A star-forming region in a nearby galaxy, imaged by the Hubble Space Telescope.
Credit: NASA, ESA, STScI/AURA
Mopra radio telescope, near Coonabarabran, NSW.
Credit: CSIRO
SYDNEY: The universe is forming fewer stars than it used to, and scientists are putting it down to a lack of gas.
Astronomers are well aware of a reduction in star formation over time, having observed it using several independent methods, but new research has established a clear connection between the decline in star formation and a depletion of hydrogen gas - the fuel that builds a new star.
"Our result helps us understand why the lights are going out," said Robert Braun, the lead author of the new study to be published in the Monthly Notices of the Royal Astronomical Society and CSIRO Astronomy and Space Science chief scientist. "Star formation has used up most of the available molecular hydrogen gas."
Bruan said these findings could lead to further discoveries in physics due to its deep-woven connection with astronomy.
Ten times as much hydrogen
A previous study by Francois Combes, an astrophysicist and at the Paris Observatory in France, surveyed the Northern Hemisphere for hydrogen gas levels, but Braun's research includes the Southern Hemisphere in an effort to boost the argument for a connection between gas levels and star formation.
Braun said his and Combes' methods could improve the accuracy of such gas mass measurements by providing a "greater certainty of what we are measuring".
To understand the past history of star formation and its link to hydrogen gas, astronomers look at far-away galaxies and compare them to those that are close-by. Braun studied galaxies that are three to five billion light years away and compared them to those near to us today. He found that galaxies in the past had approximately 10 times as much hydrogen gas as galaxies today.
"Most of the original gas — about 70% — remains locked up, having been turned into things such as white dwarfs, neutron stars and planets," said Braun. "So the molecular gas is used up over time. We find that the decline in the molecular gas is similar to the pattern of decline in star formation, although during the time interval that we have studied, it is declining even more rapidly."
Tip of the iceberg
He focussed on the brightest and most gaseous galaxies, called ultra-luminous infra-red galaxies (ULIRGs). Due to their luminous quality, Braun was able to obtain for the first time a complete sample of this type of galaxy over the entire volume of the universe to a distance of five billion light years. He used the same tool, or tracer, to measure the gas levels across time, ensuring that the measurement of gas mass was also uniform.
This research is just at "the tip of the iceberg," commented Scott Croom, astronomer at the University of Sydney. With the advent of much larger telescopes, such as the revolutionary Atacama Large Millimetre Array (ALMA), astronomers will also be able "to look at more normal galaxies", for star formation trends, said Croom. While good for data collection, ULIRGs are considered "extreme, strongly evolving galaxies", and may not be representative of universal trends in galaxies, he added.
The findings in Braun's study support the theory that low gas levels are responsible for low star formation, as galaxies today contain less gas and are making fewer stars than in the past. Due to this depletion in the fuel for star formation, old stars will eventually die out, and new stars will stop forming. "Twelve billion years from now, the universe will be a dark, dark place," said Braun.
