CLEVELAND -- Case Western Reserve University astronomers havecaptured the deepest wide-field image ever of the nearby Virgo clusterof galaxies, directly revealing for the first time a vast, complex webof "intracluster starlight" -- nearly 1,000 times fainter than the darknight sky -- filling the space between the galaxies within the cluster.The streamers, plumes and cocoons that make up this extremely faintstarlight are made of stars ripped out of galaxies as they collide withone another inside the cluster, and act as a sort of "archaeologicalrecord" of the violent lives of cluster galaxies.
The Virgo imagewas captured through Case's newly refurbished 24-inch Burrell Schmidttelescope, built in the 1930s and located at the Kitt Peak NationalObservatory in Arizona. Over the course of 14 dark moonless nights, theresearchers took more than 70 images of the Virgo Cluster, then usedadvanced image processing techniques to combine the individual imagesinto a single image capable of showing the faint intracluster light.
"Whenwe saw all this very faint starlight in the image, my first reactionwas WOW!," project leader Chris Mihos said. "Then I began to worryabout all the things we could have done wrong." Many effects, such asstray light from nearby stars, from instruments in the observatory andeven from the changing brightness of the night sky could allcontaminate the image and lead to inaccurate results. "But as wecorrected for each of these contaminants, not only did the faintstarlight not disappear, it became even more apparent. That's when weknew we had something big."
The new image gives dramatic evidenceof the violent life and death of cluster galaxies. Drawn together intogiant 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 beingcannibalized 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 fainttribute to the violence of cluster life.
"From computersimulations, we've long suspected this web of intracluster starlightshould 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) havedeveloped computer simulations that track how clusters of galaxiesevolve over time, to study exactly how this intracluster starlight iscreated.
"With the data from the telescope, we see how a clusterlooks today," Mihos explains. "But with computer simulations, we canwatch how a cluster evolves over 10 billion years of time. By comparingthe simulation to the real features we now see in Virgo, we can learnhow the cluster formed and what happened to its many galaxies." Forexample, the fact that the intracluster light in Virgo is so complexand irregular lends credence to the theory of "hierarchical assembly,"where clusters grow sporadically when groups of galaxies fall into thecluster, rather than through the smooth, slow addition of galaxies oneby one.
To detect the faint intracluster light, upgrades wereneeded to Case's Burrell Schmidt telescope, originally part of theoriginal Warner and Swasey Observatory in Cleveland until its move toKitt Peak in 1979. The improvements included the installation of a newcamera system and upgrades to the telescope to make it morestructurally stable and reduce unwanted scattered light.
"It'slike ‘The Little Engine that Could’," says Case astronomer PaulHarding, who directed the refurbishment of the telescope. "It's thesmallest telescope on the mountain, but with these upgrades it'scapable of some pretty incredible science." The telescope's wide fieldof view -- enough to fit three full moons across the image – provedcrucial to the project, allowing the team to map out the intraclusterlight over a much larger part of the Virgo Cluster than would bepossible using larger telescopes with their much smaller fields of view.
TheVirgo Cluster of galaxies -- so named because it appears in theconstellation of Virgo -- is the nearest galaxy cluster to the Earth,at a distance of approximately 50 million light years. The clustercontains more than 2,000 galaxies, the brightest of which can be seenwith the aide of a small telescope.
The Case findings arereported in the paper "Diffuse Light in the Virgo Cluster" to bepublished in the September 20th issue of The Astrophysical JournalLetters. Along with Mihos team researchers included Case astronomersHeather Morrison and Paul Harding, and John Feldmeier, a NationalScience Foundation Fellow at the National Optical Astronomy Observatoryin Tucson, Ariz. (and formerly of Case).
The wide-field image ofthe Virgo Cluster, along with movies of computer simulations ofgalaxies and galaxy clusters, can be found athttp://astroweb.case.edu/hos/Virgo.
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