Listen to the UVA Today Radio Show report on this story by Fariss Samarrai:
February 17, 2010 — Astronomers have found the astronomical equivalent of a prehistoric dinosaur: a collection of four small, ancient galaxies in our intergalactic backyard that has waited 10 billion years to come together. These late bloomers are in the final stages of galaxy assembly.
Such encounters are normally seen billions of light-years away. But these galaxies, members of "Hickson Compact Group 31," are relatively nearby – only 166 million light-years away. They are close enough together that they could fit within our Milky Way.
New Hubble Space Telescope images of these galaxies offer a window into the universe's early years, when the buildup of large galaxies from smaller building blocks was common.
The finding, reported by University of Virginia astronomer Kelsey Johnson and her colleagues at the University of Western Ontario and Youngstown State University in Ohio, appears in the February issue of the Astrophysical Journal.
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 galaxies also are aglow with a firestorm of star birth, triggered when hydrogen gas is compressed by the close encounters between galaxies, and collapses to form stars.
But new observations have added important clues to the story of this interacting foursome, allowing astronomers to determine when the encounter began and to predict a future merger.
"Most galaxies are quite social, living in groups," Johnson said. "However, 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."
Everywhere the astronomers looked they found batches of infant star clusters and regions brimming with star birth. The entire system is rich in hydrogen gas, the stuff of which stars are made. Johnson and her colleagues used Hubble's Advanced Camera for Surveys to resolve the youngest and brightest of those clusters, which allowed them to calculate the clusters' ages, map the star-formation history, and determine that the galaxies are undergoing the final stages of galaxy assembly.
The analysis was bolstered by infrared data from NASA's Spitzer Space Telescope and ultraviolet observations from the Galaxy Evolution Explorer, which helped the astronomers measure the total amount of star formation in the system. "But only Hubble has the sharpness to resolve individual star clusters, which allowed us to age-date the clusters," said lead author Sarah Gallagher, an astronomer at the University of Western Ontario.
Hubble reveals that the brightest clusters, hefty groups each holding at least 100,000 stars, are less than 10 million years old. The stars are feeding off plenty of gas. A measurement of the gas content shows that very little has been used up – further proof that the "galactic fireworks" seen here are a recent event. The group has roughly the same amount of hydrogen gas as exists in our Milky Way galaxy.
"The type of interaction between these small galaxies indicates that they almost certainly will merge into a single galaxy with roughly the mass of our own Milky Way," Johnson said. "We are seeing a snapshot of how galaxies in the universe were assembled billions of years ago."
Why did the galaxies wait so long to interact? Perhaps, suggest Johnson and Gallagher, it is because the system resides in a lower-density region of the universe, the equivalent of a rural village. Getting together took billions of years longer than it did for galaxies in denser areas.