UNIVERSITY PARK – A team of astronomers using NASA’s Hubble Space Telescope and other space instruments has discovered that a collection of small, ancient galaxies, called the Hickson Compact Group 31 (HCG 31), finally is coming together into one larger galaxy after 10 billion years. The images the team recorded offer a window into the universe’s early years, when the buildup of large galaxies from smaller building blocks was common.
According to Jane Charlton, a Penn State professor of astronomy and astrophysics and one of the team’s leaders, the late-blooming galaxy is in its final stage of development. “Most other small galaxies in the universe came together into larger galaxies billions of years ago,” said Charlton, “but the galaxies within HCG 31 have been interacting for only a few hundred million years, which is just a blink of an eye in cosmic history.” The team’s results appear in the February issue of The Astrophysical Journal. More information can be found online.
Astronomers have known for decades that the HCG 31 galaxies are tugging on each other through gravitational pulls, but new observations by the Hubble telescope have added important clues to the story of this interacting foursome, allowing the team to determine when the encounter began and to predict a future merger. According to Sarah Gallagher, an astronomer at the University of Western Ontario and principle author of the HCG 31 publication, “We found the oldest stars in a few ancient globular star clusters that date back to about 10 billion years. Therefore, we know the system has been around for a while.”
The galaxies have been stretched like taffy as a result of their gravitational effect on each other.
“Our ability to observe HCG 31 is important because it can teach us about how galaxies change their forms over time from spirals like the Milky Way to giant elliptical galaxies,” said Charlton.
The HCG 31 group of galaxies also is aglow with a firestorm of star birth, which is triggered when hydrogen gas first is compressed by the close encounters between galaxies and then collapses to form stars.
“Galaxy formation normally takes place billions of light-years away — too far for us to see the intergalactic show that we believe the hydrogen gas as it forms stars puts on,” said Charlton. “We are able to observe the ‘fireworks’ that result from the compression of hydrogen gas within HCG 31 because the system is so close to us, only 166 million light-years away.”
Everywhere the astronomers looked within HCG 31, they found batches of infant star clusters and regions brimming with star birth. The team used Hubble’s Advanced Camera for Surveys to resolve the youngest and brightest of those clusters, which allowed them to calculate the ages of the clusters, to map the star-formation history, and to determine that the galaxies are undergoing the final stages of galaxy assembly.
The team bolstered their analysis with infrared data from NASA’s Spitzer Space Telescope and ultraviolet observations from the Galaxy Evolution Explorer (GALEX), which helped the astronomers to measure the total amount of star formation in the system.
Hubble revealed 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 hydrogen gas. Yet, a measurement of the gas content shows that very little has been used up — further proof that the “galactic fireworks” 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,” said Gallagher. “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. So it’s hard to imagine how this system wouldn’t wind up as a single elliptical galaxy in another billion years.” Why did the galaxies take so long to interact? Perhaps, the astronomers said, 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.
A high-resolution image related to this story is online.
Barbara Kennedy, Penn State University