Glow in the dark

Cosmology

The Economist, Jul 12th 2012, 11:57 by T.C.

http://www.economist.com/blogs/babbage/2012/07/cosmology

LAST week scientists at CERN, Europe's main particle-physics lab, finally ran the Higgs boson to ground. The discovery of the Higgs, whose existence was first predicted in 1964, is a powerful demonstration of the predictive powers of the Standard Model of particle physics. But other scientists have powerful theories of their own, even if they get less press than particle physicists do. A paper just published in the Monthly Notices of the Royal Astronomical Society reports another predictive triumph, this time for astronomers.

Sebastiano Cantalupo, of the University of California, Santa Cruz, and his colleagues may have become the first people ever to spot "dark galaxies", the antediluvian ancestors of the bright islands that contain almost all of the stars in the modern universe, and of which Earth's own Milky Way is one.

Despite their name, dark galaxies are not as tenebrous as other "dark" astronomical phenomena. Unlike dark energy or exotic dark matter—so called because, since they do not interact with photons of electromagnetism, their presence can only be discerned through their gravitational effects—they are made up of humdrum hydrogen and helium gas. But they are relatively small, and their weak gravity means their gas is so dispersed that stars condense out of it only very slowly.

Some characteristics of bigger, brighter, modern galaxies—for example, the relationship between a galaxy's mass and its star-formation rate—could be explained if there are sources of gas feeding them. Dark galaxies could fit the bill. Astronomers think that their sluggishness at converting gas into stars meant that they remained gaseous while the earliest generations of stars formed, becoming, in effect, leftover stockpiles of gas that more active galaxies could tap. And although none has been observed until now, they were independently predicted by theoretical models that describe how the various forms of large-scale structure visible in today's universe—galaxies, clusters, superclusters and the enormous, thread-like structures called filaments—condensed out of tiny fluctuations within the thin, almost uniform soup of the early universe.

All this has meant that scientists were relatively confident dark galaxies must have existed. But their predicted dimness was always going to make them hard to pin down. Like their particle-physicist comrades, then, Dr Cantalupo and his team required cutting-edge kit to perform their search. They used a custom-built filter attached to the European Southern Observatory's Very Large Telescope (VLT). Despite having a name even more prosaic than CERN's Large Hadron Collider, where the Higgs discovery was made, the VLT (pictured) is one of the most powerful optical telescopes in the world. 

Given that dark galaxies give off almost no light, the researchers used their filter to look for something else that might, literally, shed light onto their cosmic quarry. In this case, the source of illumination was a nearby quasar. The ultraviolet radiation from this very bright and distant galactic nucleus (HE0109-3518, for the curious) caused the gas in the proto-galaxy to fluoresce. 

After controlling for various sorts of false positives, the astronomers were left with a dozen strong candidates, each with about a billion times the mass of the sun and a rate of star formation around 200 times lower than a more familiar spiral galaxy—precisely as predicted. One for the astronomers, then, just so the particle physicists don't get too smug.