Spaceflight Now: Breaking News

Galaxy groups surveyed beyond local neighborhood
Posted: February 25, 2001

There is striking evidence that two galaxy subclusters have recently collided in Abell 754. White lines show the distribution of hot, X-ray emitting gas. Photo: Ann Zabludoff
In a cosmically short time, probably in a few billion years, our Milky Way galaxy will smash into the Andromeda galaxy. Pulled together by gravity, the two spiral galaxies will violently merge perhaps into another kind of galaxy, an elliptical galaxy.

The 10 or more dim, dwarf satellite galaxies that orbit the Milky Way are at present too far away to interact with the dim, dwarf satellite galaxies circling Andromeda. When the Andromeda and Milky Way collide, their respective dwarf satellite populations may become a single large satellite population around the merged central galaxy.

Astronomers really don't know if the environment in our galactic neighborhood typifies the environment of other groups of galaxies. But it is a basic and compelling question, because if our local neighborhood environment is common in the universe, then this may be the type of environment where most galaxies evolve, says Ann Zabludoff.

"The current guess is that our local galactic neighborhood is very common for galaxies, that most galaxies lie in local group type environments or perhaps even slightly more complex and populated neighborhoods," says Zabludoff of the University of Arizona department of astronomy and Steward Observatory.

She talked on "The Role of Collisional Groups in Galaxy Evolution" at the American Association for the Advancement of Science meeting in San Francisco in a session, "Rebuilding the Galactic Neighborhood."

Most nearby galaxies, like the Milky Way, belong to "poor" groups of galaxies, that is, they contain fewer than five bright galaxies. With few nearby bright galaxies to observe, and hampered by the inability to discern whether nearby groups were real systems instead of chance projections of galaxies along the line-of-sight, astronomers have struggled to learn about what kind of matter is contained in galaxy groups, how it is distributed, or how galaxy groups evolve.

In 1994, Zabludoff, then at the Carnegie Observatories in Pasadena, and her colleague John Mulchaey began one of the first multi-fiber spectroscopic surveys to look in detail at galaxy groups beyond our local Milky Way- Andromeda group. Multi-fiber spectroscopy is a technique that allows astronomers to measure the spectra of a hundred or more galaxies at once.

When Zabludoff and Mulchaey saw two, or maybe four bright galaxies that looked like a galaxy group on the sky, they also wanted to detect any fainter galaxies in the system.

"And this is what the multi-fiber spectrograph allows us to do very efficiently, which is to see how many galaxies seemingly near the bright galaxies are actually associated with those bright galaxies," Zabludoff said of their ongoing survey.

"And we found a lot," she added. "We realized for the first time there are a lot of faint galaxies as well as bright galaxies in the other poor group environments. We realized that our local group isn't necessarily a special place.

"And we now have a lot more galaxies to work with, so we can measure the motions of more galaxies in a given system to learn much more about the mass in that system."

She studies the number and nature of satellite galaxy populations in other galactic neighborhoods similar to our own, for example. How fast the satellite galaxies orbit their parent galaxies is a pivotal clue as to how massive the parent galaxies are.

"Some of our most interesting results are on how dark matter is distributed in galaxy groups ouside of our own," Zabludoff said. The distribution of dark matter has consequences for how often the galaxies in groups collide and merge.

Astronomers initially thought that galaxy collisions and mergers happen rapidly, and that galaxy groups should disappear very quickly. They were puzzled at seeing as many galaxy groups as they saw.

"Our new evidence suggests that galaxies collide and merge more slowly than people have assumed. This might possibly explain why this environment seems to be so common for galaxies," Zabludoff said.

Zabludoff also searches for evidence that tells how efficiently galaxies form.

"We are trying to ascertain whether there is evidence that globs of cold gas between the galaxies might contribute significantly to the baryons that make up the mass of these groups," Zabludoff said. (Baryons are such particles as protons and neutrons.)

If the cold gas clumps are associated with galaxy groups, they could be gas reservoirs from which galaxies draw material to form stars. If they are just freely floating blobs in between the galaxies, they could be just the leftovers of galaxy group formation, in other words, clumps of gas too tenuous to make stars and become galaxies.

"So far, we haven't found anything that isn't associated with the galaxies themselves. Our findings suggest these gas clumps aren't just drifting in between the galaxies."

But perhaps there are significant numbers of cold gas blobs lurking just below the limit that telescopes can detect. Zabludoff and her colleagues are pushing the detection limit with the Very Large Array (VLA), which is a radio telescope in New Mexico, in the search for fainter and smaller clumps of cold gas. Whether or not these possible scrap leftovers from galaxy formation exist are clues to how efficiently galaxies form.

Zabludoff and her colleagues also can now explore in greater detail what is happening in galaxy systems more evolved than ours -- those where galaxies already are crashing into one another, pulling each other apart, changing their shape and changing their stars -- a possible preview of the fate of the Milky Way.

Zabludoff and collaborating astronomers use the Chandra and Newton X-ray telescopes orbiting Earth, the VLA, and optical telescopes at Steward Observatory in Arizona and Las Campanas Observatory in Chile in the research. Along with Mulchaey, her collaborators include Jacqueline van Gorkom of Columbia University and Eric Wilcots of the University of Wisconsin. Their work is funded principally by NASA.