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Star Formation Reduced in Galactic Groups

By Stephen Luntz

Galaxies in groups lose hydrogen gas as they move through the intergalactic medium, making it harder for them to form stars.

The process has previously been known in large galactic clusters, but has now been extended to galaxies in groups similar to our own.

Galaxies are formed out of huge clouds of gas, much of which gradually forms into stars. However, in clusters and groups of galaxies there is intergalactic medium stretching between the galaxies.

Within clusters, which can have thousands of galaxies, the intergalactic medium becomes very hot. “When a galaxy moves through this hot medium, most of its hydrogen gas can be easily removed,” says Dr Luca Cortese of Swinburne University’s Centre for Astrophysics and Supercomputing.

The intergalactic medium in small groups of galaxies is much cooler, and was not expected to have the capacity to remove gas from the disks of galaxies. Nevertheless, when Cortese and Dr Barbara Catinella used the Arecibo Telescope to examine galaxies around 500 million light years away they found that these grouped galaxies still have less hydrogen gas than those flying solo.

“It is unclear which physical process is responsible for the observed gas deficit in group galaxies. The milder intergalactic medium in groups might not suffice,” says Catinella. She hopes the completion of the Square Kilometre Array will allow us to answer this question.

Catinella says that if the intergalactic medium is not hot enough to strip the gas it is not clear what process could be responsible. Once removed, the hydrogen becomes part of the intergalactic medium that drew it away from the galaxy.

If the intergalactic gas becomes dense enough, a disturbance may trigger the formation of stars or even a small galaxy, Catinella says, and some may eventually settle on old galaxies.

While the intergalactic medium in small galactic groups was not thought capable of removing the hydrogen, the finding is not a total surprise as it was already known that star formation is reduced in groups, although less so than in clusters.

The research was published in Monthly Notices of the Royal Astronomical Society.