NAOJ

A group of researchers from the Stony Brook University (the State University of New York) and the National Astronomical Observatory of Japan has discovered 854 ultra-dark galaxies in the Coma Cluster by analyzing archival data from the Subaru Telescope. The discovery of 47 such mysterious dark galaxies was a surprising find in 2014, and the new discovery of more than 800 suggests galaxy clusters as the key environment for the evolution of these mysterious dark galaxies. "Not only do these galaxies appear very diffuse, but they are very likely enveloped by something very massive,” said Jin Koda.



These galaxies appear diffuse and remarkably extended as seen by the light of the stars they contain. Many are similar in size to the Milky Way but have only 1/1,000 of the stars that our galaxy does. The stellar population within such fluffy extended galaxies is subject to rapid disruption due to a strong tidal force detected within the cluster. Something invisible must be protecting the fragile star systems of these galaxies, something with a high mass. That "something" is likely an excessive amount of dark matter. The component of visible matter, such as stars, is calculated to contribute only 1 percent or less to the total mass of each galaxy. The rest — dark matter — accounts for more than 99 percent.



The Subaru Telescope with its large aperture and wide-field camera, used under excellent seeing conditions, revealed that these dark galaxies contain old stellar populations and shows a spatial distribution similar to those of other brighter galaxies in the Coma Cluster. That suggests they have been a long-lived population of galaxies within the cluster. The amount of visible matter they contain, less than 1 percent, is extremely low compared to the average fraction within the universe.

