Dark galaxies have gained their moniker due to their scarcity of bright stars. Theoretical models suggest that most galaxies started out this way and were no more than a cluster of intergalactic dust and diffuse gas which later got turned into stars.
Since dark galaxies don’t emit a lot of visible light, spotting them can be a tricky business, unless you’re using a cosmic “flashlight” to look for them in the quiet darkness of the universe.
This is precisely what an international team of astronomers has just done, and they wrote all about it in a paper, published this week in The Astrophysical Journal.
To scope out the hiding place of dark galaxies, the team, led by two physicists at ETH Zurich in Switzerland, turned to the brightest light sources available in the cosmos, in other words, quasars.
Quasars are essentially supermassive black holes that emit exceptionally large amounts of energy and exceed the mass of the sun by millions and up to billions of times.
This makes them ideal choices for bringing out to light their significantly darker neighbors. According to ETH Zurich, quasars helped the team peek into their nearby cosmos and identify “at least six strong candidates for dark galaxies.”
With the help of quasars, the team “was able to search the sky for potential dark galaxies with unprecedented efficiency,” the university reports in a news release.
This isn’t the first time astronomers have used quasars to shine a light on their surroundings. These supermassive black holes “emit intense ultraviolet light, which in turn induces fluorescent emission in hydrogen atoms known as the Lyman-alpha line,” ETH Zurich explains.
“As a result, the signal from any dark galaxies in the vicinity of the quasar gets a boost, making them visible,” notes the university.
The novelty is that the team managed to peer further into the cosmos than anyone ever before and “searched the neighborhood of quasars at greater distances than has been possible in earlier observations,” details the news release.
The astronomers used the European Southern Observatory’s Very Large Telescope (VLT) in the Atacama Desert of northern Chile to scour the skies and look for dark galaxies near quasars at vast distances from our planet.
Using the VLT’s Multi Unit Spectroscopic Explorer (MUSE) instrument, the team observed six quasars for a period of 10 hours each and picked up 200 sources of Lyman-alpha emissions.
Further analysis revealed that, of the 200, six were “unlikely to be normal star-forming stellar populations, making them robust candidates for dark galaxies,” stated ETH Zurich.