The Large and Small Magellanic Clouds, two of the Milky Way's better-known satellite galaxies.
Credit: Bessell and Sutherland, Mt Stromlo Observatory, Canberra.
BRISBANE: Newton's theory of gravitation may have to be modified to explain the behaviour of the Milky Way's satellite dwarf galaxies, claims a team of astronomers.
The number of companion dwarf galaxies, and the way they are distributed and moving cannot be explained by the laws of physics as they are currently understood, they argue in reports published in The Astrophysical Journal and the Monthly Notices of the Royal Astronomical Society.
Foundations of cosmological theory
"Satellite galaxies hold important clues not only to the physical processes acting at early cosmological times, but also on the foundation of cosmological theory," said Helmut Jerjen, senior author of both papers and an astrophysicist at the Mt. Stromlo Observatory in Canberra, Australia.
Jerjen and his co-workers – at the University of Bonn in Germany and the University of Vienna in Austria – made the discovery by surveying the scientific literature on all the companion galaxies currently known.
Theory predicts that the satellite galaxies should be evenly distributed around the Milky Way, but, in fact, most of them lie in a kind of disc on the same plane, perpendicular to the spiral of the Milky Way, Jerjen said.
Furthermore, the galaxies are all moving in the same direction around the Milky Way, like planets around a star, which means they may have formed when a larger galaxy crashed into the Milky Way and disintegrated, Jerjen added.
Such a collision would have stripped the dark matter out of the shattered galaxy, before new dwarf galaxies condensed out the gaseous material left behind. The new galaxies would all continue to move in the same direction as the parent galaxy had been when it hit the Milky Way, but should contain very little dark matter.
Modifications to the theory
However, stars within the galaxies are moving faster than would be predicted from the amount of visible matter the galaxies contain, suggesting that they must also contain dark matter, which affects their gravity.
This finding could even hint that some of the fundamental principles of physics have been misunderstood, Jerjen said, and if that's that case, Newton's theory of gravitation might need to be modified for the fourth time in 100 years.
The theory has previously been modified by Einstein's special theory of relativity and later, general relativity, to explain discrepancies in gravity of objects at high velocities and in the presence of large masses. It was modified again with quantum mechanics to explain physics on sub-atomic scales.
The next step is to look for more satellite galaxies, Jerjen said, because the main source of information about nearby dwarf galaxies, the Sloan Digital Sky Survey, only covers the sky of the Northern Hemisphere.
A new project covering the Southern Hemisphere sky, the Stromlo Missing Satellites Survey, will be critical for studying the strange behaviour of the dwarf galaxies and formulating theories to explain it.
Emma Ryan-Weber, an astrophysicist at Swinburne University of Technology in Melbourne, Australia, said that there's not enough evidence to warrant a re-write of Newton's theory just yet, though.
"While everyone agrees that the distribution of satellites around the Milky Way is a bit odd, I think most astronomers would say that it's a bit extreme to modify Newtonian gravity just to explain one system," she said.
Daniel Zucker, an astrophysicist at Macquarie University, in Sydney, agreed. “To say that the conclusions of the two papers... require alternate theories of gravity to what is currently accepted would be premature,” Zucker said.
However, if further studies do confirm the arrangement of satellite galaxies in discs, “it could pose serious problems for the leading paradigm of galaxy formation,” he said.
