

“Dust absorbs ultraviolet rays very effectively”

Science et Avenir: How can infrared help us unravel the secrets of these galaxies that populated the young universe?

Matthew Bethermin: For long periods of time, we used telescopes like Hubble to trace the formation of stars and therefore the galaxies that shelter them. These instruments make it possible to observe the light of large very bright stars in the visible and ultraviolet. But this is only possible for galaxies that contain very little dust because it absorbs ultraviolet rays very efficiently. However, we now know that there are galaxies that contain a lot of dust and that cannot be studied with this method. To fully evaluate star formation, it is necessary to track the emission of this dust that absorbs the star’s energy and re-emits it at much longer wavelengths in the infrared and microwaves.

“Half of the radiation produced during the history of the universe is contained in the infrared background”

Is it dust that is responsible for the diffuse infrared background? Why is it so important to our understanding of the universe?

Dust is not the only component of the infrared background. If we exclude the cosmological diffuse background emitted immediately after the Big Bang, half of the radiation generated during the history of the universe is contained in the infrared background and therefore belongs to dust. Therefore most of the stars emitted, mainly in optics, were absorbed by the dust and converted to infrared. One aspect of my work involved demonstrating from the archives of the Spitzer and Herschel telescopes that this dust was indeed contained in much larger galaxies, as well as determining when they formed. We now know that these galaxies existed between 6 and 10 billion years ago and that they formed a lot of stars. Up to a hundred times greater than the Milky Way. It is also suspected that they were formed too early and that they matured too early.

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Matthew Bethermino. MB credit

How to explain the high rate of star formation in these galaxies?

By studying and modeling these objects more specifically, we realized that they formed in a halo of relatively massive dark matter, within which much of the matter collapsed, making it possible to form very large galaxies very quickly. I was then able to study the gas inside some of the most massive and distant galaxies with the ALMA radio telescope to show that they contain a lot of the molecular cold gas that is the fuel needed to form stars. With another project, Alpine, we systematically studied more than a hundred less massive galaxies: enough to see that they also contain a lot of gas and dust.

For example, will we one day be able to examine their shapes in more detail to understand them?

Some studies are actually beginning to appear on unique objects, but this is only the beginning of this investigation. To achieve this, very long observation times are required, i.e. monopolizing the ALMA telescope in its extended version, between 5 and 10 hours for only one. With Alpine Collaboration we’ve even requested a tour of our 14 best items to try to see the details inside. We hope that we will be authorized to carry out this research in the coming years.

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