The stars in this compact galaxy appear to be about 2 billion years old or more. The galaxy itself is seen as it was just 2.7 billion years after the big bang, suggesting it formed and matured very quickly in the early universe, perhaps by collapsing directly from a massive cloud of matter rather than by growing up gradually through mergers between smaller objects (Image: IfA, U of Hawai/W. M. Keck Observatory)

Astronomers have found a new clutch of tiny, dense galaxies that thrived in the early universe. But they still can’t explain why the compact objects are nowhere to be found today.

The ultra-dense galaxies, which were already full of old stars when the universe was less than 3 billion years old, were first reported in 2008. The most extreme have masses that rival the Milky Way’s, but are just one-tenth as wide.

“They’re packing essentially as much mass as the normal galaxies that we see around us today in [a volume] about a thousand times smaller,” says Alan Stockton of the University of Hawaii in Honolulu.


Using the Keck II Telescope on Mauna Kea in Hawaii, Stockton and colleagues have found new examples of these galaxies at distances of about 11 billion light years away. Stockton presented images of the objects on Tuesday at a meeting of the American Astronomical Society in Pasadena, California.

The galaxies are so massive and some have such fragile disc shapes that they seem to have formed directly from the collapse of massive clouds of matter instead of being built up gradually by a series of mergers of smaller objects, Stockton says.

Slow expansion?

Galaxies of this sort are no longer around, but it’s not clear what happened to them. Some have suggested that the galaxies disappeared because they collided with other galaxies to snowball into the large galaxies we see today.

But there may be problems with that explanation, Stockton says. The compact galaxies are so dense they are hard to modify by mergers with other galaxies, particluarly lightweight ones, he says. “The very compact galaxies here are essentially like little cannonballs,” Stockton told reporters. “You can imagine merging a cannonball with, say, a pillow. Not much is going to happen to the cannonball.”

“Of course, if two of these dense stellar systems do merge, the resulting remnant will have a larger effective radius than either of the merging galaxies,” Stockton told New Scientist. “But it would require many such mergers to bring their sizes up to typical elliptical galaxies in the present-day universe – probably more than is realistically feasible.”

Alternatively, the tiny galaxies might grow by slowly accumulating an envelope of diffuse gas around them through successive interactions with other galaxies, Stockton says. Others have suggested that the galaxies might puff up on their own, as aged stars explode or slough off their outer layers, ejecting gas to the outer edges of the galaxy.

But it’s unclear whether there is enough time for such gradual processes to work. Last year, another team found that compact galaxies were still present when the universe was almost 5 billion years old, narrowing the amount of time that such galaxies would have to puff up to present sizes.