Something missing? (Image: NASA/ESA/STScI/AURA/A. Aloisi)
LIKE moths about a flame, thousands of tiny satellite galaxies flutter about our Milky Way. For astronomers this is a dream scenario, fitting perfectly with the established models of how our galaxy's cosmic neighbourhood should be. Unfortunately, it's a dream in more ways than one and the reality could hardly be more different.
As far as we can tell, barely 25 straggly satellites loiter forlornly around the outskirts of the Milky Way. "We see only about 1 per cent of the predicted number of satellite galaxies," says Pavel Kroupa of the University of Bonn in Germany. "It is the cleanest case in which we can see there is something badly wrong with our standard picture of the origin of galaxies."
It isn't just the apparent dearth of galaxies that is causing consternation. At a conference earlier this year in the German town of Bad Honnef, Kroupa and his colleagues presented an analysis of the location and motion of the known satellite galaxies. They reported that most of those galaxies orbit the Milky Way in an unexpected manner and that, taken together, their results are at odds with mainstream cosmology. There is "only one way" to explain the results, says Kroupa: "Gravity has to be stronger than predicted by Newton."
Challenging Newton's description of gravity is controversial. But regardless of where the truth lies, the Milky Way's satellite galaxies have become the latest battleground between the proponents of dark matter and theories of modified gravity.
Our standard picture of the universe comes from many decades of observations. It asserts that visible matter - the kind of stuff that you, me, the planets and stars are made of - is outweighed by a factor of 6 or 7 by invisible, cold dark matter. No one knows what dark matter is made of, but its existence has been postulated to explain how the stars in spiral galaxies can orbit at such breakneck speeds without being flung off into the void. There isn't enough ordinary matter out there to hold on to everything, so the extra gravitational grip provided by large amounts of dark matter stops these speeding stars flying off into space.
Dark matter is also thought to have played a key role in shaping the early universe. In the aftermath of the big bang, it was the dark stuff that first began to clump together under the force of gravity because its lack of interaction with light meant it was not blasted apart by the big-bang fireball. Later on, normal gaseous matter fell into these clumps - dubbed dark matter haloes - where it congealed into stars to make visible galaxies.
A key feature of this dark matter scenario is that dark matter haloes of all sizes form. According to the standard model of cosmology, a halo as large as the one thought to have seeded the Milky Way should be surrounded by thousands of mini haloes, which themselves should have seeded small satellite galaxies.
So why don't we see them? It could simply be because most of the satellite galaxies contain only a few thousand stars and their faintness makes them extremely hard to spot (see New Scientist, 15 August, p 10).
Another problem is that it is not obvious to the human eye that an apparent group of stars in the sky is a bound collection rather than a chance alignment of stars at wildly different distances. Proving their connectedness requires computerised search techniques and detailed analyses of the colours of the stars to give their relative distances and types - a painstaking and expensive business.
Nevertheless, the rate of discovery of satellite galaxies has been boosted in the past five years by a detailed search by the Sloan Digital Sky Survey. Whereas only nine satellites were discovered in the 30 years before SDSS, another 15 have been found since. The biggest are about 1000 light years across - less than 1 per cent of the diameter of the Milky Way's disc - and the smallest about 150 light years across. Despite this progress, the total number of satellites known falls far short of that predicted by the cold dark matter paradigm.
The missing-satellites problem is not the only puzzle. Kroupa and his Bonn colleague Manuel Metz, together with Gerhard Hensler at the University of Vienna, Austria, and Helmut Jerjen of Mount Stromlo Observatory near Canberra, Australia, have studied the location and motion of the small number of known satellite galaxies. They found that a high proportion of the galaxies appear to be confined to a plane perpendicular to the disc of the Milky Way. What's more, most of the galaxies orbit the Milky Way in the same direction. "This is completely incompatible with the dark matter model of the Milky Way's formation," says Kroupa. He points out that the satellites should be more like a swarm of bees, moving on random orbits and distributed in a spherical shell around our galaxy.
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