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A Cool Theory about Galaxy Formation

By Dave Reneke

A surprising finding about galaxy formation, and the discovery of starspots on Proxima Centauri.

The surprise finding that giant galaxies may grow from cold gas that condenses as stars rather than forming in hot, violent mergers has been made with CSIRO and US radio telescopes by an international team including four CSIRO researchers.

The biggest galaxies are found at the hearts of clusters among huge swarms of galaxies. “Until now we thought these giants formed by small galaxies falling together and merging,” said Prof Ray Norris of CSIRO and Western Sydney University.

But Dr Bjorn Emont’s team from the Centro de Astrobiología in Spain saw something very different when they looked at an embryonic cluster 10 billion light years away. This protocluster was known to have a giant galaxy called the Spiderweb forming at its centre. They found that the Spiderweb is wallowing in a huge cloud of very cold gas that could be up to 100 billion times the mass of our Sun. Most of this gas must be hydrogen, the basic material from which stars and galaxies form.

Earlier work by another team had revealed young stars all across the protocluster. The new finding suggests that rather than forming from infalling galaxies, the Spiderweb may be condensing directly out of the gas. The astronomers didn’t see the hydrogen gas directly but located it by detecting a tracer gas, carbon monoxide (CO), which is easier to find.

The Very Large Array telescope in the USA showed that most of the CO could not be in the small galaxies in the protocluster, while CSIRO’s Australia Telescope Compact Array saw the large cloud surrounding the galaxies. “This is the sort of science the Compact Array excels at,” Norris said.

Prof Matthew Lehnert from the Institut Astrophysique de Paris described the gas as “shockingly cold” – about –200°C. “We expected a fiery process with lots of galaxies falling in and heating gas up,” he said.

Where the CO came from is a puzzle. It seems to be a byproduct of previous stars, but nobody as yet can say for sure where it came from or how it accumulated in the cluster core.

The astronomers feel that to answer that question we’d have to look even deeper into the universe’s history. And that is almost certainly where the next phase begins.

Proxima Centauri Might More Sun-Like

At first glance, Proxima Centauri seems nothing like our Sun. It’s a small, cool, red dwarf star only one-tenth as massive and one-thousandth as luminous as the Sun. However, new research shows that it is Sun-like in one surprising way. It has a regular cycle of starspots – dark blotches on the surface where the temperature is a little cooler.

A star is made of ionised gases called plasma. Magnetic fields can restrict the plasma’s flow and create spots. Changes to a star’s magnetic field can affect the number and distribution of starspots.

A new study has now found that Proxima Centauri undergoes a regular dramatic cycling lasting 7 years from peak to peak. At least one-fifth of the star’s surface is covered in spots at once. Some of these spots are much bigger relative to the star’s size than the spots on our Sun.

Astronomers were surprised to detect a stellar activity cycle in Proxima Centauri because its interior is expected to be very different from the Sun’s. The outer third of the Sun experiences a broiling motion called convection, similar to water boiling in a pot, while the Sun’s interior remains relatively still.

Many astronomers think the shear arising from this difference generates the Sun’s magnetic activity cycle. In contrast, the interior of a small red dwarf like Proxima Centauri should be convective all the way into the star’s core. As a result, it shouldn’t experience a regular cycle of activity.

“The existence of a cycle in Proxima Centauri shows that we don’t understand how stars’ magnetic fields are generated as well as we thought we did,” says Smithsonian researcher and co-author Jeremy Drake.

The study didn’t determine whether Proxima Centauri’s activity cycle would affect the potential habitability of its Earth-like planet Proxima b, but it’s felt that flares or a stellar wind, both driven by magnetic fields, could scour the planet and strip away any atmosphere. In that case, Proxima b might be like the Earth’s Moon, located in the habitable zone but not at all friendly to life.

Direct observations of Proxima b won’t be possible for a long time. Until then, our best bet is to study the star and continue to plug that information into theories about star–planet interactions.

David Reneke is an astronomy lecturer and teacher, a feature writer for major Australian newspapers and magazines, and a science correspondent for ABC and commercial radio. Subscribe to David’s free Astro-Space newsletter at