NASA's Hubble Space Telescope has identified 18 "tiny" galaxies which existed 9 billion years ago and are brimming with star birth.
The galaxies - located in a field known as the Great Observatories Origins Deep Survey (GOODS) - are among 69 dwarf galaxies found in the GOODS (marked by green circles in the large image) and other fields.
While dwarf galaxies are the most common type of galaxy in the universe, the rapid star-birth observed in these newly found examples may force astronomers to reassess their understanding of the ways in which galaxies form.
To be sure, the galaxies are a hundred times less massive, on average, than the Milky Way, yet churn out stars at such a furious pace that their stellar content would double in just 10 million years. By comparison, the Milky Way would take a thousand times longer to double its star population.
The universe is estimated to be 13.7 billion years old, and these newly discovered galaxies are extreme even for the young universe - when most galaxies were forming stars at higher rates than they are today.
Astronomers using Hubble's instruments could spot the galaxies because the radiation from young, hot stars has caused the oxygen in the gas surrounding them to light up like a bright neon 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," explained Arjen van der Wel of the Max Planck Institute for Astronomy in Heidelberg, Germany. "We weren't looking specifically for these galaxies, but they stood out because of their unusual colors."
In addition to the images, Hubble also captured spectra that show the oxygen in a handful of galaxies and confirmed their extreme star-forming nature.
"Spectra are like fingerprint," said Amber Straughn at NASA's Goddard Space Flight Center in Greenbelt, Md. "They tell us the galaxies' chemical composition."
Interestingly enough, the resulting observations are somewhat at odds with recent detailed studies of the dwarf galaxies that are orbiting as satellites of the Milky Way.
"Those studies suggest that star formation was a relatively slow process, stretching out over billions of years," noted Harry Ferguson of the Space Telescope Science Institute (STScI) in Baltimore, Md.
"[The discovery] 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."
Indeed, the observations suggest that the newly discovered galaxies were very common 9 billion years ago. However, it is a mystery why the newly found dwarf galaxies were making batches of stars at such a high rate.
Computer simulations show star formation in small galaxies may be episodic. Gas cools and collapses to form stars, which then reheat the gas and blow it away, as in supernova explosions. 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 what the simulations can reproduce," added van der Wel.