NOT all galaxies grow up to be productive star-forming members of the universe. But just as delinquent youth can reveal a lot about the human condition, these delinquent galaxies could tell us about the conditions required for star formation.
Galaxies begin as clouds of hydrogen and dark matter that contract under their own gravity until they become dense enough to create stars. Usually that triggers a runaway reaction, with more and more stars forming until the ratio of star mass to gas mass is more than 2 to 1.
But certain galaxies appear to stop developing almost as soon as they start, with the most extreme examples containing 10 or more times as much gas as stars. Nor do these 鈥済as galaxies鈥� have the excuse of youth on their side. They are roughly the same age as the Milky Way, giving them plenty of time to have completed the process of star formation, Brad Warren of the Australian National University in Canberra told the meeting.
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Warren, along with Helmut Jerjen, also of ANU, and B盲rbel Koribalski of the Australia Telescope National Facility in Sydney, discovered the gas galaxies after scrutinising the results of the HI Parkes All-Sky Survey. HIPASS is an ongoing 鈥渂lind鈥� survey: rather than homing in on objects that emit visible light, the sky is scanned piece by piece by the Parkes radio telescope in New South Wales. When Warren and his colleagues studied the data, they discovered that many of the dimmest galaxies contain huge amounts of radio-emitting gas. They are now studying 20 of the gassiest galaxies.
鈥淭hey have a large number of galaxies at the extreme end of the spectrum. That gives them the hope of working out what conditions are needed for star formation,鈥� says Erwin de Blok of Cardiff University in the UK.
One theory is that gravitational interaction with other galaxies is needed to kick start the process of contraction and star formation 鈥� a theory supported by the initial findings, since most of the gas galaxies appear to have no near neighbours. Another possibility is that gas galaxies undergo such fierce bursts of star formation early on that the resulting stellar winds blow apart the gas before it has time to form new stars.
The gas galaxies also raise questions about the universe at large. 鈥淭he real puzzle is why don鈥檛 you see galaxies that are entirely hydrogen and dark matter with no stars at all. There is something profound underlying all this,鈥� says Ken Freeman, also of ANU.
