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Galaxies recycle gas to maintain star formation
Posted: 19 March 2012

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The first direct evidence for distant galaxies recycling their gas to provide the next generation of stars has been detected by astronomers using the Keck I telescope in Hawaii.

Studying the amount of gas and dust in an ordinary galaxy like our own Milky Way presents an interesting observation: there is not enough raw material to maintain its level of star formation for long.

"We know that the Milky Way would use up its available gas in less than two billion years at its current star formation rate," says Kate Rubin of the Max Planck Institute for Astronomy. Our Galaxy turns the equivalent of roughly one solar mass worth of matter into new stars every year. "If the Milky Way has maintained its current rate of star formation for the past five billion years, it would have needed to replenish its gas supply at least twice during that time."

The six galaxies with detected inflows, as seen by the Hubble Space Telescope, which was used to confirm galaxy orientation. Star formation is occurring in small "knots" in several of the galaxies' spiral arms. Image: K. Rubin, MPIA.

Rubin and colleagues find evidence that this new fuel could come in the form of recycled gas once ejected by the galaxy, pushed out by massive supernovae events or by the light pressure of massive newborn stars, but which does not leave the gravitational clutches of the galaxy completely, instead falling back towards its centre millions of years later.

"Based on the inflow speeds we measure, it would take about 500 million years for the gas to travel all the way from a galaxy's outskirts to the centre," says Rubin, who used the Low Resolution Imaging Spectrometer on the Keck I telescope to make observations of the gas motion. "So, this gas would certainly be able to reach the galaxy quickly enough to supply the necessary fuel for continued star formation."

While gas inflow has already been observed in local galaxies, the new study provides the first evidence for this process occurring in distant galaxies. Rubin's team examined the gas of 100 galaxies located between five and eight billion light years and found six galaxies that displayed evidence of gas adrift in space flowing back onto the galaxy. The team say that because of the galaxies' orientation relative to the observer, as many as 40 percent of the galaxies might display this inflow behaviour and thus evidence of galactic "recycling".

"There are also many reasons why we might miss inflows in the remaining 60 percent, even though the galaxies are mostly still forming stars quite vigorously, so they are not yet 'dead'," says Rubin. "It could be that the inflowing gas is too diffuse to be detected in our data or that the inflow is occurring along a sightline that is inaccessible to us. Also, many of the galaxies we looked at have somewhat disturbed morphologies, which could be a sign of past merger activity that could act to trigger star formation."

The results are reported in the journal Astrophysical Journal Letters.