NASA photograph of Hickson Compact Group 31

Putting together a galaxy takes time—about 10 billion years.

That’s what astronomers at UVA and elsewhere have concluded in observing four small, ancient galaxies in the final stages of forming a single, larger entity.

The galaxies are part of Hickson Compact Group 31, which is about 166 million light years away. New images from the Hubble Space Telescope have allowed scientists to observe the foursome, determine when they began interacting and predict their future merger.

In galactic terms, they are late bloomers, says UVA astronomer Kelsey Johnson.

“Most galaxies are quite social, living in groups,” she says. “The galaxies in Hickson Compact Group 31 appear to have lived most of their lives as wallflowers, but in their middle age are now the life of the party. The level of interaction between these galaxies would have been typical in the earlier universe, but it is very unusual today.”

What took so long for them to interact? The system resides in a lower-density region of the universe, the equivalent of a rural village. Johnson and her colleagues suspect getting together took billions of years longer than it did for galaxies in denser areas.