February 15, 2013
Dark Matter Rival Theory Predicts Properties Of Satellite Galaxies
April Flowers for redOrbit.com - Your Universe Online
A research collaboration between Case Western University and Weizmann Institute of Science has used modified laws of gravity to closely predict a key property measured in faint dwarf galaxies that are satellites of the nearby giant spiral galaxy Andromeda.
The study centers around the property of velocity dispersion, which is the average velocity of objects within a galaxy relative to each other. Velocity dispersion is by used by astronomers to determine the accelerations of objects within the galaxy as well as to estimate the mass of the galaxy itself.
The scientists used Modified Newtonian Dynamics (MOND) to calculate the velocity dispersion for each dwarf galaxy. MOND is a hypothesis that attempts to resolve what appears to be an insufficient amount of mass in galaxies needed to support their orbital speeds.
According to MOND, under a certain condition, Newton's law of gravity must be modified. The MOND 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." McGaugh teamed up with Mordehai Milgrom, professor of physics and astrophysics at Weizmann Institute in Israel. Milgrom is considered the father of MOND.
MOND theory arose from the observed fact that galaxies rotate faster than predicted by the law of gravity without flying apart, forcing astronomers and physicists to try to explain this unexpected phenomenon. In 1932, Dutch astronomer Jan Oort conceived the concept of dark matter. Researchers theorize that dark matter is gathered in and around galaxies, adding the mass needed to hold them together.
Milgrom was dissatisfied with that theory and offered MOND as an alternative theory for explaining the unusual rotational behavior of galaxies. According to MOND, Newton's force law must be tweaked at low acceleration — eleven orders of magnitude lower than what we feel on the Earth's surface. As Newton's law states, acceleration above that threshold is linearly proportional to the force of gravity. Below that threshold, however, it is not. By tweaking the force law under that limitation, the mass discrepancy is resolved.
McGaugh initially subscribed to the theory of dark matter early in his career, but over time came to doubt the accuracy of the theory, believing that it came up short in a number of aspects. However, in recent years he says he has found increasing evidence that supports Milgrom´s explanation.
The research team tested MOND with dwarf spheroidal galaxies, which are very low-surface brightness galaxies that are actually satellites of other larger galaxies. As galaxies go, 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 exlained.
The team used the luminosity of the galaxies — an indicator of stellar mass — and the theory of MOND to make their calculations and predict the velocity dispersions of 17 faint galaxies. In 16 of the 17, the predictions closely matched the velocity dispersions measured by other researchers. In the case of the 17th galaxy, the data from independent observers differed from one another.
"Many predictions were bang on," said McGaugh. "Typically, the better the data, the better the agreement." MOND was also used to predict velocity dispersions for 10 more faint dwarf galaxies in Andromeda. The team is currently awaiting additionally measurements to refute or corroborate these predictions.
The findings of this study will be published in the next edition of the“¯Astrophysical Journal and are already available online through the electronic preprint archive“¯http://arxiv.org/abs/1301.0822.