Carnegie Institution

Carnegie Institution

Press Release

Contact John Mulchaey at The Carnegie Observatories, 626-304-0257, e-mail Mulchaey@ociw.edu

EMBARGOED FOR RELEASE 12:30 PM MDT, TUESDAY, JUNE 4, 2002

NEW LIGHT ON DARK MATTER

June 4, 2002, Albuquerque, New Mexico. Astronomers are announcing today that they have obtained the most accurate determination to date of the amount of dark matter in a small group of galaxiesãan important step toward a precise measurement of the total matter-density of the universe. The new results are being presented by Dr. John S. Mulchaey of The Carnegie Observatories in Pasadena, California, at the meeting of the American Astronomical Society in Albuquerque, New Mexico.

The measurement was made using X-ray pictures of the NGC 4325 group of galaxies taken with the European Space Agency's XMM-Newton space telescope. The group lies 350 million light-years from Earth in the direction of the constellation Virgo.

EVIDENCE FOR DARK MATTER

XMM-Newton's X-ray images show that the small group of galaxies is immersed in a huge cloud of hot gas about 1.9 million light-years in diameter.  The temperature of the gas determined from the XMM-Newton observations is about 10 million degrees Kelvin (about 18 million degrees Fahrenheit).

Given the high temperature of the gas, the cloud would have dispersed into space long ago if it were not being held together by gravity of an immense mass. According to Mulchaey, the cloud is maintained by a balance between the force of gravity exerted by the mass, which tries to compress the cloud, and the pressure of the hot gas, which keeps the cloud from collapsing.

Mulchaey and collaborators  calculated the total mass of the system based on the temperature of the gas. They  then compared the total mass with the "visible" mass of the hot gas and galaxies. The mass required to contain the hot gas is about 10 times greater than the visible mass of the hot gas and galaxies combined, indicating that approximately 90% of the mass in the group is in the form of "dark matter"ãmatter that does not emit any radiation that can be detected by current telescopes.

Although the nature of dark matter is not currently known, most astronomers believe it is in the form of an unknown type of elementary particle.

Gas clouds like the one in the NGC 4325 group have been the subject of intense study by the astronomical community for several decades.  The new XMM-Newton observations are important because they provide the first detailed maps of the spatial variations of the temperature in the cloud for a small group of galaxies. Previously, astronomers had to assume a single temperature for the gas cloud to make a mass estimate. The XMM-Newton observation verifies that the gas temperature is fairly constant throughout the cloud.

Mulchaey also notes that the most detailed X-ray studies have usually been restricted to rich clusters of galaxies. Although these systems contain more galaxies than a typical galaxy group, they are very rare and not representative of the universe in general. "Most galaxies are in small groups," notes Mulchaey. "Therefore, groups provide a much better picture of what the universe looks like on average."

The XMM-Newton observations of the NGC 4325 group imply that we live in a low-density universe, a result consistent with recent findings based on the cosmic microwave background radiation and the properties of distant supernovae. "Our observations are completely consistent with the idea that the mass-density of the universe is only about 30% of the value needed to stop the universe from expanding forever," said Mulchaey. He further notes, however, that similar measurements must be made for many more groups before a firm conclusion can be reached.

IMPLICATIONS FOR OUR OWN LOCAL GROUP

Our own Milky Way Galaxy is a member of a small group of galaxies known as the Local Group. Mulchaey speculates that a gas cloud similar to that seen in the NGC 4325 group is likely to surround the Local Group.

However, a number of different observations indicate that the mass of the Local Group is somewhat lower than that of the NGC 4325 group. Since the temperature of the hot gas is related to the mass, a lower mass implies a lower temperature for the Local Group cloud.  "In fact, based on what we know about the Local Group, it seems likely that if a gas cloud does exist it would not be hot enough to produce a significant amount of X-ray emission," says Mulchaey.

Mulchaey and others are currently planning observational programs to detect the cooler gas clouds around groups like the Local Group. "We've now found gas clouds in many groups; it would be nice to know if our own system has a similar component," he says.

Mulchaey is a staff astronomer at The Carnegie Observatories, in  Pasadena, California.  His collaborators on this project are Drs. Richard F. Mushotzky of NASA Goddard Space Flight Center, Greenbelt, Maryland, and Ann I. Zabludoff of the University of Arizona, Tucson, Arizona.  Partial support for this project was provided by NASA's Long-Term Space Astrophysics (LTSA) program.

For more information, contact John S. Mulchaey (626-304-0257, mulchaey@ociw.edu)

Figure caption: An X-ray picture (false color) taken with the European Space Agency's XMM-Newton telescope overlaid on an optical image of the NGC 4325 group of galaxies reveals a huge cloud of hot gas surrounding the group. The gas cloud is 1.9 million light-years in diameter and has a temperature of about 10 million degrees Kelvin (about 18 million Fahrenheit). The existence of this gas cloud implies that about 90% of the mass in this group is in the form of dark matter. The optical image of the field was taken with the 1-meter (40-inch) Swope Telescope at Las Campanas Observatory, in Chile. This photograph was presented to the American Astronomical Society meeting in Albuquerque, New Mexico, on June 4, 2002.  PHOTO CREDIT: Dr. John S. Mulchaey, The Carnegie Observatories. 

EDITORS: This photograph can be obtained over the Internet via http://www.carnegieinstitution.org/mulchaey.html   after 12:30 PM MDT on June 4, 2002.