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Most of us expect galaxies to be filled with the light of thousands of millions of stars, but another type of galaxy is thought to exist – one that contains hardly any stars at all. Known as dark galaxies, astronomers hypothesize that these hidden galaxies, barely detectable due to the lack of starlight within them, are scattered throughout the universe just waiting to be discovered.
It’s suggested that these mysterious dark galaxies are made up of primarily dark matter, hydrogen and dust. Whilst astronomers have identified many possible candidates for dark galaxy classification, upon further inspection many objects which astronomers think could be a dark galaxy candidate instead turn out to be a tidal gas cloud – which do not contain dark matter.
The theory behind dark galaxies
The prediction of the existence of dark galaxies was first made by Dr Neil Trentham, Dr Ole Moller and Dr Enrico Ramirez-Ruiz, all from the University of Cambridge. In April 2001 they published their hypothesis in Monthly Notices of the Royal Astronomical Society. Titled ‘Completely dark galaxies: their existence, properties and strategies for finding them’ their work details the properties they expect these dark galaxies to possess, as well as potential observational strategies. Trentham said: “Observationally, a picture is emerging that there is a lot of dark matter in the universe and that most galaxies possess a great deal of it. On the theory side, the cold dark matter theory predicts that there are many low-mass galaxies for every massive one, but we don't see many of them around. That could simply be because very few stars - perhaps none at all - have formed in them. So the question is, ‘How do we look for these completely dark galaxies?’”
Identification of dark galaxies
Unlike our own Milky Way or Cygnus A, dark galaxies are not studded with billions of stars and are therefore very difficult to locate initially. Usually, astronomers can relatively accurately predict the number of stars within a galaxy by measuring tis dark-matter halo, which includes the visible and characteristic galactic disc but also extends outside the edge of this disc. But this doesn’t work with potential dark galaxies, due to the amount of dark matter and lack of stars.
Astronomers have certainly been looking for them though, and have identified a number of potential candidates for classification as dark galaxies, with one of these known as HI1225+01, a gas cloud situated on the edge of the Virgo cluster. Comprising of two large clumps of gases, with one of these having “no identified stellar emission” which makes it a prime candidate for that coveted dark galaxy classification. Research into this gas cloud is still ongoing.
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Another object with the potential to be classified as a dark galaxy is AGC229385, which was identified by the ALFALFA extragalactic HI survey. Again detected by the ALFALFA survey, VIRGOHI21 is also suggested to be a strong candidate for a dark galaxy.
In August 2016, astronomers reported that they had found a huge galaxy which is made up of almost completely dark matter, and may well be a candidate for a dark galaxy. Dragonfly 44, whilst being roughly the same mass as the Milky Way, appears to contain only one star for every hundred found in the Milky Way. Roberto Abraham, co-author if the paper detailing the discovery of Dragonfly 44 said:
"We have no idea how galaxies like Dragonfly 44 could have formed. The Gemini data show that a relatively large fraction of the stars is in the form of very compact clusters, and that is probably an important clue. But at the moment we're just guessing."
Searching for dark matter
Pieter van Dokkum, from Yale University, said of the discovery of Dragonfly 44: “This has big implications for the study of dark matter. It helps to have objects that are almost entirely made of dark matter so we don’t get confused by stars and all the other things that galaxies have. The only such galaxies we have had to study before were tiny. This finding opens up a whole new class of massive objects that we can study.
Ultimately what we really want to learn is what dark matter is. The race is on to find massive dark galaxies that are even closer to us than Dragonfly 44, so we can look for feeble signals that may reveal a dark matter particle.”
Finding more potential candidates for dark galaxies could be the perfect project for a citizen science initiative such as Galaxy Zoo. This project has already helped astronomers to discover objects worthy of further observation which might otherwise take years or even decades to be picked up during the course of normal research. A great example of this is ‘Hanny’s Vorweep’, which was flagged up by Galaxy Zoo volunteer Hanny Van Arkel from the Netherlands. She noticed an object which didn’t conform to any of the examples she had been given and flagged this up with the research team. It took scientists a year of research to discover that Hanny’s Vorweep was in fact an entirely new object that had never before been observed.
It’s thought that these dark galaxies could well be the building blocks of classic galaxies such as our own Milky Way, so discovering them may also help us shed light not only on what mysterious dark matter actually is, but also how our own galaxy formed.