Chandra puts further limits on nature of dark matter
Posted: January 8, 2002

NASA scientists using the Chandra X-ray Observatory have measured the distribution of dark matter in an elliptical galaxy on the smallest scale yet and found that this mysterious substance exists in high concentrations throughout most of galaxy.

Chandra's image of the elliptical NGC 4636. Credit: NASA/SAO/CXC/C.Jones et al
The detailed measurement implies that dark matter is not "self-interacting" in massive cosmic structures, such as elliptical galaxies or galaxy clusters. The finding strikes another blow at the "self-interacting dark matter" theory and thus narrows the field of candidates that can explain the enigmatic nature of dark matter, one of the most pressing questions in astronomy.

Drs. Michael Loewenstein and Richard Mushotzky of NASA's Goddard Space Flight Center in Greenbelt, Md., present the results of their Chandra observation of elliptical galaxy NGC 4636 Monday at the 199th Meeting of the American Astronomical Society in Washington, D.C.

"We still don't know what dark matter is, but we now have a much better idea of what it isn't," said Loewenstein.

Scientists say that approximately 90 percent of the matter in the Universe is in some dark, as yet detected, form that makes its presence felt only through gravity. All matter, by virtue of its mass, exerts a gravitational attraction. Dark matter seems to be the gravitational glue holding together galaxies and galaxy clusters, for neither type of cosmic structure has enough visible mass to keep it from flying apart.

Big bang nucleosynthesis theory provides an estimate on the amount of "ordinary" matter in the Universe, and this rules out the possibility that dark matter is from dim stars, dark chunks of solid material or black holes. Dark matter must be exotic, that is, not made of protons and electrons. Various observations with radio, optical and X-ray telescopes aim to determine the distribution and nature of dark matter.

Loewenstein and Mushotzky derived the total mass-to-light ratio in galaxy NGC 4636 and found that 50 to 80 percent of the matter (or mass) in the galaxy "proper" is non-luminous, from the core out to about 35,000 light years -- farther than the distance from the Milky Way galaxy's core to Earth.

This implies that dark matter is highly concentrated and suggests that dark matter (and least in large galaxies and, by extension, galaxy clusters) is not noticeably self-interacting. That is, particles of dark matter are not likely to collide with one another like billiard balls and "puff out" or become diffuse throughout the galaxy. In September, scientists from the Massachusetts Institute of Technology drew a similar conclusion about dark matter in galaxy clusters, also from Chandra data.

Loewenstein and Mushotzky's observation does not rule out other "cold dark matter" theories. These include exotic elementary particles such as WIMPS, axions, cosmic strings or supersymmetry particles.

Quantum mechanics and general relativity, the two pillars of physics that attempt to explain the workings of the Universe from the atomic to the cosmic, do not predict the existence of dark matter. Defining dark matter, therefore, would be tantamount to establishing a more complete theory of the Universe, which may be flush with the rewards that quantum mechanics and general relativity have brought.

Chandra observed NGC 4636, some 50 million light-years from Earth in the constellation Virgo, with its Advanced CCD Imaging Spectrometer for 64,000 seconds. Data from the European Space Agency's XMM-Newton observatory was also used. Loewenstein joins NASA Goddard as a research associate with the University of Maryland, College Park.