Astronomers have measured the motions of stars for the first time in a very distant galaxy, speeding around its host at twice the speed of our Sun through the Milky Way.

The speeding stars may help astronomers understand how such compact galaxies form so early in the Universe and then evolve into the galaxies we see in today's 13.7 billion year old Universe.

"This galaxy is very small, but the stars are whizzing around as if they were in a giant galaxy that we would find closer to us and not so far back in time," says Pieter van Dokkum, professor of astronomy and physics at Yale University. The stars are clocking up speeds of over 1.6 million kilometres per hour.

Van Dokkum and colleagues used data from the Hubble Space Telescope to confirm the size of the galaxy, and the eight metre Gemini South telescope in Chile to collect enough light to determine the motions of the stars. These near-infrared spectroscopic observations of galaxy 1255-0 spanned 29 hours to allow the faint light to be collected.

"By looking at this galaxy we are able to look back in time and see what galaxies looked like in the distant past when the Universe was very young," says team member Mariska Kriek of Princeton University. Galaxy 1255-0 was born when the Universe was just three billion years old.

The team hope that the results will shed light on how such compact, massive galaxies form, and why they are not seen in today's local Universe. "One possibility is that we are looking at what will eventually be the dense central region of a very large galaxy," says Marijn Franx of Leiden University. "The centre of big galaxies may have formed first, presumably together with the giant black holes that we know exist in today's large galaxies that we see nearby."

The next step will be to capture these galaxies in the process of forming, such as with Hubble's new Wide Field Camera 3. "The ancestors of these extreme galaxies should have quite spectacular properties as they probably formed a huge amount of stars, in addition to a massive black hole, in a relatively short amount of time," says van Dokkum.

Recent work revealed that the oldest most luminous galaxies in the early Universe are also very compact, yet exhibit masses similar to those of today's elliptical galaxies. The most massive galaxies we see in the local Universe which have a mass similar to 1255-0 are typically five times larger than a young compact galaxy, so understanding how galaxies grew so much in the past 10 billion years is a key piece of evidence in eventually solving this puzzle.

The results of this study are presented in the August 6 issue of the journal Nature, along with a companion paper in the Astrophysical Journal.

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