Abell 754
By Lori Stiles

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 Saturday 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

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

“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.”

The survey has spawned a number of related projects, which are summarized on
Zabludoff’s website at http://atropos.as.arizona.edu/aiz/

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.



[Image 1]

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.
(IMAGE: Ann Zabludoff)

[Image 2]

Ann Zabludoff (PHOTO: Lori Stiles)