Most people think of galaxies as huge islands of stars, gas and dust that populate the universe in visual splendor. Theory, however, has predicted there are other types of galaxies that are devoid of stars and made predominately of dense gas. These "dark" galaxies would be unseen against the black backdrop of the universe.
Now, an international team of astronomers has detected several dark galaxies by observing the fluorescent glow of their hydrogen gas, illuminated by the ultraviolet light of a nearby quasar. But what exactly are dark galaxies, and what role do they play in the evolution of our universe?
"Dark galaxies are composed of dark matter and gas, but for some reason they have not been able to form stars," said Martin Haehnelt, Kavli Institute for Cosmology at the University of Cambridge. "Some theoretical models have predicted that dark galaxies were common in the early universe when galaxies had more difficulty forming stars – partly because their density of gas was not sufficient to form stars – and only later did galaxies begin to ignite stars, becoming like the galaxies we see today."
Haehnelt is a member of the scientific team that detected these galaxies. According to Haehnelt, one can begin to understand the importance of dark galaxies by looking at our own Milky Way. "We expect the precursor to the Milky Way was a smaller bright galaxy that merged with dark galaxies nearby. They all came together to form our Milky Way that we see today."
Another member of the team, Sebastiano Cantalupo of the University of California, Santa Cruz, agreed that dark galaxies are the building blocks of modern galaxies. "In our current theory of galaxy formation, we believe that big galaxies form from the merger of smaller galaxies. Dark galaxies bring to big galaxies a lot of gas, which then accelerates star formation in the bigger galaxies."
The techniques used for detecting dark galaxies also may provide a new way to learn about other phenomena in the universe, including what some call the "cosmic web" – unseen filaments of gas and dark matter believed to permeate the universe, feeding and building galaxies and galaxy clusters where the filaments intersect.
"I wonder if we can indeed use this technique to see the emission of filamentary gas in the cosmic web, and if so, how close are we to seeing that?" said team member Simon Lilly of the Swiss Federal Institute of Technology in Zurich, Switzerland. "That has been something of a Holy Grail for many, many years and I think this most recent discovery of dark galaxies is a significant step toward the goal."