A team of astronomers led by Dr. David Pooley of Eureka Scientific has made an important determination of the amount of dark matter in massive galaxies using NASA's Chandra X-ray Observatory, providing independent evidence for the dark matter. In addition, these results help map out the distribution of dark matter in elliptical galaxies, which is vital in understanding both galaxy formation and the nature of dark matter.

The team studied 14 massive galaxies averaging about 6 billion light-years away, which appear almost directly in front of even more distant galaxies nearly three times farther away. Those more distant galaxies each harbor a supermassive black hole in the center, known as a quasar, which produces an enormous amount of light.

Because of the special configuration of these galaxies, the light from the distant quasar is "gravitationally lensed" by the intervening galaxy to produce four images of the quasar as seen from Earth's vantage point.

According to Einstein's general theory of relativity, the gravitational field of a massive object, like one of these galaxies, can bend the path of light in its vicinity, acting as a lens and producing multiple images, or mirages, of the background object.

"We compared what those four images were supposed to look like according to lensing theory to what we actually saw with Chandra," said Dr. Pooley, who presented the results at the American Astronomical Society meeting in Seattle. "We found some major differences."

Based on the configuration of the images and the lensing galaxy, general relativity can predict the amount of matter needed in the lensing galaxy to produce the images of the background quasar. However, it does not address the form of the matter.

The team's explanation for the anomalies seen with Chandra lies in the makeup of the matter in the lensing galaxies. The aggregate gravitational field from all the matter in the lensing galaxy produces the effect strong enough to make the four distinct images of the background quasar. Along those four paths through the lensing galaxy, the light from the quasar can be further affected.

The stars in a galaxy have their own gravitational fields, which further perturb the light from the background quasar. "It's lensing on top of lensing," said Dr. Pooley. The degree to which the light is further affected is a sensitive function of the number of stars and the amount of dark matter at the locations in the galaxy where the quasar's light passes through.

A galaxy made entirely of stars, without any dark matter, would not produce what Dr. Pooley and his team saw with Chandra. Neither would a galaxy made entirely of dark matter.

To produce what was observed, the galaxies must consist of approximately 85-95% dark matter in the regions where the background quasar's light passes through. Typically, those regions are located about 15,000 to 25,000 light-years from the centers of the lensing galaxies, similar to the distance of the Solar System from the center of the Milky Way.

"This is one of the most direct measurements of the amount of dark matter at a specific location in a galaxy," said Dr. Pooley.

Other studies have determined the amount of dark matter in clusters of galaxies, such as the Bullet Cluster. In those cases, the dark matter being studied is mainly outside of the individual galaxies in the vast expanse between them. These results, on the other hand, are probing well inside individual galaxies.

Having determined the amount of matter in stars and dark matter at these locations, the team's next step is to determine the mass-to-light ratio. It will be the most direct measurement yet of an important quantity used in almost all areas of astronomy.

Earlier work in this area was presented by Professor Paul Schechter of the Massachusetts Institute of Technology and Professor Joachim Wambsganss of the University of Heidelberg, who worked with observations in optical light.

Those results were ambiguous because the size of the region emitting the optical light was unknown, but these new X-ray results give a very clean determination of the amount of dark matter. Both Schechter and Wambsganss are members of the team presenting the X-ray results, along with Professor Saul Rappaport of MIT and Dr. Jeffrey Blackburne of Ohio State University.

These results will be further refined as more gravitationally lensed quasars are discovered and then observed with Chandra. The discovery of additional systems will be greatly aided by emerging large-area surveys undertaken by ground-based observatories, such as the Panoramic Survey Telescope and Rapid Response System project and the Large Synoptic Survey Telescope project.