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Telescopes and space missions

Telescopes and space missions

Galaxy survey casts doubt on cold dark matter

24 Oct 2008 Hamish Johnston
Different but similar? A selection of galaxies from the survey

The physical properties of most galaxies in the universe can be explained in terms of just a single parameter. That’s the controversial conclusion of a team of astronomers in the UK and US, who have studied some 200 galaxies using radio and optical telescopes. The team believes that their discovery could mean that cold dark matter — an invisible substance that some astrophysicists have invoked to explain the formation and motion of galaxies — does not exist. However, not all astrophysicists are convinced.

One of the most important questions in cosmology is how galaxies emerged from the primordial fireball that followed the Big Bang. Physicists believe that ordinary matter — the stuff that makes up stars — would have been distributed evenly throughout the universe by the intense radiation present after the Big Bang. That makes it very unlikely that dense patches of matter would form and rapidly grow into galaxies, which is what seems to have happened less than a billion years after the Big Bang.

The solution to this puzzle could lie with cold dark matter (CDM) — invisible stuff that interacts via gravity but not electromagnetic radiation. CDM, astrophysicists argue, would initially not have been evenly spread by radiation but would instead have been pulled together into clumps by gravity. These denser regions of CDM would then evolve into galaxies by dragging in surrounding matter and CDM. This view is backed up by indirect observations of dark matter, which suggest that it accounts for most of the mass of a typical galaxy.

Many astrophysicists believe that this process can be described by the “hierarchical theory of galaxy formation”, in which progressively larger clumps of matter — and ultimately galaxies — attract each other and merge in often violent collisions. But because of this chaotic formation process, there should therefore be a wide variety of galaxy types in the Universe. That view appeared to be backed up with the observation that galaxies have various properties — such as their radii, mass, rate of rotation and luminosity — that can not be related in a simple way.

Six into one

However, in the last decade or so, astronomers have found that there are correlations between some properties of galaxies. The radius of a galaxy, for example, can be predicted by measuring its luminosity. Now, though, Mike Disney at Cardiff University and colleagues claim to have shown that six important properties of galaxies are controlled by a single parameter. Although the team has yet to identify this parameter, they believe that it is related to the mass of the galaxies (Nature 455 1082).

According to Disney, the discovery makes it very unlikely that galaxies formed according to the hierarchical theory of galaxy formation. Indeed, he goes one step further and says that the team’s results put the very existence of CDM into question — a statement that is certain to rile many astrophysicists.

The team used observational data from the Parkes radio telescope in Australia to identify about 300 objects that looked like they could be galaxies from the radio waves given off by neutral hydrogen. Julianne Dalcanton and colleagues at the University of Washington then searched through optical data from the Sloan Digital Sky Survey (SDSS) for the same objects and found that 200 of them were indeed galaxies.

Single parameter

Using data from both telescopes, the team classified the galaxies in terms of six independent properties. These were two optical radii (which define the sizes of the regions of a galaxy that emit 50% and 90% of the object’s light); the luminosity; the mass of neutral hydrogen in the galaxy; the dynamical mass (which includes dark matter); and the colour of the galaxy.

The team then carried out a statistical analysis of the data and found five correlations between these six properties — leading them to conclude that the structure of these galaxies is controlled by just one parameter. Although the team was unable to conclude exactly what this parameter is, Disney says that it seems to have a strong relationship with the mass of the galaxies.

Disney argues that the team’s finding is at odds with the hierarchical theory of galaxy formation, according to which the structure of a galaxy would be strongly influenced by the nature of the collisions that formed it. “If this were the case we would have expected to see 4-5 independent parameters”, he says. And because the hierarchical theory has CDM at its root, Disney believes that the team’s survey provides strong evidence that CDM does not exist. “Maybe our observations could be explained by CDM, but I wouldn’t bet on it”, he said.

Not everyone is convinced

Not surprisingly, not everyone agrees with Disney. Richard Bower of the University of Durham in the UK told that CDM advocates are aware of correlations between galaxy properties and are trying to explain them. “The theory has been tested successfully against a number of individual correlations”, said Bower, but admitted that CDM advocates have yet to demonstrate that the theory can deal with this method of analysis.

Bower says that the merging of galaxies is now thought to be less important than it was in earlier versions of hierarchical theory. As a result, he is confident that galaxy formation can be explained using CDM. Bower also questioned the significance of the dominance of the mass parameter over the others, noting that the masses of galaxies varied over a much wider dynamic range than the other parameters so it was not surprising that the mass correlation was the strongest.

Disney is now exploring theories of galaxy formation that don’t involve CDM, but rather more conventional matter such as “hydrogen snow”.

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