FOR RELEASE: 9:20 A.M. EST, January 10, 2006
MINIATURE GALAXY REVEALED IN NEAREST GALAXY CLUSTER

A team of astronomers has discovered a new dwarf galaxy and up to four possible star clusters in a region of the Virgo cluster of galaxies previously thought to be empty. The exceptional resolution of the Advanced Camera for Surveys (ACS) instrument on the Hubble Space Telescope has also allowed the team to study individual stars in these objects. Study of these stars is important towards understanding how galaxies evolve in large clusters. An analysis of these stars, which lie over 55 million light years from us, is being presented today at the American Astronomical Society meeting in Washington, DC.

The Virgo IntraCluster Stars project (VICS for short) involves 13 astronomers from around the world. Presenting the results today are Patrick Durrell, an assistant professor of physics and astronomy at Youngstown State University in Youngstown, OH; Ben Williams, a research associate at Penn State University in University Park, PA; and Robin Ciardullo, professor of astronomy and astrophysics at Penn State University. The main purpose of the team's work is to study intracluster stars. Unlike most stars in the universe, intracluster stars do not belong to any single galaxy. Instead, they are orphans, drifting through the intergalactic space of a galaxy cluster. At a distance of 55 million light years, the Virgo cluster is the nearest large cluster of galaxies, making it the easiest place to search for such objects. Even so, the detection of these stars in Virgo required 37 orbits of observations (or over 25 hours of data) with the Advanced Camera for Surveys of the Hubble Space Telescope.

The VICS field was chosen to be as far from any known galaxy as possible, yet still be in the central region of the Virgo Cluster. While the field area is small (roughly 1\% the size of the Full Moon), the team found it contains thousands of intracluster stars. But even more surprising was the team's discovery of four isolated intracluster globular star clusters, and a previously unknown dwarf galaxy.

Figure 1 :  Optical image of the core of the Virgo Cluster of galaxies, showing the many large galaxies present. The VICS field (denoted by the box) is located in a relatively empty region of the cluster between the largest Virgo galaxies, M87 (lower left), and M86 and M84 (two largest galaxies in upper right). It is within this field where a team of astronomers is using the Hubble Space Telescope to search for stars that have been ripped away from their parent galaxies. Their search has revealed many intracluster stars, four new globular clusters free floating in the Virgo cluster space, and a previously undiscovered dwarf galaxy. This material was presented to the American Astronomical Society meeting in Washington, DC on January 10, 2006.
PHOTO CREDIT: Image from the Digitized Sky Survey.  The DSS was produced at the Space Telescope Science Institute under U.S. Government grant NAG W-2166.  The images of these surveys are based on photographic data obtained using the Oschin Schmidt Telescope on Palomar Mountain and the UK Schmidt Telescope.  The plates were processed into the present compressed digital form with the permission of these institutions.  The Second Palomar Observatory Sky Survey (POSS-II) was made by the California Institute of Technology with funds from the National Science Foundation, the National Aeronautics and Space Administration, the National Geographic Society, the Sloan Foundation, the Samuel Oschin Foundation, and the Eastman Kodak Corporation.
High-resolution TIFF image (0.9M)
Lower-resolution JPG image (110k)

Dwarf galaxies are the most numerous type of galaxy in the universe. Of these, the smallest and faintest are called dwarf spheroidals; a typical dwarf spheroidal galaxy is ten thousand times fainter than the Milky Way, and contains roughly a million stars in a region of space just a few thousand light years across. Consequently, these objects are extremely difficult to find: at the distance of the Virgo Cluster, they appear only as faint `smudges' on most astronomical images.

The VICS data, however, shows a dwarf spheroidal galaxy in its full splendor. The brightest individual stars of the galaxy are clearly seen. By measuring the brightnesses and colors of these stars, the VICS team has been able to show that the structure and composition of the galaxy is similar to that of dwarf spheroidals in our own Local Group of galaxies. This result is somewhat surprising, given how dense the core of Virgo is compared to our own galaxy neighborhood.

Large galaxy clusters such as Virgo are expected to contain thousands of such dwarf galaxies. The detailed images of the VICS field has given astronomers their first look at how these galaxies evolve in a dense galaxy cluster. ``It is only with the capabilities of the Hubble Space Telescope that we are even able to see the stars in this galaxy, let alone study them in as much detail as we have been able to do,'' says Durrell, who is presenting the dwarf galaxy results at the meeting.
Figure 2:  Color image of the entire VICS field, based on 37 orbits of data from the Hubble Space Telescope. Many of the objects seen in image are distant background galaxies, but some are stars, star clusters, and galaxies which belong to the relatively nearby Virgo galaxy cluster, 55 million light years away. The inset at the upper right shows an enlarged region of the VICS field (indicated by the box) which includes the recently-discovered dwarf galaxy. (The three small red objects are background galaxies.) This is the first time that astronomers have been able to see and measure the individual stars of a dwarf spheroidal galaxy within a large galaxy cluster; such data reveal the galaxy's distance, age, structure, and chemical composition. At the distance of Virgo, the box is 3,000 light years across. This material was presented at the American Astronomical Society meeting in Washington, DC on January 10, 2006.
PHOTO CREDIT :  Ben Williams & Robin Ciardullo (Penn State University), Patrick Durrell (Youngstown State University)

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High-resolution JPG image (both) - 986k
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VICS field only - Hi-resolution JPG (11M)
High-resolution TIFF image (inset) - 0.9M

Another bonus was the team's discovery of what may be four globular star clusters in the VICS field. Globular clusters are dense, roughly spherical associations of typically a few hundred thousand stars of the same age. Such clusters are normally found in the halos of galaxies, but these four systems are orphans. ``These images are giving us our first measurements of the structure of star clusters that have been torn from their home galaxies in the distant past,'' remarked Williams, ``and the structures provide clues about how these clusters came to be in such an empty region of space.''

``What is interesting about this project is that we set out with one primary goal, only to find much more than we had hoped for,'' says Ciardullo, who is the VICS project leader. ``And we've really only started.''

Other team members include Steinn Sigurdsson and Matt Vinciguerra from Penn State University, PA; John Feldmeier from the National Optical Astronomy Observatories in AZ; George Jacoby, Director of the WIYN Observatory in AZ; Ted von Hippel from the University of Texas; Henry Ferguson from the Space Telescope Science Institute in MD; Nial Tanvir from U. Hertfordshire, UK; Magda Arnaboldi from the Osservatorio Astronomico di Torino, Italy, and the European Southern Observatory; Ortwin Gerhard from the Max-Planck-Institut fuer extraterrestrische Physik, Germany; Alfonso Aguerri from the Instituto de AstrofÌsica de Canarias, Spain; and Ken Freeman from Mount Stromlo Observatory, Australia.