The Hubble Space Telescope has uncovered an extraordinary population of tiny young galaxies 9 billion light-years away that are brimming with star formation. The galaxies are churning out stars at such a rate that the number of stars in them will double in just 10 million years, one-thousandth of the time it has taken the Milky Way to double its stellar population. Astronomers believe this rapid star birth represents an important phase in the formation of dwarf galaxies, the most common type in the cosmos. These newly discovered dwarf galaxies are approximately a hundred times smaller than the Milky Way. They have turned up in the Hubble images because the radiation from young hot stars has caused the oxygen in the gas surrounding them to light up like a fluorescent sign. “The galaxies have been there all along but, up until recently, astronomers have been able only to survey tiny patches of sky at the sensitivities necessary to detect them,” said Arjen van der Wel of the Max Planck Institute for Astronomy, the lead author of a paper soon to appear in the Astrophysical Journal. “We weren’t looking specifically for these galaxies, but they stood out because of their unusual colors.” Eighteen dwarf galaxies have been unveiled by the Hubble Space Telescope. Shown in the postage-stamp-size images, they existed 9 billion years ago and are brimming with star birth. They are typically a hundred times less massive than the Milky Way galaxy but are churning out stars at such a furious pace that their stellar population will double in just 10 million years. The large image shows the location of the galaxies. (Images: NASA, European Space Agency, Arjen van der Wel (Max Planck Institute for Astronomy), Harry Ferguson and Anton Koekemoer (Space Telescope Science Institute), and the CANDELS team). The observations were part of the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS), an ambitious three-year survey to analyze the most distant galaxies in the universe. CANDELS is the first census of dwarf galaxies at such an early epoch in the history of the universe. “In addition to the images, Hubble has captured spectra from a handful of these galaxies that show us the detailed physics of what’s happening within them and that confirm their extreme star-forming nature,” said co-author Amber Straughn of Goddard Space Flight Center in Greenbelt, Md. The observations of ancient galaxies are somewhat at odds with recent detailed studies of the dwarf galaxies that are currently orbiting the Milky Way. “Those studies suggest that star formation was a relatively slow process, stretching out over billions of years,” said Harry Ferguson of the Space Telescope Science Institute (STScI) in Baltimore, a co-leader of the survey. “The CANDELS finding that there were galaxies of roughly the same size forming stars at very rapid rates at early times is forcing us to re-examine what we thought we knew about dwarf galaxy evolution.” The CANDELS team uncovered the 69 young dwarf galaxies in near-infrared images taken with Hubble’s Wide Field Camera 3 and Advanced Camera for Surveys. The observations concentrated on two regions of the sky called the Great Observatories Origins Deep Survey-South and the UKIDSS Ultra Deep Survey (part of the UKIRT Infrared Deep Sky Survey). The observations suggest that the newly discovered galaxies were very common 9 billion years ago. However, it’s a mystery why the newly found dwarf galaxies were making batches of stars at such a high rate. Computer simulations show that star formation in small galaxies may be episodic. Gas cools and collapses to form stars. The stars then reheat the gas through, for example, supernova explosions, which blow the gas away. After some time, the gas cools and collapses again, producing a new burst of star formation, continuing the cycle. “While these theoretical predictions may provide hints to explain the star formation in these newly discovered galaxies, the observed ‘bursts’ are much more intense than those reproduced by the simulations,” van der Wel of Max Planck Institute said. The James Webb Space Telescope, an infrared observatory scheduled to be launched later this decade, will be able to probe these faint galaxies at an even earlier era to see the glow of the first generation of stars, providing detailed information of the galaxies’ chemical composition. “With Webb, we’ll probably see even more of these galaxies, perhaps even pristine galaxies that are experiencing their first episode of star formation,” STScI’s Ferguson said. “Being able to probe down to dwarf galaxies in the early universe will help us understand the formation of the first stars and galaxies.” For more information, visit: www.eso.org