News in Science

Mystery of the first stars deepens

Stellar Mystery It usually takes lots of dust and gas to make stars and galaxies, but a new study has discovered that there was very little dust around to make stars in the very early universe.

The findings, reported in the journal Nature, raise huge questions about how the first stars were made.

Existing models based on observation in the local universe explain how stars are formed, but don't work under the different condition of the early cosmos, say the study's lead author, Dr David Fisher of the University of Maryland and Swinburne University.

"The universe seems to know how to make galaxies, after all we're here," says Fisher. "So it could make stars early on, but we just don't understand how."

To try to answer that question, astronomers look for local galactic analogues which have similar physical conditions to those that existed when the first galaxies formed.

These conditions include a large rate of star formation, and a low concentration of certain elements. The early universe consisted of the elements hydrogen and helium with trace amounts of lithium and beryllium. The first stars, composed of only these elements, manufactured all the other elements that exist today, which astronomers call metals.

Scientists have also found evidence suggesting that the early universe had very little dust, which made it hard to explain how stars formed at that time.

"Our models of star formation are based around there being a lot of dust to cool the gas, acting as a facilitator to get the gas to condense into star forming regions," says Fisher.

"Without it, our models have a lot of trouble making stars."

Perfect match

Fisher and colleagues were using the European Space Agency's Herschel space telescope to observe dust in an analogue galaxy called 1-Zwicky18, located 58 million light years away, which has many of the characteristics they expected to find in far more primitive galaxies.

"1-Zwicky18 has active star formation occurring, and is metal poor, having about 4 per cent of the metals we have in our local galaxy," says Fisher.

As expected there was also very little dust in this galaxy but Fisher and colleagues were surprised at just how little.

Fisher and colleagues took their dust measurements from 1-Zwicky18 and applied them to an early galaxy named Himiko, which formed just 850 million years after the big bang.

"We measured the lowest dust mass that had ever been measured, and much lower than what we were expecting to get by a factor of about 100," says Fisher.

"It's a big mystery," he says. "This problem of how to make stars in the early universe may be even greater than we thought it was."

Wrong telescopes

Fisher believes the lack of dust will also make early galaxies far more difficult to study, even with the new generation of giant radio telescopes now under construction, such as ALMA, the Atacama Large Millimetre submillimetre Array in Chile.

"It's saying these galaxies are going to be much fainter than we were expecting," says Fisher.

"We probably won't be able to detect them very well, even with ALMA, which is the most powerful telescope on Earth right now.

"People have been using ALMA to try to observe dust in early universe galaxies, and they're failing to find detections in all but the most extreme super bright galaxies, because there's much less dust than the theories are telling us."

However, the findings are good news for the next generation of super large optical telescopes now under construction, because first generation stars shine mostly in ultraviolet.

"The fact that there's no dust is a good thing for ultraviolet light because dust would block this light, making it difficult to observe," says Fisher.

"And so since there's essentially no dust in these galaxies in the early universe, they should be brighter than a comparable mass galaxy in the local universe."