Giant galaxies may grow from cold gas

Giant galaxies may grow from cold gas that condenses as stars, according to an international research team, contradicting the theory that they are formed by smaller galaxies falling together in a hot, violent merger.

The news was announced in the journal Science by researchers led by Dr Bjorn Emonts, from the Centro de Astrobiología in Spain, who saw something very strange when they looked at a protocluster 10 billion light-years away. This protocluster was known to have a giant galaxy called the Spiderweb forming at its centre.

Dr Emonts’ team 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 Earth’s Sun. Most of this gas must be hydrogen, the basic material from which stars and galaxies form.

The astronomers located the hydrogen gas by detecting a tracer gas, carbon monoxide (CO), which is easier to find. The Very Large Array telescope in the US 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.

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, according to team member Professor Ray Norris from CSIRO and Western Sydney University.

Co-author Professor Matthew Lehnert, from the Institut Astrophysique de Paris, described the gas as “shockingly cold” — about -200°C.

“We expected a fiery process — lots of galaxies falling in and heating gas up,” he said.

As for where the carbon monoxide came from, that’s still a mystery.

“It’s a by-product of previous stars, but we cannot say for sure where it came from or how it accumulated in the cluster core,” Dr Emonts said.

“To find out, we’d have to look even deeper into the universe’s history.”

Top image: An artist’s impression of the Spiderweb galaxy sitting in a cloud of cold gas (blue). Modified by CSIRO from ESO. ©ESO/M Kornmesser under CC BY 4.0