MessageToEagle.com - Using modified laws of gravity, researchers from Case Western Reserve University and
Weizmann Institute of Science closely predicted a key property measured in faint dwarf galaxies that are satellites
of the nearby giant spiral galaxy Andromeda.
The predicted property in this study is the velocity dispersion, which is the average velocity of objects within a
galaxy relative to each other. Astronomers can use velocity dispersion to determine the accelerations of objects
within the galaxy and, roughly, the mass of a galaxy, and vice-versa.
To calculate the velocity dispersion for each dwarf galaxy, the researchers utilized Modified Newtonian Dynamics,
MOND for short, which is a hypothesis that attempts to resolve what appears to be an insufficient amount of mass
in galaxies needed to support their orbital speeds.
Click on image to enlarge
M31: The Andromeda Galaxy
Andromeda is the nearest major galaxy to our own Milky Way Galaxy. Our Galaxy is thought to look much like Andromeda. Together these two galaxies dominate the Local Group of galaxies. The diffuse light from Andromeda is caused by the hundreds of billions of stars that compose it. The several distinct stars that surround Andromeda's image are actually stars in our Galaxy that are well in front of the background object. Andromeda is frequently referred to as M31 since it is the 31st object on Messier's list of diffuse sky objects. M31 is so distant it takes about two million years for light to reach us from there. Although visible without aid, the above image of M31 is a digital mosaic of 20 frames taken with a small telescope. Much about M31 remains unknown, including how the center acquired two nuclei.
Credits: Robert Gendler
MOND suggests that, under a certain condition, Newton's law of gravity must be altered. That hypothesis is less
widely accepted than the hypothesis that all galaxies contain unseen dark matter that provides needed mass.
"MOND comes out surprisingly well in this new test," said Stacy McGaugh,
astronomy professor at Case Western Reserve.
"If we're right about dark matter, this shouldn't happen."
Astronomers and physicists need some way to explain why galaxies rotate faster than predicted by the law of gravity
without flying apart. That spurred researchers to theorize that dark matter, first assumed by Dutch astronomer Jan
Oort in 1932, is gathered in and around galaxies, adding the mass needed to hold galaxies together.
Dissatisfied with that hypothesis, Milgrom offered MOND, which says that Newton's force law must be tweaked at
low acceleration, eleven orders of magnitude lower than what we feel on the surface of Earth.
Acceleration above that threshold is linearly proportional to the force of gravity -- as Newton's law states -- but below the threshold,
is not, he posits. When the force law is tweaked under that limitation, the modification can resolve the mass discrepancy.
Early in his career, McGaugh believed in dark matter. But, over time, he's found the hypothesis comes up short in
a number of aspects while he's found increasing evidence that supports MOND.
In this paper, researchers tested MOND with dwarf spheroidal galaxies. These very low-surface brightness galaxies
are satellites of larger galaxies. By the standards of galaxies they are tiny, containing only a few hundred thousand stars.
"These dwarfs are spread exceedingly thin. Their light is spread over hundreds to thousands of light-years.
These systems pose a strong test of MOND because their low stellar density predicts low accelerations," McGaugh said.
McGaugh and Milgrom used the luminosity of the galaxies, an indicator of stellar mass, and MOND to make their
calculations and predict the velocity dispersions of 17 faint galaxies. In 16 cases, the predictions closely
matched the velocity dispersions measured by others. In the last case, the data from independent observers
differed from one another.
"Many predictions were bang on," McGaugh said. "Typically, the better the data, the better the agreement."
The scientists also used MOND to predict velocity dispersions for 10 more faint dwarf galaxies in Andromeda.
They are awaiting measurements to refute or verify this prediction.