Credit: Pieter van Dokkum, Roberto Abraham, Gemini, Sloan Digital Sky Survey

There was a dark twin in the Milky Way. Dim massive galaxy Dragonfly 44 ( Dragonfly 44 - approx. Translator ) consists of dark matter at a record 99.99% and can help rewrite our theories about the formation of galaxies. Dragonfly 44 is similar to the Milky Way in mass, but differs in the number of stars and structure.

“If you take the Milky Way and leave one out of every 100 stars, you’ll have about the same thing,” says Peter van Dokkum of Yale University. - We'll also have to take these remaining stars and mix in a blender.
This galaxy is not spiral, like the Milky Way, but not a flat disk.

Dragonfly eye

And she is not alone. Van Dokkum and his colleagues noticed the galaxy and its neighbors in 2014 using an array of telephoto lenses.

“By combining them into this array, which looks like an insect's eye, we collect much more light,” says van Dokkum. Therefore, it was called Dragonfly.

When the team aimed the Dragonfly at the Coma Cluster, a huge cluster of galaxies 320 million light years away, they found 47 obscure spots : galaxies that could be as large as the Milky Way, 100,000 light years from edge to edge, but containing so few stars that their light was no brighter than the light from dwarf galaxies.

It may have two explanations: galaxies can be firmly connected by dark matter, a still undetected substance, which is supposed to be 85% of the mass of the universe. Or, these galaxies are unstable - and Coma’s brutal cluster is now tearing them to shreds.

How can there be so much dark matter?

To understand this, van Dokkum and his colleagues watched Dragonfly 44, one of the largest galaxies, with a spectrograph on the 10-meter Keck II telescope on Mauna Kea, Hawaii. This allowed the team to track how fast the stars move through the galaxy and calculate its mass: the higher the speed, the greater the mass.

They found that the stars were moving at 47 km / s, making the Dragonfly 44 about a trillion times more massive than the Sun. With such a small amount of normal matter, it must be 99.99% dark matter to survive, which is much more than the average for the universe. The galaxy broke a record of another similar dark galaxy in the Virgo cluster discovered earlier this year, which is 99.96% dark matter.

But astronomers do not understand how these dark galaxies are formed. “It’s hard to argue with observations, but the conclusions from the article contradict my understanding of how galaxies are formed,” says Marla Geha of Yale University. “I hope that such objects are quite rare and / or are formed only under special conditions, like a dense cluster of galaxies. Otherwise, we may need to rewrite the theory of the formation of galaxies. ”

Link to the original publication: Astrophysical Journal Letters , DOI: 10.3847 / 2041-8205 / 828/1 / L6. arXiv