Galaxy clusters much more common than thought

Data mining exercise reveals a whole new class of astronomical structure. Andrew Masterson reports.

The MACS J0717 galaxy cluster, 5.6 billion light years from Earth, as seen by NASA's Chandra X-ray Observatory.

The MACS J0717 galaxy cluster, 5.6 billion light years from Earth, as seen by NASA's Chandra X-ray Observatory.

X-ray: NASA/CXC/SAO/van Weeren et al.; Optical: NASA/STScI; Radio: NSF/NRAO/VLA

A re-examination of data gathered a decade ago by astronomers using a 3.9-metre telescope located at the Siding Spring Observatory in the Australian state of New South Wales has revealed that the number of galaxy clusters in the universe has been underestimated by as much as a third.

The finding is remarkable, because galaxy clusters – collections of individual galaxies bound together by gravity – are the largest structures in the universe and, by dint of containing billions or trillions of stars, relatively easy to see.

The word “relatively”, in this case, is particularly apt, because stars, and whatever planets and other rocky bits and bobs accompany them, comprise only a very small fraction of any cluster’s mass.

This was a discovery first made by American astronomer Fritz Zwicky in 1933, when he analysed the movements of stars within an enormous agglomeration called the COMA cluster and concluded that the mass of all the visible matter therein was insufficient to account for his findings. Something else – and something huge, at that – must have been in play.

And thus, the concept of dark matter entered the cosmological discourse.

Current estimates suggest that in most galaxy clusters, the galaxies themselves – at least as defined by visible matter – comprise only 1% of the total mass. Hot gas clouds account for another 9%, and dark matter makes up the remaining 90%.

The latest research, however, led by Luis Campusano from the Universidad de Chile, in Chile, suggests that in a substantial tranche of cases these percentages need to be revised, with the visible matter component declining even further.

Campusano and colleagues revisited data gathered during a major galaxy redshift survey known as 2dFGRS, which was completed in 2003. The project looked at 191,440 galaxies.

By carefully mining the information, and discarding some standard definitions, the astronomers identified 341 clusters – 87 of them previously unknown.

The newly discovered groups, classified by the researchers as “late-type-rich clusters”, are described as being “high mass-to-light ratio systems”, which means that they contain fewer stars than other clusters. The stars are also less densely packed, meaning the galaxies contained in each cluster are less luminous than normal.

Campusano and colleagues looked only at galaxies contained in the nearby universe – another highly relative term – but assume the results probably hold for the rest of the cosmos.

The discovery – published in The Astrophysical Journal – seems likely to prompt a surge in newly focussed practical and theoretical astronomy. Not only are galaxy clusters about 33% more common than previously assumed, the astronomers report, but the newly defined “class of late-type-rich clusters is not predicted by current theory”.

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