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Jurassic Space: Ancient Galaxies Come Together After Billions of Years

Release date: Feb 18, 2010 9:00 AM (EST)

Imagine finding a living dinosaur in your backyard. Astronomers have found the astronomical equivalent of prehistoric life in our intergalactic backyard: 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 these galaxies by NASA's Hubble Space Telescope offer a window into what commonly happened in the universe's formative years when large galaxies were created from smaller building blocks. The Hubble 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. Astronomers know the system has been around for a while, because the oldest stars in a few of its ancient globular clusters are about 10 billion years old. The encounter, though, has been going on for about a few hundred million years, the blink of an eye in cosmic history. Everywhere the astronomers looked in this compact group they found batches of infant star clusters and regions brimming with star birth. Hubble reveals that the brightest clusters, hefty groups each holding at least 100,000 stars, are less than 10 million years old. The entire system is rich in hydrogen gas, the stuff of which stars are made. Astronomers used Hubble's Advanced Camera for Surveys to resolve the youngest and brightest of those clusters, which allowed them to calculate the clusters' ages, trace the star-formation history, and determine that the galaxies are undergoing the final stages of galaxy assembly.

The composite image of Hickson Compact Group 31 shows the four galaxies mixing it up. The bright, distorted object at middle, left, is actually two colliding dwarf galaxies. The bluish star clusters have formed in the streamers of debris pulled from the galaxies and at the site of their head-on collision. The cigar-shaped object above the galaxy duo is another member of the group. A bridge of star clusters connects the trio. A longer rope of bright star clusters points to the fourth member of the group, at lower right. The bright object in the center is a foreground star. The image was composed from observations made by the Hubble Space Telescope's Advanced Camera for Surveys, NASA's Spitzer Space Telescope, and the Galaxy Evolution Explorer (GALEX).

The Full Story
Release date: Feb 18, 2010
Jurassic Space: Ancient Galaxies Come Together After Billions of Years

Imagine finding a living dinosaur in your backyard. Astronomers have found the astronomical equivalent of prehistoric life in our intergalactic backyard: 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," says 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."

Everywhere the astronomers looked in this group 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. Gallagher and her team used Hubble's Advanced Camera for Surveys to resolve the youngest and brightest of those clusters, which allowed them to calculate the clusters' ages, trace 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 (GALEX) and NASA's Swift satellite. Those data helped the astronomers measure the total amount of star formation in the system. "Hubble has the sharpness to resolve individual star clusters, which allowed us to age-date the clusters," Gallagher adds.

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 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. The group has about five times as much hydrogen gas as our Milky Way Galaxy.

"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 says. "The galaxies are relatively small, comparable in size to the Large Magellanic Cloud, a satellite galaxy of our Milky Way. Their velocities, measured from previous studies, show that they are moving very slowly relative to each other, just 134,000 miles an hour (60 kilometers a second). So it's hard to imagine how this system wouldn't wind up as a single elliptical galaxy in another billion years."

Adds team member Pat Durrell of Youngstown State University: "The four small galaxies are extremely close together, within 75,000 light-years of each other – we could fit them all within our Milky Way."

Why did the galaxies wait so long to interact? Perhaps, says Gallagher, 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.

Hickson Compact Group 31 is one of 100 compact galaxy groups catalogued by Canadian astronomer Paul Hickson.

Gallagher's results appear in the February issue of The Astronomical Journal.

Her science team consists of Pat Durrell (Youngstown State University), Debra Elmegreen (Vassar College), Rupali Chandar (University of Toledo), Jayanne English (University of Manitoba), Jane Charlton, Caryl Gronwal, and Jason Young (Penn State), Panayiotis Tzanavaris (NASA's Goddard Space Flight Center), Kelsey Johnson (University of Virginia), Claudia Mendes de Oliveira (University of São Paulo), Brad Whitmore (STScI), Ann Hornschemeier (NASA's Goddard Space Flight Center), Aparna Maybhate (STScI), and Ann Zabludoff (University of Arizona).