Do dwarf galaxies favour MOND over Dark Matter? (Forwarded)

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CONTACTS

Garry W. Angus
School of Physics and Astronomy
SUPA, University of St Andrews
St Andrews
KY16 9SS
UK
Tel: +44 1334 461668
Website: http://star-www.st-and.ac.uk/~gwa2/

EMBARGOED UNTIL 0001 BST, 2 April 2008

Ref.: PN 08/18 (NAM 09)

Do dwarf galaxies favour MOND over Dark Matter?

A detailed analysis of eight dwarf galaxies that orbit the Milky Way
indicates that their orbital behaviour can be explained more accurately with
Modified Newtonian Dynamics (MOND) than by the rival, but more widely
accepted, theory of dark matter. The results will be presented by Garry
Angus, of the University of St Andrews, at the RAS National Astronomy
Meeting in Belfast on Wednesday 2nd April.

"MOND was first suggested to account for things that we see in the distant
universe. This is the first detailed study in which we've been able to test
out the theory on something close to home. The MOND calculations and the
observations appear to agree amazingly well. We've also found some exciting
tidal effects predicted by MOND that we should be able to test through
future observations and simulations," said Angus.

Essentially, MOND adds a new constant of nature (a0) to physics, besides the
speed of light, Planck's constant among others. Above it, accelerations are
exactly as predicted by Newton's second law, which says that a force equals
an object's mass times its acceleration. Below it, gravity decays with
distance from a mass, rather than distance squared. This constant is so
small that it goes unnoticed with the large accelerations that we experience
in day-to-day life. For instance, when we drop a ball the gravity is 100
billion times stronger than a0 and the accelerated motion of the Earth round
the Sun is 50 million times stronger. However, when objects are accelerating
extremely slowly, as we observe in galaxies or clusters of galaxies, then
the constant makes a significant difference to the resulting gravitational
forces.

When MOND is applied to the population of dwarf galaxies, one effect is that
tidal forces from the Milky Way, which have a negligible effect in classical
Newtonian Mechanics, can actually make a big difference. This is
particularly significant for the dwarfs orbiting our Galaxy most closely.

"In these dwarf galaxies, the internal gravity is very weak compared to the
gravity of the Milky Way. MOND suggests that the Milky Way is a bit like a
bank that loans out gravity to nearby dwarf galaxies to make them more
stable. However, there are conditions on the loan: if the dwarf galaxies
start to approach the bank, the loan is gradually reduced or even cancelled
and the dwarfs must pay it back. In two galaxies, we've seen what could be
signs that they've come too close too quickly and are unable to repay the
loan fast enough. This appears to have caused disruption to their
equilibrium," said Angus.

Angus used MOND to calculate the ratio of mass to amount of light emitted by
the stars in the dwarf galaxies from the observed random velocities of the
stars collected independently. He also calculated the orbital paths of the
stars in the dwarf galaxies. In all eight cases, the MOND calculations for
the orbits were within predictions. For six of the eight galaxies, the
calculations were also a good match to expected values for mass-to-light
ratios; however for two galaxies, Sextans and Draco, the ratios were very
high, which could well suggest tidal effects. The value for Sextans could
also be due to poor quality measurements of the galaxy's luminosity, which
are improving all the time for these ultra dim objects.

"These tidal effects can be tested by updating the 13 year old luminosity of
Sextans and making accurate observations of the orbits of Draco and Sextans
around the Milky Way. We also need to carry out some detailed simulations to
understand the exact mechanisms of the tidal heating," said Angus.

If Newton's gravity holds true, the dark matter needed in the dwarfs has
constant density in the centre which is contrary to theoretical predictions,
which suggest density should rise to the centre.

"Even without direct detection, the dark matter theory is difficult to prove
or refute and although we may not be able to prove whether MOND is correct,
by carrying out these kind of tests we can see if it continues to hold up or
if it is definitely ruled out," said Angus.

IMAGES
[http://www.ras.org.uk/images/stories...satellites.jpg
(424KB)]
Satellite galaxies of the Milky Way.

NOTES FOR EDITORS

RAS NATIONAL ASTRONOMY MEETING

The RAS National Astronomy Meeting (NAM 2008) is hosted by Queen's
University Belfast. It is principally sponsored by the RAS and the STFC. NAM
2008 is being held together with the UK Solar Physics (UKSP) and
Magnetosphere, Ionosphere and Solar-Terrestrial (MIST) spring meetings.

MODIFIED NEWTONIAN DYNAMICS (MOND)

The MOND theory was first put forward in 1983 by Mordehai Milgrom, now at
the Weizmann Institute of Science in Rehovot, Israel.

The constant, a0 is equal to 1.2 x 10**-10 m/s2. Milgrom points out that
this value is also the acceleration that you get by dividing the speed of
light by the lifetime of the universe.

DWARF GALAXIES

There are fourteen dwarf galaxies known to orbit the Milky Way, including
the Large and Small Magellanic Clouds. The eight selected for this study are
all spherical in shape and lie around 240-750 million trillion (x10**16)
kilometres outside the Milky Way.