Hidden Galaxies Detected

Artist's impression showing how a background quasar can hide a galaxy. The light that travels from the quasar is filtered by the foreground galaxy on its way to Earth. Credit: European Southern Observatory

Deer can't see cars at night because of blindingly bright headlights. And until now, astronomers couldn't see foreground galaxies outshined by the dazzling quasars behind them.

A new technique can pick apart the intense pattern of light emitted by quasars, finding irregularities in the image where "invisible" galaxies are absorbing some of the quasar light.

"The difficulty in actually spotting and seeing these galaxies stems from the fact that the glare of the quasar is too strong compared to the dim light of the galaxy," said Nicholas Bouche, an astronomer at the Max Planck Institute for Extraterrestrial Physics in Munich, Germany.

Bouche and his team's findings will be detailed in an upcoming issue of Astrophysical Journal.

Very large help

Quasars are small, distant and extremely bright cosmic beacons that produce more light than typically comes from an entire large galaxy. In spite of their brightness, however, some of the light is soaked up by intervening objects during its long journey to Earth's telescopes.

To locate the so-called "invisible" galaxies, Bouche and his team looked through huge catalogues of quasar data and picked out those with "dips" in their light signatures. Then, using the European Southern Observatory's (ESO) Very Large Telescope (VLT), located in the mountains of northern Chile, the team searched for galaxies close to the pulse of quasar light.

The astronomers capitalized on the VLT's special infrared spectrometer, called SINFONI, to pick apart 20 patches of sky around the quasars to search for galaxies from the time when the universe was about 6 billion years old, almost half its current age. Seventy percent of the time, they found a galaxy hiding in the "headlights" of a quasar.

So far, the astronomers who pioneered the technique have detected 14 hidden galaxies by targeting the VLT on unusual quasar light signatures.

Galaxy hunt

Bouche said he is surprised by not only the amount of galaxies he and his colleagues have found hiding near quasars, but also by the types of these galaxies.

"These are not just ordinary galaxies," he said. "They are ... actively forming a lot of new stars and qualifying as 'starburst galaxies.'"

These types of galaxies are forming the equivalent of about "20 suns per year," noted team member Celine Peroux, an astronomer at the Institute of Astronomy in Cambridge.

The team thinks their finding will spur a new hunt for galaxies in the universe.