First supernovae blew early galaxies apart

by Heather Catchpole
Cosmos Online

SYDNEY: The universe's first stars blew small galaxies apart when they exploded, effectively quashing all nearby star formation, say Japanese astrophysicists.

The theory, based on analytical calculations of the energy and disruptive effects of early supernovae, adds another piece to the puzzle of what the first stars were like and how they influenced galaxy formation.

The first stars formed around 200 million years after the Big Bang in clumps of dark matter called dark matter haloes – the basic building blocks of galaxies.

Running out of gas

These stars were massive, probably 10 to 100 times bigger than the Sun. Like most massive stars, they would have burnt through their fuel within a few tens of millions of years and then exploded as either a type II supernova or a pair instability supernova.

The study, published in the current issue of the Astrophysical Journal, looks at what happened to the dark haloes near these massive explosions. Previously, experts were divided as to whether the first supernovae kick started star formation in the haloes or suppressed it.

Astrophysicists Masaru Sakuma from the University of Tsukuba, in Tsukuba, and Hajime Susa from Konan University in Kobe, Japan, say their model shows the shockwave from these supernovae would have expanded the gas shell within the stars' own galaxies, creating a gaseous 'wind' that stripped the gas out of nearby dark haloes.

First clues

This 'wind' would have swept the gas from dark haloes within a radius of up to 5,000 light-years around the supernova, depending on the force of the initial explosion and the mass of the dark haloes, the researchers say.

"[If] a neighbouring halo is located very close to the centre of the supernova explosion, the gas in the halo would be evacuated by the shock momentum... supernova feedback has basically negative effects on the star formation in surrounding halos," the researchers write.

Commenting on the research, Australian theoretical astrophysicist Stuart Wyithe, from the University of Melbourne, said the research answered a "little bit" of the big question of how the death of the first stars affected the early universe.
"Early galaxies are analogous to buckets of gas, with the gas confined by gravity rather than by walls. The supernova explosion blows the gas out of the first bucket. This moving gas then acts like a wind which in turn blows the gas out of small nearby galaxies," Wyithe said.

"[The research] doesn't address other issues about how much gas there is in the first place and what happens when there are many haloes [surrounding the supernovae]," he said. But added that the study is "solid piece of work."

"Analytic calculations like these can sometimes give quite good information, and I think in this case it does," said Wyithe. "It's not understood fully what role the first stars played in the reionisation of the early universe. This is a first step on the way to understanding that."