A team of astronomers from US and Canada has discovered a ghost galaxy, dubbed Dragonfly 44, which they claim is made up of almost 99.99% dark matter.
The galaxy is located about 330 light years away from Earth, in the Coma Cluster, and is about the same size as the Milky Way. The galaxy lacks stars and is so dim that it evaded being discovered by astronomers in the past so many decades. According to researchers, the presence of such a huge amount of dark matter in the galaxy enabled it from being ripped apart.
Scientists believe about 27% of the mass and energy in the Universe is made up of dark matter, 5% made up the kind of normal matter, and remaining 68% made of dark energy.
While this invisible, dark matter doesn’t emit any observable light or radiation, it definitely exerts a gravitational force which scientists have been able to detect. It is believed that this gravitational force also helps keep galaxies intact, otherwise they would rip apart because of the great speeds at which they rotate.
Dragonfly 44 was actually discovered in 2014 by a team of researchers from the US and Canada. These astronomers were using the WM Keck Observatory and the Gemini North Telescope in Manuakea, Hawaii to observe the Coma Cluster when they found Dragonfly 44. The scientists observed that the stars in the galaxy were not normal set of stars – but that the galaxy was actually made up of dark matter. Only 0.001% of the galaxy was made of up of the matter like dust, gas and stars. In comparison, our galaxy Milky Way has a hundred times more stars than the number of stars present in Dragonfly 44.
“If the Milky Way is a sea of stars, then these newly discovered galaxies are like wisps of clouds,” said researcher Pieter van Dokkum, a member of the team from Yale University.
“We are beginning to form some ideas about how they were born, and it’s remarkable they have survived at all.”
“They are found in a dense, violent region of space filled with dark matter and galaxies whizzing around, so we think they must be cloaked in their own invisible dark matter ‘shields’ that are protecting them from this intergalactic assault.”
The team tested out their hypothesis by calculating the mass of Dragonfly 44. They measured the velocities of stars in the galaxy for many hours, and then calculated the Dragonfly’s mass. The team found that stars in Dragonfly 44 are moving at a speed of 47 km/s and that the galaxy is about 1 trillion times more massive than the Sun.
According to van Dokkum, the motion of stars indicated that Dragonfly 44 has many times more mass than the total mass of the stars within the galaxy. Such a huge amount of mass suggests that Dragonfly 44 is made up of 99.99 percent of dark matter.
“We have no idea how galaxies like Dragonfly 44 could have formed.” said co-author Professor Roberto Abraham, from the University of Toronto in Canada.
Researchers say chances are there are many more such ghost galaxies waiting to be discovered in the universe.
The results of the research have been published in Astrophysical Journal Letters.
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