Chandra's Find of Lonely Halo Raises Questions About Dark Matter(Forwarded)

Steve Roy
Marshall Space Flight Center, Huntsville, AL
Phone: 256-544-6535


Megan Watzke
Chandra X-ray Observatory Center, CFA, Cambridge, MA
Phone: 617-496-7998


Science Contacts:
Ewan O'Sullivan, 617-496-2197,
Trevor Ponman, +44 (0)121 414 6448,

October 26, 2004

CXC: 04-11

Chandra's Find of Lonely Halo Raises Questions About Dark Matter

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 relatively nearby 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 -- perhaps a major modification -- 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/photo/2004/ngc4555/
and
http://www.msfc.nasa.gov/news/news/p...tos04-253.html

"Andrew Yee" ha scritto nel messaggio
. ..
snip
I'm not a dark matter fan but this Chandra observation is almost a
demonstration of dark matter existence!

Luigi Caselli