Astronomers at the European Southern Observatory (ESO) have spotted a dark galaxy for the very first time. The team was able to discover this once theoretical object by utilizing the presence of a bright quasar to reflect off the gas-rich contents of the galaxy.
Dark galaxies are thought to be the building blocks of larger, star-filled galaxies. They are smaller and rich in gasses that have yet to form into stars. It’s hypothesized that the high content of gasses contributes to the formation of stars once they are consumed.
Without stars, dark galaxies emit very little light. Despite techniques that may have hinted at their existence in the past, this is the first time one was fully observed.
“Our approach to the problem of detecting a dark galaxy was simply to shine a bright light on it.” explains Simon Lilly, co-author of the paper. “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 mapped out regions surrounding quasar HE 0109-3518. Using the sensitivity of ESO’s Very Large Telescope (VLT), they set off to search for the faint fluorescent glow emitted from dark galaxies. The intense radiation causes the hydrogen gas within the dark galaxy to emit ultraviolet light that was then detected by the VLT.
“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, lead author of the study.
A total of 12 objects surrounding the quasar were narrowed down from an initial 100 in the neighboring area where the light emitted may have been a result of star formation as opposed to reflection from the quasar. The dark galaxy is estimated at approximately 1 billion solar masses, in line with theoretical assumptions previously held.
“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”, concludes Sebastiano Cantalupo.
Integral field spectrography is to be commissioned for on the VLT next year and will contribute to their further study of dark galaxies.