This Is What Happens When a Galaxy Runs Out of Gas

Just like your car, it stalls. And now we have photographic evidence of the stalling.

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A false-color ultraviolet image of the dwarf galaxy IC3418 with its ram-pressure-stripped tail of young stars, shown on a false-color X-Ray image of the Virgo cluster and showing hot gas in the cluster. The size of IC3418 here is magnified by a factor of 10 relative to the cluster, in order to make it easier to see. (Yale)

The Virgo Cluster is the nearest large galaxy collection to the Local Group, the group that includes the Milky Way. It contains some 1,000 galaxies. Deep inside it, there's a galaxy to which humans have given a practical yet clunky name: IC3418.

Sometime between 200 and 300 million years ago, the core of IC3418 stopped making stars. The tail of IC3418, however, is dotted with a series of fireballs: bright blobs of gas lit up by stars that were likely formed in the last few million years. IC3418 is likely undergoing a process known as "ram pressure stripping": a galaxy being stripped of its gases by "winds" that are more powerful than the galaxy's gravitational pull. Under the stripping process, while existing stars stay intact, the galaxy's interior gas gets swept away. Which is significant -- because a galaxy devoid of gases is a galaxy that is, effectively, dead. "Stars, planets, and life can form only if a galaxy has gas to make them," explains the Yale astronomer Jeffrey D.P. Kenney.

IC3418, in other words, is running out of gas. It's running on fumes. And that, in turn, is significant because Kenney and his colleagues argue that they've captured images of IC3418 in the process of dying.

If that is, indeed, what the astronomers have witnessed, it will be the first time humans have obtained such an image.

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Dwarf Elliptical Galaxy NGC 205, a galaxy within the Local Group, pictured at lower left (J.-C. Cuillandre)

In a presentation today to the 222nd American Astronomical Society meeting in Indianapolis, Kenney and his team presented the paper summarizing their findings. (The document's delightfully predicated title? "Transformation of a Virgo Cluster Dwarf Irregular Galaxy by Ram Pressure Stripping: IC3418 and its Fireballs.") Their work, which they based on both optical imaging and spectroscopy, builds on existing evidence about IC3418: while previous research has certainly speculated that the galaxy is experiencing ram pressure stripping, Kenney and his colleagues are making the explicit case that IC3418 is, indeed, an example of a galaxy near death. (Their work relies on images from both the WIYN 3.5-meter telescope in Arizona and the twin Keck 10-meter telescopes in Hawaii.)

The work gives new status to IC3418. It also, however, offers scientists the potential for a better understanding of how dwarf elliptical galaxies, a subspecies of the universe's most common type of galaxy, come to exist. "We think we're witnessing a critical stage in the transformation of a gas-rich dwarf irregular galaxy into a gas-poor dwarf elliptical galaxy -- the depletion of its lifeblood," Kenney explained. He added: "Until now, there has been no clear example of this transformation happening."