Halo Lopped Off

A halo of stars surrounding a galaxy in the relatively nearby Virgo cluster is missing, possibly torn away by a neighboring galaxy or snuffed out by the collapse of the cluster itself.

The victim is the giant elliptical galaxy Messier 87, which lives about 50 million light-years away in the center of the Virgo, the closest galaxy cluster to Earth.

"We were surprised to see that the star in the galactic halo in M87 stopped after a certain radius in the center," Ortwin Gerhard, a researcher with Germany's Max Planck Institute for Extraterrestrial Physics, told Discovery News.

Astronomers were looking at planetary nebula, the exploded remains of stars, using a light-splitting spectrograph at the European Southern Observatory's Very Large Telescope in Chile when they made their discovery.

"It was more like an accident," said Gerhard, co-author of paper scheduled to be published in the journal Astronomy and Astrophysics. "We were studying the Virgo cluster trying to find stars that do not belong to galaxies, but lie between them."Planetary nebulas are shells of gas illuminated by the energy of stellar core explosions. In M87's case, they are bright enough to be seen as individual points of light, though the measurements are time-consuming and painstaking.

Astronomers in Europe and the United States partnered to first find planetary nebulas in the Virgo Cluster and then determine their speeds using an instrument called FLAMES, which can make simultaneous measurements of light sources over an area of the sky about the size of the moon.

"Planetary nebulas are dying stars," said John Feldmeier, an astronomer at Youngstown State University in Ohio. "We like them because they're very common. They act like tracers for the other stars that are there but too hard to observe."

Folding the data into computer models, scientists realized that M87's gravitational reach extended about one-third as far as predicted and few clues to explain why.If M87's outer edge was stripped away by the cluster or another galaxy, straggler stars should still be visible.

"We don't see them and we should be able to because they don't move so fast," Gerhard said. "Either it would have had to happen a long time ago so that the stars would have had time to disappear or they were never there."

Perhaps M87 star formation was cut short by the galaxy cluster collapsing, or perhaps its small feeder galaxies were not able to support stellar nurseries, Gerhard added.

"We really don't understand how galaxies form and change over time," said Feldmeier.

The scientists plan follow-up studies to try to refine M87's boundaries even further.