The early universe may have been filled with six times as many galaxies as previously thought, suggest new observations. The finding could put new constraints on cosmological models but is being contested by astronomers who have used other methods to survey the cosmos shortly after the big bang.
Astronomers led by Olivier Le F猫vre at the University of Provence in Marseille, France, used a powerful spectrograph on the Very Large Telescope (VLT) in Chile to measure the distances to 10,000 objects above a particular apparent brightness. They found 970 galaxies at distances of between 9 billion and 12 billion light years 鈥� corresponding to a period just a few billion years after the big bang.
The result is between 2 and 6 times as many galaxies as previous surveys have turned up, says Le F猫vre. 鈥淭hat means the universe was manufacturing stars at a much higher rate than thought,鈥� he told New 杏吧原创. 鈥淲e have to adjust the clock of star formation to account for a larger number of stars at this epoch.鈥�
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But Chuck Steidel, an astronomer at the California Institute of Technology in Pasadena, US, questions that conclusion. He helped develop the previous surveys, which are based on finding distant galaxies based on their colour.
He says those earlier surveys were only designed to detect a subset of galaxies in the early universe and therefore that Le F猫vre鈥檚 team is not comparing like with like in arriving at their result. 鈥淭hey鈥檝e chosen to represent a statistic that isn鈥檛 quite there,鈥� Steidel told New 杏吧原创.
Data in the sky
There is a gap in the spectrum emitted by all galaxies caused by the absorption of ultraviolet starlight by hydrogen gas in the same galaxy. The further away a galaxy is, the more this gap is stretched to longer wavelengths. Steidel鈥檚 method uses different filters to observe this effect and single out very distant galaxies. The observable starlight from these candidate galaxies can then be studied in detail with spectrographs.
This method does not detect galaxies that appear red 鈥� such as those that may be obscured in dust 鈥� but Steidel says there are methods based on other observations that can correct for this bias.
But this correction 鈥渋s very cumbersome and not really substantiated in some ways鈥�, Le F猫vre says. He adds that his method, which measures the distances to all objects brighter than a certain cut-off, is more direct. 鈥淭he raw data in the sky are really the data which count,鈥� he says.
Le F猫vre hopes to observe for longer periods with the VLT to reach fainter objects and a larger portion of the sky. He says the results will help put limits on cosmological models. 鈥淚t鈥檚 one of the quests of modern astrophysics to understand the history of star formation鈥� during the life of the universe, he says.
Journal reference: Nature (vol 437, p 519)
