Galactic cannibalism strikes again

An arc of blue stars that stretches for thousands of light-years sits just above the nearby galaxy Centaurus A. Astronomers analyzing the arc have discovered that it’s the stellar remains of a tiny galaxy that was swallowed by Centaurus A only a few hundred million years ago. In the December Astronomical Journal, the scientists report that this relatively recent example of galactic cannibalism is another indication that material ripped from smaller galaxies is a key contributor to the formation of halos–the tenuous outer perimeters of galaxies.

GALACTIC REMAINS. Arc of stars (arrows) sits above the galaxy Centaurus A. Peng/JHU/NOAO/AURA

“This adds a nice example in the local universe to the growing evidence that galactic halos are built up from the accretion of dwarf satellite galaxies,” says study coauthor Eric Peng of the Johns Hopkins University in Baltimore. Halos are intriguing, he adds, because their ancient stellar denizens provide data about galaxies as they were when they were forming billions of years ago.

Other astronomers had detected the arc but didn’t recognize it as the remnant of a galactic merger. Using a new wide-field camera attached to a 4-meter telescope at the Cerro Tololo Inter-American Observatory near La Serena, Chile, Peng’s team viewed Centaurus A through several color filters. Those images revealed the predominance of young stars in the arc, which probably were born following a recent merger with Centaurus A.

Peng says he would now like to measure the velocities of star clusters in the arc and map their course. That would enable his team to better determine how long ago the galactic merger occurred and what was the motion of the galaxy as it was swallowed.


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