NEW DISTANCE MEASUREMENT TO A VIRGO GALAXY GIVES NEW MEASUREMENT OF UNIVERSE AGE

BLOOMINGTON, Ind. -- A team of four Canadian and American astronomers has used the Hubble Space Telescope, along with an elegantly simple observational technique, to measure the distance to the Virgo Cluster of galaxies.

The distance to the Virgo Cluster is a linchpin for establishing the correct distances to remote galaxies, the expansion rate of the universe, and most important of all, the age of the universe.

The results were presented today (Jan. 7) at the national meeting of the American Astronomical Society in Washington, D.C., by team members Patrick Durrell of Case Western Reserve University in Cleveland, Ohio, and Michael Pierce of Indiana University, Bloomington.

The distance to the Virgo Cluster that the team determined favors a relatively "short" distance to remote galaxies, implying an age for the universe of about 12 billion years at most, which is at the low end of the range currently being debated, Pierce said.

Last June, the team used the WFPC2 camera on the Hubble Space Telescope 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 nine hours spread over 12 orbits of the spacecraft. It was the first time this method has been applied to targets as distant as the Virgo galaxies, which are key stepping stones to establishing the cosmological distance scale, Pierce said.

The distance obtained for the target galaxy is 50 million light years, give or take 10 percent (5 million light years). If this distance is assumed to apply to the Virgo Cluster as a whole, then cosmological distances can be determined.

The target object, known only by its catalog name of VCC 1104, is a dwarf elliptical galaxy. Almost all of the stars in these small, simple galaxies are known to be extremely old, formed more than 10 billion years ago in the earliest stages of the galaxies' history, Pierce said. Astronomers know quite accurately how bright the most luminous stars in these galaxies actually are. The purpose of obtaining the 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.

The team leader is William Harris of McMaster University in Hamilton, Ontario, Canada. The other team member is Jeff Secker of Washington State University, Pullman, Wash.

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 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 to bring the galaxy into close enough view that we could see those individual stars.''

Pierce said, "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 to the Virgo Cluster is established, the overall scale of the universe can be set.

"A useful analogy," he continued, "might be to consider a ruler marked in arbitrary divisions. You could still use the ruler to accurately measure lengths and distances, without knowing the conversion to inches or centimeters, but it would be hard to make sense of the measurements. If the conversion to inches or centimeters could be determined, then any distance or length measured with such a ruler could be converted into something we could better understand.''

In this case, the ruler's arbitrary division is the distance to the Virgo Cluster, and the team's goal was to measure this distance, a controversial topic for many years.

Said team member Pat Durrell, "We've pushed the Hubble telescope to its limit and have shown that this method works at the large distances that we need in order 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, contact William Harris (905-525-9140, ext. 22744, harris@physics.mcmaster.ca), Patrick Durrell (216-368-6699, durrell@huascaran.astr.cwru.edu), Michael Pierce (812-855-0274, mpierce@astro.indiana.edu) or Jeff Secker (509-335-3136, secker@delta.math.wsu.edu).

EDITOR'S NOTE: There is a web site photograph featuring the Hubble Space Telescope photo of a Virgo Cluster galaxy. If used, it should be credited to Indiana University and NASA/Space Telescope Science Institute. Below is a caption to accompany the photograph.

PHOTO CAPTION: A small elliptical galaxy in the Virgo Cluster of galaxies is resolved into a multitude of stars for the first time. Measuring the apparent brightness of these stars allows the distance of this galaxy to be measured, providing a calibration of the scale used to measure the size and rate of expansion of the universe. The inset at upper left shows an enlarged area from the galaxy (indicated by the box). This photograph, made with the Hubble Space Telescope, was presented at the American Astronomical Society meeting in Washington, D.C., on Tuesday (Jan. 7).

(Hal Kibbey, Office of Communications and Marketing, 812-855-0074 or 812-855-3911, hkibbey@indiana.edu

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