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