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Radio Observations Reveal Hidden Secret Of Star-Forming Galaxies

Almost 100 years after Cecilia Payne-Gaposchkin worked out that hydrogen was the main component of stars, astronomers are getting a better idea at what type of hydrogen is actually being used to light up the cosmos.

Thanks to new research from the International Centre for Radio Astronomy Research, astronomers think that they have found that atomic hydrogen is present in the same proportion in galaxies throughout cosmic history. As reported in the Astrophysical Journal Letters, this has important implications on how stars formed and galaxies evolved.

In galaxies, hydrogen can be found in its atomic form, with single atoms spread out in large gas clouds, or in its molecular form, where two atoms are bound together. So far, none of this is more than just high-school chemistry but things got complicated when we looked for the most star-forming galaxies in the universe and compared them to our own.

Observations show that 70 percent of the Milky Way’s hydrogen content is atomic and our galaxy forms about one star like the Sun per year. But galaxies forming stars hundreds of times faster, during the peak of the cosmic star formation over 6 billion years ago, have 10 times more molecular hydrogen than the Milky Way.

This, combined with the fact that atomic hydrogen is notoriously difficult to detect, led to a lot of uncertainties. Is molecular hydrogen better for forming stars? Are there processes breaking the molecules as galaxies age?

We don’t have the tech to study atomic hydrogen that far away in the universe, so Dr Luca Cortese from the University of Western Australian and colleagues used (relatively) nearby galaxies as proxies. These objects are about 3 billion years younger than the Milky Way.

“What we found is that despite hosting 10 billion solar masses of molecular gas these young galaxies turn out to be very, very rich in atomic hydrogen as well,” Dr Cortese said in a statement. “The balance between atomic and molecular hydrogen is pretty much the same as in the Milky Way. In other words, it’s still dominated by atomic gas.”

The findings show that atomic hydrogen is likely to play an important role in far-away galaxies even if molecular hydrogen ends up be more abundant. To find out just how much atomic hydrogen there is out there we will have to wait for future radio telescopes, like the Square Kilometer Array, to come online.

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