|
|
NGC 720:
Chandra Casts Cloud On Alternative To Dark
Matter
A Chandra image of NGC 720 shows a galaxy enveloped in
a slightly flattened, or ellipsoidal cloud of hot gas
that has an orientation different from that of the
optical image of the galaxy. The flattening is too
large to be explained by theories in which stars and
gas are assumed to contain most of the mass in the
galaxy.
According to the standard theory of gravity, the X-ray
producing cloud would need an additional source of
gravity - a halo of dark matter - to keep the hot gas
from expanding away. The mass of dark matter required
would be about five to ten times the mass of the stars
in the galaxy.
An alternative theory of gravity called MOND, for
Modified Newtonian Dynamics, does away with the need
for dark matter. However, MOND cannot explain the
Chandra observation of NGC 720, which shows that the
dark matter halo has a different shape from that of the
stars and gas in the galaxy. This implies that dark
matter is not just an illusion due to a shortcoming of
the standard theory of gravity - it is real.
The Chandra data also fit predictions of a cold dark
matter model. According to this model, dark matter
consists of slowly moving particles which interact with
each other and "normal" matter only through gravity.
Other dark matter models, such as self-interacting dark
matter, and cold molecular dark matter, are not
consistent with the observation in that they require a
dark matter halo that is too round or too flat,
respectively.
|
Fast Facts for NGC
720:
|
|
Credit
|
X-ray: NASA/CXC/UCI/D.Buote et al.,
Optical: DSS U.K.Schmidt Image/STScI
|
|
Scale
|
Images are 5 x 4.2 arcmin.
|
|
Category
|
Normal Galaxies & Starburst Galaxies
|
|
Coordinates
(J2000)
|
RA 01h 53m 0.4s | Dec -13º 44'
18"
|
|
Constellation
|
Cetus
|
|
Observation
Date
|
Chandra: October 12, 2000, DSS:
August 08, 1982
|
|
Observation
Time
|
Chandra: 9.6 hours, DSS: 1 hour
|
|
Obs.
ID
|
492
|
|
Color
Code
|
Intensity (Chandra: 0.3-3keV
bandwidth)
|
|
Instrument
|
ACIS
|
|
Reference
|
D. Buote et al., 2002 Astrophys. J. 577, 183.
|
|
Distance Estimate
|
About 80 million light years
|
|
Release Date
|
October 22, 2002
|
|
|
|
