Dark matter continues to confound astronomers, as NASA‚s Chandra X-ray
Observatory demonstrated with the detection of an extensive envelope of
dark matter around an isolated elliptical galaxy. This discovery
conflicts with optical data that suggest a dearth of dark matter around
similar galaxies, and raises questions about how galaxies acquire and keep
such dark matter halos.

The observed galaxy, known as NGC 4555, is unusual in that it is a fairly
large, elliptical galaxy that is not part of a group or cluster of
galaxies. In a paper to be published in the November 1, 2004 issue of the
Monthly Notices of the Royal Astronomical Society, Ewan O’Sullivan of the
Harvard-Smithsonian Center for Astrophysics in Cambridge, MA and Trevor
Ponman of the University of Birmingham, United Kingdom, use the Chandra
data to show that the galaxy is embedded in a cloud of
10-million-degree-Celsius gas.

This hot gas cloud has a diameter of about 400,000 light years, about twice
that of the visible galaxy. An enormous envelope, or halo, of dark matter
is needed to confine the hot cloud to the galaxy. The total mass of the
dark matter halo is about ten times the combined mass of the stars in the
galaxy, and 300 times the mass of the hot gas cloud.

A growing body of evidence indicates that dark matter – which interacts
with itself and “normal” matter only through gravity – is the dominant
form of matter in the universe. According to the popular “cold dark
matter” theory, dark matter consists of mysterious particles left over
from the dense early universe that were moving slowly when galaxies and
galaxy clusters began to form.

“The observed properties of NGC 4555 confirm that elliptical galaxies can
posses dark matter halos of their own, regardless of their environment,”
said O’Sullivan. “This raises an important question: what determines
whether elliptical galaxies have dark matter halos?”

Most large elliptical galaxies are found in groups and clusters of
galaxies, and are likely the product of the merger of two spiral galaxies.
In such an environment, the dark matter halos can be stripped away by
gravitational tidal force and added to other galaxies or the group as a
whole.

Therefore, it is difficult to determine how much dark matter the original
galaxies had, and how much they have lost to the group as a whole through
interactions with their environment.

The importance of the issue of the intrinsic amount of dark matter
associated with an elliptical galaxy has recently increased owing to a
report by an international team of astronomers led by Aaron Romanowsky of
the University of Nottingham, United Kingdom. This team found little, if
any evidence of dark matter in three elliptical galaxies. Two of these
were in loose galaxy groups, and one was isolated. Their result, based on
optical data from the 4.2 meter William Herschel Telescope on the Spanish
island of La Palma, is in clear conflict with the X-ray data on NGC 4555.
The optical technique used to search for dark matter in the nearby
elliptical galaxies could not be applied to NGC 4555 because it is more
than 3 times as far away from Earth.

Either the galaxies observed by Romanowsky and colleagues have lost their
dark matter halos through earlier interactions with other galaxies, or
their dark matter halos are much more extended, or they formed without
dark matter halos. The first option is possible for the galaxies in
groups, but very unlikely for the isolated galaxy. The second and third
options are still open, but would require a modification ˆ perhaps a major
modification ˆ of the cold dark matter theory of galaxy formation.

“This is clearly a question which deserves further consideration,” said
O’Sullivan. “It seems likely that much more theoretical and observational
work on elliptical galaxies will be required before this issue can be
resolved.”

Chandra observed NGC 4555 with its Advanced CCD Imaging Spectrometer
(ACIS) in February 2003. NASA’s Marshall Space Flight Center, Huntsville,
Ala., manages the Chandra program for NASA’s Office of Space Science,
Washington.

Northrop Grumman of Redondo Beach, Calif., formerly TRW, Inc., was the
prime development contractor for the observatory. The Smithsonian
Astrophysical Observatory controls science and flight operations from the
Chandra X-ray Center in Cambridge, Mass.

Additional information and images are available at:
http://chandra.harvard.edu
and http://chandra.nasa.gov