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X marks the galactic spot

An ethereal "X" or boxlike structure marks the chaotic center of the edge-on

galaxy NGC 4710. Click on the picture for a larger version.

One of the "X-Files" that astronomers keep in their filing cabinets relates to the mysterious X shape seen at the center of some galaxies — but this particular mystery may be close to being explained. At least that's what Paul Goudfrooij of the Baltimore-based Space Telescope Science Institute and his colleagues are hoping.

If you squint your eyes, you should be able to make out the faint X-shaped pattern splaying out above and below the center of the edge-on galaxy in the picture above. The image of the galaxy NGC 4710, which is about 60 million light-years away in the constellation Coma Berenices, was  released just today by the European Space Agency's Hubble Space Telescope team.

The picture is part of a Hubble survey aimed at determining how such structures, which astronomers call boxy bulges, arise in galaxies that have dense bars of stars, gas and dust in their middle. "The overall idea is to test the origins of bulges in spiral galaxies," Goudfrooij told me this week.

The X-shaped bulge is actually created by streams of stars that have been knocked out of the main plane of the galaxy due to dynamical instability. All those inclined stellar orbits produce an optical effect vaguely reminiscent of the boxy look of the famous Red Rectangle nebula.

Thanks to computer simulations, astronomers have figured out two leading scenarios for building bulges. In one scenario, the process happens very early in a galaxy's evolution, even before it develops its disk and spiral arms. In the other scenario, the bulges gradually build up and dissipate later in the life cycle of a galaxy. "They just form every now and then during the evolution of a galaxy," Goudfrooij explained.

The bulges that form gradually "typically are apparent in the late-type galaxies with small bulges [and] not as prominent in early-type galaxies with large bulges," he said.

Goudfrooij and his colleagues are looking for a way to determine which galaxies are going through which scenario, and they think globular star clusters could provide a good way to tell the difference. The pressures involved in the rapid-formation scenario should squeeze lots more star clusters into existence. In the other scenario, "when you build up a bulge through the more gradual way, you wouldn't expect to form many globular clusters in that process," Goudfrooij explained.

In the case of NGC 4710, the smallest of the boxy-bulge galaxies studied by Goudfrooij's team, it turns out that relatively few globular clusters formed. That matches what the astronomers were hoping to find. The team is still in the process of writing up their findings, but Goudfrooij said the work could lead to a more complete picture of galactic evolution - not only for spiral galaxies, but also for elliptical galaxies that are basically big balls of stars.

NGC 4710 is something of a hybrid: It's known as a lenticular galaxy, which exhibits some of the characteristics of spiral galaxies as well as elliptical galaxies. And that makes it well-suited for this kind of study. As any fan of the fictional FBI agent Fox Mulder knows, there's nothing like an alien hybrid to spice up "The X-Files."

More galactic X-Files:

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