Dark galaxies spotted for the first time
July 11, 2012

Dark Galaxies Observed For First Time

Lee Rannals for redOrbit.com - Your Universe Online

Dark galaxies may have been spotted for the very first time using the European Space Observatory's Very Large Telescope (VLT).

An international team of astronomers believe they have detected these elusive objects by observing them as they were lit up by glowing quasars.

Dark galaxies are very small, gas-rich galaxies in the early universe that are inefficient at forming stars. Scientists believe these galaxies are building blocks of today's bright, star-filled galaxies.

Astronomers think that these galaxies may have fed large galaxies with much of the gas that later formed into the stars that exist today.

Dark galaxies do not emit much light because they are nearly devoid of stars, making them hard to detect.

Astronomers have been trying to develop new techniques to confirm the existence of these galaxies, and this new study marks the first time the objects have been seen directly.

Our approach to the problem of detecting a dark galaxy was simply to shine a bright light on it.” Simon Lilly, co-author of the paper, said in a press release. “We searched for the fluorescent glow of the gas in dark galaxies when they are illuminated by the ultraviolet light from a nearby and very bright quasar. The light from the quasar makes the dark galaxies light up in a process similar to how white clothes are illuminated by ultraviolet lamps in a night club.”

The team used ESO's VLT to detect the extremely faint fluorescent glow of the dark galaxies. They used the FORS2 instrument to map a region of the sky around the bright quasar, looking for the ultraviolet light that is emitted by hydrogen gas when it is subjected to intense radiation.

“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,” Sebastiano Cantalupo, lead author of the study, said in a press release.

The team detected nearly 100 gaseous objects which lie within a few million light-years of the quasar. They narrowed down their search to 12 objects after a careful analysis designed to exclude objects where emission might be powered by internal star-formation in the galaxies rather than light from the quasar.

The astronomers were able to determine some of the properties of the dark galaxies. They believe the mass of the gas in them is about 1 billion times that of the Sun.

They were able to estimate that the star formation efficiency is suppressed by a factor of more than 100 relative to typical star-forming galaxies found at a similar stage in cosmic history.

“Our observations with the VLT have provided evidence for the existence of compact and isolated dark clouds. With this study, we´ve made a crucial step towards revealing and understanding the obscure early stages of galaxy formation and how galaxies acquired their gas”, Sebastiano Cantalupo said in the release.

The researchers paper will appear in the Monthly Notices of the Royal Astronomical Society.