November 12, 2011

The Hubble Space Telescope, despite being in the last phase of its tenure in orbit, continues to offer new glimpses into the universe. Now the telescope has peered deep into space and 10 billion years into the past and delivered new images of 18 dwarf galaxies that are, to the excitement of scientists, creating stars at an extremely rapid rate. In fact, the galaxies are churning out stars at such a rate that the number of stars in them would double in just 10 million years. To form stars so rapidly you must have an enormous amount of gas condensing in the cores of these galaxies. By comparison, our galaxy, the much older and larger Milky Way, has taken a thousand times longer to double its star count. The observations were part of the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS), an ambitious three-year project that studies the most distant galaxies in the universe. CANDELS is using Hubble to image a much larger region of space than was observed before, and is going extremely deep into the infrared end of the light spectrum to pick up the faintest specks of light. The new discovery came when CANDELS found a population of small "dwarf" galaxies that were unusually bright in one particular wavelength. 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. Galaxies with rapidly forming stars characteristically feature huge quantities of oxygen, an element astronomers were not expecting to find. Many current models suggest dwarf galaxies form stars more slowly, over billions of years, to gradually build up to what we see today. However, the implications of these observations is that galaxies form the bulk of their stars extremely rapidly in a burst of star formation. This turns our picture of star formation on its head. Astronomers are able to make detailed analyses of dwarf galaxies because there are other, much closer, examples orbiting the Milky Way, which can be used for comparison. On top of that, there was the significant fact that so many dwarf galaxies were discovered apparently behaving the same way at the same time indicating that this must be a very common process. It's not yet clear exactly how old these dwarf galaxies are, though they must be significantly younger than the Milky Way. Similarly, astronomers cannot be sure how many generations of stars have appeared previously in those galaxies, since the new wave of star births is outshining any older siblings they have. The big mystery is why this population of young galaxies was creating stars at such a high rate. Alas, the Hubble may not be around to unravel that one. The aging telescope has now been in orbit around the Earth since 1990, and received one last upgrade - which included the Wide Field Camera 3 used by CANDELS - in 2009.