A Galactic Striptease

Pity C153. Astronomers using a broad suite of instruments have caught this spiral galaxy performing an involuntary striptease act as it plunges through the heart of a massive galaxy cluster at nearly 2,000 kilometers per second.

Images and spectra in radio, optical, and X-ray wavelengths show a galaxy’s worth of gas being stripped from C153, creating a 200,000-light-year-long wake that resembles the tail of a comet. Countless other galaxies have undoubtedly endured the same humiliation, which explains why extremely massive galaxy clusters in the local universe contain so few spirals.

"The galaxy has lost all of its gas; it’s a shadow of its former glory," said William Keel (University of Alabama) at a press conference held during the 2004 American Astronomical Society conference in Atlanta. Keel leads a team that has observed C153 for nearly a decade with the Hubble Space Telescope and ground-based instruments.

"It’s a tragedy on a galactic scale," added Q. Daniel Wang (University of Massachusetts), who observed C153’s fleeing gas with the Chandra X-ray Observatory. The Chandra observations indicate that C153 is losing gas at the whopping rate of 10 solar masses per year. "This is a fast way to transform a galaxy from gas-rich to gas-poor," said Wang.

C153’s problem began several hundred million years ago as it fell toward Abell 2125, a cluster 3 billion light-years away at the border of Ursa Major and Draco. This cluster is one of the richest known, containing roughly 100 times the mass of the much closer Virgo Cluster. Astronomers are catching the galaxy right as it’s passing through the densest region of hot gas in the cluster’s center. Pressure from the hot gas is stripping C153’s cooler gas. But this has little effect on the galaxy’s stars, which remain speeding on their way unperturbed.

Five billion years ago, huge clusters such as Abell 2125 were rich in spirals. But today the spirals are gone. Rich clusters instead contain elliptical and SO galaxies — flywheel-shaped galaxies that have huge central bulges and puny disks. "This is one of the processes that has to explain galaxy transformations in clusters," said Keel. "We are seeing a galaxy being shut down. If we could look at C153 in a few billion years, it will be a big, fat SO galaxy."