Brightest quasars quash star formation
20 November 2012
The brightest quasars can totally ionise the hydrogen gas in their host galaxies, rendering that gas undetectable by radio telescopes. That's the conclusion reached by Dr Matthew Whiting (CSIRO) and Dr Steve Curran (University of Sydney) – and it has implications for surveys to be done with ASKAP and the SKA.
Curran and Whiting were trying to solve a problem that has become noticeable only in recent years.
"When astronomers have gone looking for “neutral hydrogen” (atomic hydrogen gas, the raw material for making stars) in distant galaxies, they haven’t been able to find it," Whiting said.
"We thought it was a problem with our telescopes not being sensitive enough, but it looks like the gas just isn't there."
The research duo have come up with a model that explains how ultraviolet radiation from the brightest of quasars can strip the electrons off hydrogen gas throughout even the largest of galaxies.
"When the gas is in this state it’s too agitated to collapse and form stars," said Whiting, "When it's in this state it is also not emitting or absorbing the 21-cm wavelength that can be detected with radio telescopes."
This is an important constraint to know about when designing surveys for galaxies containing a lot of hydrogen gas, Whiting says.
"Concentrating on quasars that are faint in the ultraviolet will be crucial for searches made with the SKA, as we will want to look where there is likely to be gas to see, and so maximise the discovery potential of this major facility. And some of the surveys planned with ASKAP, particularly FLASH, are being designed with this constraint in mind."
After the 'quasar phase' is over, the ionised gas would recombine into atomic hydrogen and star formation could start up again, Whiting said. "After the excitement, galaxies would get their normal lives back."
Publication:
S.J. Curran & M. T. Whiting “Complete ionisation of the neutral gas: why there are so few detections of 21-cm hydrogen in high redshift radio galaxies and quasars” in The Astrophysical Journal 759-117
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