Case Western Reserve U. astronomers find vast stellar web spun bycolliding galaxies (Forwarded)

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September 19, 2005

CASE ASTRONOMERS FIND VAST STELLAR WEB SPUN BY COLLIDING GALAXIES

Virgo image gives evidence of violent life, death of cluster galaxies

CLEVELAND -- Case Western Reserve University astronomers have captured
the deepest wide-field image ever of the nearby Virgo cluster of
galaxies, directly revealing for the first time a vast, complex web of
"intracluster starlight" -- nearly 1,000 times fainter than the dark
night sky -- filling the space between the galaxies within the cluster.
The streamers, plumes and cocoons that make up this extremely faint
starlight are made of stars ripped out of galaxies as they collide with
one another inside the cluster, and act as a sort of "archaeological
record" of the violent lives of cluster galaxies.

The Virgo image was captured through Case's newly refurbished 24-inch
Burrell Schmidt telescope, built in the 1930s and located at the Kitt
Peak National Observatory in Arizona. Over the course of 14 dark
moonless nights, the researchers took more than 70 images of the Virgo
Cluster, then used advanced image processing techniques to combine the
individual images into a single image capable of showing the faint
intracluster light.

"When we saw all this very faint starlight in the image, my first
reaction was WOW!," project leader Chris Mihos said. "Then I began to
worry about all the things we could have done wrong." Many effects, such
as stray light from nearby stars, from instruments in the observatory
and even from the changing brightness of the night sky could all
contaminate the image and lead to inaccurate results. "But as we
corrected for each of these contaminants, not only did the faint
starlight not disappear, it became even more apparent. That's when we
knew we had something big."

The new image gives dramatic evidence of the violent life and death of
cluster galaxies. Drawn together into giant clusters over the course of
cosmic time by their mutual gravity, galaxies careen around in the
cluster, smashing into other galaxies, being stripped apart by
gravitational forces and even being cannibalized by the massive galaxies
which sit at the cluster's heart. The force of these encounters
literally pulls many galaxies apart, leaving behind ghostly streams of
stars adrift in the cluster, a faint tribute to the violence of cluster
life.

"From computer simulations, we've long suspected this web of
intracluster starlight should be there," says Mihos, associate professor
of astronomy at Case, "but it's been extremely hard to map it out
because it's so faint." Mihos and graduate students Craig Rudick (Case)
and Cameron McBride (University of Pittsburgh, and former Case
undergraduate) have developed computer simulations that track how
clusters of galaxies evolve over time, to study exactly how this
intracluster starlight is created.

"With the data from the telescope, we see how a cluster looks today,"
Mihos explains. "But with computer simulations, we can watch how a
cluster evolves over 10 billion years of time. By comparing the
simulation to the real features we now see in Virgo, we can learn how
the cluster formed and what happened to its many galaxies." For example,
the fact that the intracluster light in Virgo is so complex and
irregular lends credence to the theory of "hierarchical assembly," where
clusters grow sporadically when groups of galaxies fall into the
cluster, rather than through the smooth, slow addition of galaxies one
by one.

To detect the faint intracluster light, upgrades were needed to Case's
Burrell Schmidt telescope, originally part of the original Warner and
Swasey Observatory in Cleveland until its move to Kitt Peak in 1979. The
improvements included the installation of a new camera system and
upgrades to the telescope to make it more structurally stable and reduce
unwanted scattered light.

"It's like 'The Little Engine that Could'," says Case astronomer Paul
Harding, who directed the refurbishment of the telescope. "It's the
smallest telescope on the mountain, but with these upgrades it's capable
of some pretty incredible science." The telescope's wide field of view
-- enough to fit three full moons across the image -- proved crucial to
the project, allowing the team to map out the intracluster light over a
much larger part of the Virgo Cluster than would be possible using
larger telescopes with their much smaller fields of view.

The Virgo Cluster of galaxies -- so named because it appears in the
constellation of Virgo -- is the nearest galaxy cluster to the Earth, at
a distance of approximately 50 million light years. The cluster contains
more than 2,000 galaxies, the brightest of which can be seen with the
aide of a small telescope.

The Case findings are reported in the paper "Diffuse Light in the Virgo
Cluster" to be published in the September 20th issue of The
Astrophysical Journal Letters. Along with Mihos team researchers
included Case astronomers Heather Morrison and Paul Harding, and John
Feldmeier, a National Science Foundation Fellow at the National Optical
Astronomy Observatory in Tucson, Ariz. (and formerly of Case).

The wide-field image of the Virgo Cluster, along with movies of computer
simulations of galaxies and galaxy clusters, can be found at
http://astroweb.case.edu/hos/Virgo.