Washington: Astronomers have found the astronomical equivalent of prehistoric life in our intergalactic backyard, in the form of a group of small, ancient galaxies that has waited 10 billion years to come together.
These “late bloomers” are on their way to building a large elliptical galaxy.
Such encounters between dwarf galaxies are normally seen billions of light-years away and therefore occurred billions of years ago.
But these galaxies, members of Hickson Compact Group 31, are relatively nearby, only 166 million light-years away.
New images of this foursome by NASA’s Hubble Space Telescope offer a window into the universe’s formative years when the buildup of large galaxies from smaller building blocks was common.
Astronomers have known for decades that these dwarf galaxies are gravitationally tugging on each other.
Their classical spiral shapes have been stretched like taffy, pulling out long streamers of gas and dust.
The brightest object in the Hubble image is actually two colliding galaxies.
The entire system is aglow with a firestorm of star birth, triggered when hydrogen gas is compressed by the close encounters between the galaxies, and collapses to form stars.
The Hubble observations have added important clues to the story of this interacting group, allowing astronomers to determine when the encounter began and to predict a future merger.
“We found the oldest stars in a few ancient globular star clusters that date back to about 10 billion years ago. Therefore, we know the system has been around for a while,” said astronomer Sarah Gallagher of The University of Western Ontario in London, Ontario, leader of the Hubble study.
“Most other dwarf galaxies like these interacted billions of years ago, but these galaxies are just coming together for the first time. This encounter has been going on for at most a few hundred million years, the blink of an eye in cosmic history. It is an extremely rare local example of what we think was a quite common event in the distant universe,” she added.
Everywhere the astronomers looked in this group, they found batches of infant star clusters and regions brimming with star birth.
The stars are feeding off of plenty of gas.
A measurement of the gas content shows that very little has been used up — further proof that the “galactic fireworks” seen in the images are a recent event.
“This is a clear example of a group of galaxies on their way toward a merger because there is so much gas that is going to mix everything up,” Gallagher said.