ALMA Finds Dark Dwarf Galaxy

We all know about stars, nebulae and galaxies. But these luminous objects are just the bright tip of a huge, dark cosmic iceberg. Most of the mass in the Universe consists of mysterious dark matter. Dark matter cannot be seen, and no one knows its composition. Cosmologists believe that clumps of dark matter may swarm around galaxies like our own galaxy, the Milky Way. In some of those clumps, stars may have been born. If so, we see them as faint dwarf galaxies. But if the dark matter clumps do not contain stars, astronomers can’t see them. So how do we know if these dark dwarf galaxies are really there? The answer is gravity. Dark matter may be invisible, but it does have gravity. As a result, it can slightly bend the paths of light rays traveling through the Universe. Now, by analyzing existing ALMA observations, astronomers have found a dark dwarf galaxy through its light-bending effect. Back in 2014, ALMA already observed a so-called Einstein ring, known as SDP.81. Here, light from a very remote galaxy is bent into an almost perfect ring by the gravity of another galaxy much nearer to us. The precise shape and structure of the ring is determined by the distribution of mass in the foreground galaxy. An international team of astronomers has now studied the shape of the Einstein ring in very much detail. Using large supercomputers, they then calculated that the foreground galaxy must be accompanied by a much smaller clump of matter, weighing less than one thousandth the mass of the Milky Way. Since nothing shows up on photographs made with large telescopes, this is probably a dark dwarf galaxy. In the future, similar observations may uncover many other dark dwarf galaxies in the Universe. If they are indeed as numerous as cosmologists believe, that will confirm some of their current ideas of dark matter. If not, the mystery becomes even bigger.

What? SDP.81 is one of the best-known examples of an Einstein ring. The ring is actually the distorted image of a galaxy at a distance of some 12 billion light-years. The distortion is due to the gravity of a much nearer galaxy, at a ‘mere’ 4 billion light-years from the Earth. The foreground galaxy acts like a lens. Long ago, Albert Einstein already predicted the existence of these ‘gravitational lenses’. That’s why the ring is called an Einstein ring. SDP.81 is located in the constellation Hydra, the Water Snake. Who? Many astronomers and physicists have worked together in analyzing the Einstein ring SDP.81. They published their results in The Astrophysical Journal. The team leader (and first author of the article) is Yashar Hezaveh of Stanford University in California (USA). Yashar compared the discovery of the dark dwarf galaxy to detecting rain drops on a window pane because they also distort the images of background objects.