London, August 19 : New research has shortened the timescale during which the mysterious bloating of dense, compact galaxies in the early universe happened.
In April, astronomers reported finding extremely compact galaxies as far back as 10 billion years ago, or 3.7 billion years after the big bang.
The galaxies contained the same number of stars as modern, blob-shaped galaxies known as ellipticals - but were two to three times smaller on average.
Now, according to a report in New Scientist, observations have turned up compact galaxies roughly a billion years later, when the universe was almost 5 billion years old.
Some, dubbed 'red nuggets', are even more compact than those seen in the earlier study - weighing as much as modern ellipticals, but measuring only a tenth their size.
"There's nothing like this in the nearby universe," said astronomer Roberto Abraham of the University of Toronto in Canada. "These things are a complete, out-of-left-field surprise," he added.
Astronomers do not see any of these galaxies in surveys of the present-day universe, so they reason something must have happened to cause them to get larger without forming more stars.
"The ideal way is to figure out a way to puff them up, to make them bigger without changing their mass," said Abraham. "But it's hard to come up with a model that can do that," he added.
That's because the galaxies had to puff up very quickly.
Abraham and colleagues, led by Ivana Damjanov of the University of Toronto, studied galaxies at different eras and found that the compact galaxies had disappeared from the universe only 1.6 billion years after their own observations of the red nuggets.
Various explanations have been proposed for the galaxies' bloating, but none accounts for all of the observations.
One possibility is that the galaxies got larger by colliding and combining with other galaxies. Such mergers tend to produce newborn stars, since the collision triggers gas clouds within the galaxies to start condensing.
But, since both the modern elliptical galaxies and the old compact varieties seem to be devoid of young stars, astronomers suggested that the mergers must have been 'dry' - occurring between galaxies with a dearth of star-forming clouds.
Still, such mergers would increase the number of stars - and therefore the mass - in the resulting galaxies - and observations show compact galaxies and ellipticals are roughly equivalent in mass.
What's more, statistics dictate that some compact galaxies should survive to the present day without merging.
According to Damjanov, a combination of mergers and expansion might solve the mystery, but more observations are needed to test the idea.