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"This discovery of dark galaxies may be the tip of the iceberg," says Jin Koda. "We may find more if we look for fainter galaxies embedded in a large amount of dark matter." (Credit: Adam Block/Wikimedia Commons)

dark matter

There’s ‘something invisible’ around 850 dark galaxies

The discovery of 854 “ultra-dark galaxies” trumps the 47 discovered in 2014—and scientists say this may be the tip of the iceberg.

The galaxies, found in the Coma Cluster using the Subaru Telescope, are similar in size to the Milky Way, but have only 1/1,000 of stars that our galaxy does.

“The findings suggests that these galaxies appear very diffuse and are very likely enveloped by something very massive,” says Jin Koda, principal investigator and associate professor of physics and astronomy at Stony Brook University.

“We believe that something invisible must be protecting the fragile star systems of these galaxies, something with a high mass,” says Koda. “That ‘something’ is very likely an excessive amount of dark matter.”

Coma Cluster
Yellow circles show two of the 47 dark galaxies discovered last year, and green circles are the ones discovered in this new study. (Credit: NAOJ/Stony Brook University)

The component of visible matter, such as stars, is calculated to contribute only one percent or less to the total mass of each galaxy. The rest—dark matter—accounts for more than 99 percent.

The Subaru Telescope 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. It suggests that there has been a long-lived population of galaxies within the cluster and the amount of visible matter they contain, less than one percent, is extremely low compared to the average fraction within the universe.

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These galaxies are dark because they have lost gas needed to create new stars during, or after, their largely unknown formation process billions of years ago.

From their preferential presence within the cluster, it’s likely that the cluster environment played a key role in the loss of gas, which affects star formation within the galaxy.

Dark matter is one of the unresolved mysteries in cosmology. Studies of such interplay between dark matter and stars and gas in galaxies are increasingly attracting attention from researchers.

“This discovery of dark galaxies may be the tip of the iceberg,” says Koda. “We may find more if we look for fainter galaxies embedded in a large amount of dark matter, with the Subaru Telescope and additional observations may expose this hidden side of the Universe.”

Scientists from the National Astronomical Observatory of Japan collaborated on the study. Findings appear in the Astrophysical Journal Letters.

Source: Stony Brook University

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