Galaxy birth: The UV vision of NASA's Galaxy Evolution Explorer reveals, for the first time, dwarf galaxies (circled) forming out of nothing more than pristine gas leftover (hydrogen, in blue) from the early universe. Dwarf galaxies are relatively small collections of stars that often orbit around larger galaxies like our Milky Way.
Credit: NASA/JPL-Caltech/DSS
BRISBANE: Experts thought that dark matter was a prerequisite for the birth of galaxies. But a new study of dwarf galaxies, which are forming from remnants of the universe's primordial gases, is turning that idea on its head.
These dwarf galaxies – which are relatively small, each containing only a few billion stars – are forming in a ring-shaped cloud of helium and hydrogen in the constellation Leo, according to a report in the British journal Nature this week.
Dominated by dark matter
The cloud, discovered 25 years ago, appears to lack dark matter, which was initially perplexing for astronomers. "Almost all other galaxies we know are dominated by dark matter," said lead author David Thilker.
In most galaxies, it is thought that invisible dark matter acted as a gravitational seed that attracted visible matter like stars, gas and dust, said Thilker, an astronomer at John Hopkins University in Baltimore, USA.
The only known exceptions were 'tidal dwarf galaxies', formed when two large galaxies collided and the stresses of the encounter dragged some matter away from their parents into new galactic offspring. These galaxies are enriched with metals from their parents, which were themselves formed with dark matter.
However, the newly analysed dwarf galaxies, appear to have neither the metal signature nor dark matter, leading researchers to conclude that there is a third, previously-undiscovered method of galaxy formation.
Amazing discoveries
The new galaxies were discovered with NASA's Galaxy Evolution Explorer spacecraft (GALEX), which is extremely sensitive to ultraviolet (UV) light and spotted the distinctive UV signature of massive, short-lived stars within in gaseous clumps in the ring.
"GALEX has consistently surprised us with its UV view of the universe," said Thilker. "I suspect there will be more amazing discoveries to come." The next step is a series of even more sensitive observations of the gaseous ring to find out just how the new galaxies are forming, he said.
One hypothesis is that the gravitational influence of a nearby galaxy, M96, might be the catalyst for the new galaxies, but no-one is yet sure, added Thilker.
Despite the fact the researchers have never seen the new method of galaxy formation before, they speculate that it may have been common in the early universe when pristine gas clouds were more abundant.
Warrick Couch, an astrophysicist at Swinburne University in Melbourne, Australia, said the work was important, because it shows that not all galaxies were formed in the distant past.
"We need to change our view that all galaxies are old and formed long ago," said Couch. "This result shows that there are dwarf galaxies that are just starting to form right now."
However, he said that the UV measurements indicating that the Ring was made of primordial gas were not definitive, and more detailed spectroscopy observations were needed to confirm the theory. "If the gas is not primordial, then other less significant scenarios come into play," Couch said.
