Predicted in theory but never before observed, these elusive objects appear to be similar to today's galaxies in that they're rich in gas. However, without any stars to light the gas, the galaxies have remained hidden from view.
For instance, without stars, the Milky Way's famous, gas-rich Orion Nebula would be dark to our telescopes, said study leader Sebastiano Cantalupo, an astronomer at University of California, Santa Cruz.
To find these cosmic ghosts, Cantalupo and colleagues took advantage of one of the brightest light sources in the cosmos—a quasar known as HE0109-3518.
A superbright galaxy 11 billion light-years away, the quasar shines with the power of a hundred trillion suns and can light up its galactic neighborhood to a radius of ten million light-years. Quasars are very distant galaxies that have actively feeding—and rapidly rotating—supermassive black holes at their hearts.
Using the European Southern Observatory's Very Large Telescope (VLT) in Chile, the astronomers took a very long exposure snapshot of the area surrounding the quasar and detected at least a dozen dark-galaxy candidates.
Only by combining the light-gathering power and exquisite sensitivity of the VLT's four 8.2-meter telescopes could the team directly observe the dim galaxies.
"Missing Link" in Galaxy Evolution?
Because of the time it takes light to travel from the dark galaxies to us, we're seeing the galaxies as they were 11 billion years ago. As such, the newfound galaxies may shed light on the evolution of the bright galaxies we see today, Cantalupo said.
"We're looking at these dark objects as 'infant' galaxies when they were at their earliest stage of their evolution. [That's] when the properties of their gas—such as composition, density, or some still unknown parameter—weren't ideal to form stars," he said.
"So these dark galaxies that we are now finally observing in the early universe are in a sense a missing link in the evolution of galaxies, and may in fact represent the 'building blocks' of our current bright galaxies we see today, including our own."
It's still a mystery how the stars form from the primordial gas in dark galaxies, but one possibility is that these galaxies remain dark until they become massive enough to ignite star formation or until they collide with each other.
"In our current picture of galaxy formation, big galaxies might acquire a large fraction of their gas through cannibalism of smaller systems," Cantalupo said.
"So in our case of small dark galaxies, they might have been essential to bring to their bright companions the gas necessary to feed their star formation."
The dark-galaxies research will appear in an upcoming issue of Monthly Notices of the Royal Astronomical Society.