Astronomers
have found a nearby galaxy that's roughly the same mass as the Milky Way, but
somehow contains less than 1 percent of its stars. The galaxy is so dim, it’s
evaded detention for decades, and now the team behind its discovery has figured
out how its lack of stars hasn’t ripped it apart - it's made from 99.99 percent
dark matter. Dark matter is estimated to make up around 27 percent of all the
mass and energy in the observable Universe, and while we can detect its
gravitational force, it doesn't appear to emit any form of light or radiation
that we can observe.
Now
scientists have found a galaxy that’s almost entirely made up of the stuff.
Listen to Neil DeGrasse Tyson about 'Dark Matter'.
"If the Milky Way is a sea of stars, then these newly discovered galaxies are like wisps of clouds," one of the researchers, Pieter van Dokkum from Yale University, said at the time. "We are beginning to form some ideas about how they were born, and it’s remarkable they have survived at all," he added. "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."
Now van Dokkum
and his team have had a chance to test out their hypothesis, and by figuring
out the mass of Dragonfly 44, they say they have enough evidence to suggest
that dark matter truly is the glue holding this whole thing together. The
researchers measured the velocities of stars in Dragonfly 44 for 33.5 hours
over a period of six nights, and used this information to calculate the mass of
the galaxy as a whole.
An increase in the velocity of an object will increase
its kinetic energy and therefore its mass, which means the faster these stars
are going will equate to a more massive galaxy. Having measured the speed of
Dragonfly 44’s stars as around 47 kilometers per second, the team calculated
that it’s around 1 trillion times more massive than our Sun - far too heavy to
be held together by its stars alone. "Motions of the stars tell you how
much matter there is," van Dokkum told Avery Thompson at Popular
Mechanics.
"They don't care what form the matter is, they just tell you that it's there. Using the Keck Observatory, we found many times more mass indicated by the motions of the stars than there is mass in the stars themselves."
Having estimated that the galaxy needs to be made up of 99.99
percent of dark matter to remain intact, the team has officially found the
darkest known galaxy in the Universe. A similarly dark galaxy in the Virgo
cluster was identified earlier this year, but its 99.96 percent dark matter
just got beat. As cool as this discovery is, it’s thrown up a whole lot more
questions than answers.
Right now, every potential candidate for dark matter
has failed to yield enough evidence to explain what it's made from, and until
recently, the only dark matter galaxies we’ve known about have been tiny.
Dragonfly 44 is huge, and no one can figure out how it got so big - and stayed
so big - with so little visible matter. But at least now we've now got an
entire galaxy full of dark matter to study, right?
"It’s hard to argue
with the observations, yet the conclusion from this paper runs counter to my
understanding of how galaxies are formed," one of the team, astronomer
Marla Geha from Yale University, who wasn’t involved in the research, told New
Scientist.
"I’m hoping these objects are rather rare and/or only form in special environments such as a dense galaxy cluster. Otherwise we may need to rewrite galaxy formation."
The research has been published in Astrophysical Journal Letters (via sciencealert.com).