The Search for Dark Galaxies



Dark galaxies are essentially devoid of stars, therefore they don’t emit any light that telescopes can catch. This makes them virtually impossible to observe unless they are illuminated by an external light source like a background quasar.  The gargantuan quasar shown above is 800 million times the mass of the Sun was discovered 13 billion light-years away.

The image below combines observations from the Very Large Telescope, tuned to detect the fluorescent emissions produced by the quasar illuminating the dark galaxies, with color data from the Digitized Sky Survey. Taken in 2012, the image shows the region of the sky around the quasar HE0109-3518. The quasar is labelled with a red circle near the center of the image. The energetic radiation of the quasar makes dark galaxies glow, helping astronomers to understand the obscure early stages of galaxy formation. The faint images of the glow from 12 dark galaxies are labelled with blue circles.



Despite substantial progress over the past half-century in understanding how galaxies form, important open questions remain regarding how precisely the diffuse gas of the intergalactic medium is converted into stars. One possibility, suggested in recent theoretical models, is that the early phase of galaxy formation involves an epoch when galaxies contain a great amount of gas but are still inefficient at forming stars. Direct proof of such a dark phase has been so far elusive, however—after all, dark galaxies do not emit much visible light. The observational discovery of such galaxies would therefore fill an important gap in our understanding of galaxy evolution.

There are ways to identify dark galaxies, however. An international team led by Dr. Raffaella Anna Marino and Prof. Sebastiano Cantalupo from the Department of Physics at ETH Zurich has now done just that, and was able to search the sky for potential dark galaxies with unprecedented efficiency. They report their results in a paper published today in The Astrophysical Journal, and have identified at least six strong candidates for dark galaxies.

"After several years of attempts to detect fluorescent emission from dark galaxies, our results demonstrate the potential of our method to discover and study these fascinating and previously invisible objects," says Sebastiano Cantalupo (University of California, Santa Cruz), lead author of the 2012 study.

To overcome the obstacle that their target objects are dark, the team used quasars as a flashlight of sorts. These emit intense ultraviolet light, which in turn induces fluorescent emission in hydrogen atoms known as the Lyman-alpha line. As a result, the signal from any dark galaxies in the vicinity of the quasar gets a boost, making them visible. Such fluorescent illumination has been used before in searches for dark galaxies, but Marino et al. searched the neighbourhood of quasars at greater distances than has been possible in earlier observations.

They acquired the full spectral information for each of the dark-galaxy candidates. Deep observations—10 hours for each of the six quasar fields they studied—enabled Marino and her colleagues to efficiently discern dark-galaxy candidates from other sources. From initially 200 Lyman-alpha emitters, a half-dozen regions remained that are unlikely to be normal star-forming stellar populations, making them robust candidates for dark galaxies.

The advances in observational capability have become possible thanks to the Multi Unit Spectroscopic Explorer (MUSE) instrument at the Very Large Telescope (VLT) of the European Southern Observatory (ESO) in Chile. In essence, previous studies were limited to imaging a relative narrow band of frequencies, for which specific filters had to be designed. The MUSE instrument instead allowed hunting 'blindly'—without filters—for dark galaxies around quasars at larger distances from Earth than had been possible so far.

The Daily Galaxy via ETH Zurich

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