Astronomers discover "dark galaxies"

Riotous star clusters are probably almost dark matter only

Some of the newly discovered dark galaxies (green) in the Coma Cluster © NAOJ
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Invisible Guardian: Astronomers have discovered more than 800 specimens of the enigmatic dark galaxies. These hardly contain stars and gas, but are nevertheless massive. The researchers therefore suspect that they consist of up to 99 percent of dark matter. This could also explain why these dark galaxies still exist despite the strong tidal forces inside the coma galaxy cluster.

Astronomers discovered that they exist only last year: in 2014, they first came across galaxies that are completely different from those known so far. Because these "dark galaxies" are normal-sized and massive, but contain extremely few stars or gas. So many of them are about the size of the Milky Way, but they have only one-thousandth of their stellar population.

Massive, but poor in stars

Now, with the help of the Subaru Telescope in Hawaii, astronomers have discovered another 854 of these dark galaxies in the Coma cluster. This cluster of galaxies is located some 300 million light-years from Earth and contains mainly elliptical galaxies. Between these brighter star clusters, the researchers now observed numerous much fainter, more washed-out galaxy spots.

"Not only do these galaxies appear very diffuse, they also have to be surrounded by something very massive, " explains study leader Jin Koda of Stony Brook University in New York. For although these galaxies contain very few stars, they have not been torn apart by the strong tidal forces in the cluster. Obviously, in these dark galaxies, there is a large mass whose gravity holds them together.

Dark matter as a stabilizer?

Astronomers suspect that dark matter may be responsible for this mysterious cohesion. "These dark galaxies must be dominated by dark matter if they survive so long in the strong tidal field in the center of the cluster, " the researchers said. They estimate that the galaxies contain only about one percent of normal matter, but the remaining 99 percent are dark matter. display

Presumably, the dark galaxies originated in the early days of the Coma Cluster. At that time, interactions with the cluster environment must have resulted in the young galaxies losing much of their gas. But with that, they lacked the raw material for star formation and they remained star-poor. Their dark matter was preserved and became extremely dominant.

Whether the mysterious stabilizer of these dark galaxies is indeed dark matter, however, can only be determined indirectly. Because the dark matter itself is invisible and so far can only be detected by its gravitational effect on normal matter. For example, astronomers do this by measuring the motion of stars in galaxies. In the case of the coma cluster, however, this is difficult because the galaxies are far away and very faint. (Astrophysical Journal Letters, 2015; arXiv: 1506.01712)

(Subaru Telescope, 23.06.2015 - NPO)