Any dictionary will tell you that a galaxy is a vast collection of stars, floating deep in space. But this definition may need revision following new research by an ANU graduate student who has discovered galaxies that consist mostly of gas, rather than stars.
In research to be presented to the General Assembly of the International Astronomical Union in Sydney today, Brad Warren will reveal his discovery of twenty gassy galaxies, which have very few stars.
"When you look for gas [in these galaxies] the signal just booms in," Mr Warren said. "But when you look for stars, all you see is a barely recognisable smudge."
The galaxies are vast discs of hydrogen, tens of thousands of light years across, weighing more than a billion suns, with a tiny number of barely visible stars in their centre.
For an unknown reason, they have not transformed their rich source of hydrogen gas into masses of stars like their brilliant, twinkling counterparts.
"Hydrogen is the most common element in the Universe and it forms the building blocks for stars," Mr Warren said.
"Most galaxies, like our own Milky Way, have transformed most of their gas into stars but the galaxies we have discovered have held back and we are not sure why.
"Discovering this missing link will give us important insights into how, when and why galaxies, such as our own, formed."
Although the existence of gassy galaxies has been documented in the past, it is the first time they have been discovered with such prominent discrepancies between the amount of hydrogen gas and stars.
"This research throws up a further challenge in the ongoing quest to discover the secrets of the Universe," Mr Warren said.
Mr Warren, from the Research School of Astronomy and Astrophysics, collaborated with fellow ANU researcher, Dr Helmut Jerjen, and Dr Baerbel Koribalski, from CSIRO's Australia Telescope National facility.
The team used three of Australia's most powerful telescopes for their research - the Parkes Radio Telescope; the Australia Telescope Compact Array near Narrabri and the University's 2.3 metre telescope at Siding Spring Observatory, Coonabarabran.
Materials provided by CSIRO Australia. Note: Content may be edited for style and length.
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