A combined Canadian/American team of four astronomers has
used the powerful Hubble Space Telescope cameras, along with an
elegantly simple observational technique, to measure the distance to
the Virgo Cluster of galaxies. The Virgo Cluster is a linchpin
for establishing the correct scale for distances to remote galaxies,
the expansion rate of the universe, and most important of all, the age
of the universe. The report of their work is being presented today
at the American Astronomical Society meeting in Washington, DC, by
team members Dr. Patrick Durrell of Case Western Reserve University
(Cleveland, OH) and Dr. Michael Pierce of Indiana University
(Bloomington, IN). The other team members are Drs. William Harris
(McMaster University, Hamilton, Ontario) and Jeff Secker (Washington
State University, Pullman WA).
Last June, the team used the WFPC2 camera on board the Hubble Space Telescope, asking it to `stare' at one particular galaxy near the center of the Virgo Cluster for an unusually long time. The resulting image was a total exposure of 32,200 seconds, or almost 9 hours spread over 12 orbits of the spacecraft. The target object, known only by its catalog name of VCC 1104, is a dwarf elliptical galaxy. The stars in these types of small, simple galaxies are known to be almost all extremely old, formed long ago in the earliest stages of the galaxys' history more than 10 billion years ago. This characteristic is exactly what makes these little galaxies so useful: astronomers know quite accurately how bright the most luminous ``red giant'' stars in these galaxies actually are. The purpose of obtaining the deep Hubble image was to measure precisely how faint these most luminous stars appear to be: the fainter they appear, the farther away the galaxy must be. Because the Virgo Cluster is fairly distant, an extremely long exposure was needed to clearly reveal these brightest old stars.
According to Harris, ``The basic idea of the measurement can be seen from a simple analogy. Suppose you take a bag of sugar and spill it out onto your kitchen counter. If you look at it from a distance across the room, all you see is a shapeless white heap. But if you gradually walk closer, you start seeing the individual grains of sugar, particularly around the edges of the heap. Finally, as you get up close to it, you can see that the entire pile is made up of separate grains. The pile of sugar is our Virgo galaxy, and the grains are its stars. We used the Hubble camera effectively to bring it into close enough view that we could see those individual stars.''
The distance to VCC 1104 they obtained is 48 million light years, give or take a measurement error of 10 percent (5 million light years). Says Secker, ``Our new result represents the first time the method has been applied to targets as distant as the Virgo galaxies, which are key stepping stones to establishing the cosmological distance scale.'' If this distance is assumed to apply to the Virgo Cluster as a whole, then the cosmological distances to remote galaxies can be established.
According to Pierce, ``Over the last few decades astronomers have developed a number of techniques for accurately measuring the relative distances of galaxies and clusters. As a result, distances to remote galaxies can be expressed in units of the Virgo Cluster distance. Once the distance of Virgo is established, the overall scale of the universe can be set. A useful analogy might be to consider a ruler marked in arbitrary divisions. You could still use the ruler to accurately measure lengths and distances but without knowing the conversion to inches or centimeters it would be hard to make sense of the measurements. If the conversion factor can be determined any distance or length measured with such a ruler could then be converted into something we could better understand. In this case, the ruler's arbitrary division is the distance to the Virgo Cluster.''
The distance determined for the Virgo Cluster - a controversial topic for many years - produces moderately ``short'' distances to remote galaxies. In turn, this implies an expansion age for the universe of only about 12 billion years at most, which is on the low end of the range being currently debated.
Says Durrell, ``We've pushed the Hubble telescope to its limit and have shown that this method works at the large distances that we need to measure the cosmic distance scale. To make our result stronger, the next step is to measure the distance to more than just one target Virgo galaxy. With distances to several more dwarf elliptical galaxies in hand, we can increase our confidence that we are not being fooled by one possibly unusual case. There is nothing to prevent us from reducing the uncertainty in cosmological distances down to a few percent with this technique.''
For More Information:
Dr. William Harris (905-525-9140 x 22744, harris@physics.mcmaster.ca)
Dr. Patrick Durrell (216-368-6699, durrell@huascaran.astr.cwru.edu)
Dr. Michael Pierce (812-855-0274, mpierce@astro.indiana.edu)
Dr. Jeff Secker (509-335-3136, secker@delta.math.wsu.edu)