Astronomers Discover ‘Ghost Galaxy’ Made Of 99.99% Dark Matter

arge, elongated galaxy, and halo of spherical clusters of stars around the galaxy’s core, similar to the halo that surrounds our Milky Way Galaxy. Dragonfly 44 is very faint for its mass, and consists almost entirely of Dark Matter. Credit: Pieter Van Dokkum, Roberto Abraham, Gemini, Sloan Digitial Sky Survey
Dragonfly 44 in Coma Berenices is very faint for its mass, and consists almost entirely of dark matter. A halo of  star clusters (white dots) surrounds around the galaxy’s core, similar to the halo that surrounds our own Milky Way galaxy. The 10 kpc scale bar at upper left stands for 10 kiloparsecs, equal to a span of 32,600 light years. Credit: Pieter van Dokkum, Roberto Abraham, Gemini, Sloan Digital Sky Survey

Dark matter is infuriating. It’s everywhere, outnumbering regular matter by 5 to 1. Yet it doesn’t reveal itself to our eyes or telescopes. Dark matter gives off no light and interacts with ordinary matter in just one way — through gravity. By measuring how much it tugs on stars and galaxies, we can estimate the mass of dark matter in a galaxy or cluster of galaxies, but that’s as far as it goes. If we look across the known universe, most ordinary matter is made up almost entirely of hydrogen and helium with a tiny fraction of all the other elements. Dark matter? No clue.


Nice explanation of how dark matter affects a star’s speed around the center of a galaxy

A leading theory says it could be exotic “weakly interacting massive particles” or WIMPs, another that it’s neutrinos. It might even be Macros, superdense chunks the size of apples to asteroids composed of chilled quarks, the wee bits that make up the familiar protons and neutrons inside atoms.

The Milky Way is embedded in a huge halo of invisible dark matter that extends well beyond the visible spiral disk. To account for the extra kick it gives stars in their revolution about the galactic center, astronomers estimate there may be up to one trillion solar masses of the stuff! When you come across those spectacular images of galactic pinwheels taken with the Hubble Space Telescope, picture them surrounded by invisible dark moats four times as big.

All galaxies are surrounded by dark matter halos including the Milky Way. Only a fraction of the galaxy's matter resides in the bright disk and halo, which contains stars, globular clusters and gas clouds. Most of it is invisible and only detected through its gravitational attraction. Credit: ESO (galaxy impression) and author
All galaxies are surrounded by dark matter halos including the Milky Way. Only a fraction of the galaxy’s matter resides in the bright disk and halo, which contains stars, globular clusters and gas clouds. Most of it is invisible and only detected through its gravitational attraction. Credit: ESO (galaxy impression) with additions by the author

As if ordinary matter’s puny contribution to the universe’s energy account wasn’t pitiful enough, an international team of astronomers has discovered a massive galaxy named Dragonfly 44 that consists almost entirely of dark matter. 99.99% to be exact. Even though the galaxy is relatively nearby at 300 million light years from Earth, it’s been missed by astronomers for decades because it’s so incredibly faint.

Dragonfly 44 was discovered just last year with the Dragonfly Telephoto Array in the springtime constellation Coma Berenices. When astronomers took a closer look, they realized it had too few stars for the glue of gravity to keep it from flying apart.  Unless something was holding it together, it would quickly rip itself to pieces. Without enough visible matter to do the job, dark matter had to be at play.

To determine the amount of dark matter in Dragonfly 44, astronomers used the giant Keck II telescope in Hawaii with its 10-meter (393.7-inch) mirror to measure the velocities of stars around it core for 33.5 hours over a period of six nights so they could determine the galaxy’s mass.

How the universe divvies up its energy/matter. Credit: NASA
How the universe divvies up its energy and matter. Credit: NASA

“Motions of the stars tell you how much matter there is,” said Pieter van Dokkum, who headed up the international team studying the galaxy. “They don’t care what form the matter is, they just tell you that it’s there. In the Dragonfly galaxy stars move very fast. So there was a huge discrepancy: using Keck Observatory, we found many times more mass indicated by the motions of the stars, then there is mass in the stars themselves.”

The mass of the galaxy is estimated to be a trillion times the mass of the Sun – very similar to the mass of our own Milky Way galaxy. However, only one hundredth of one percent of that is in the form of stars and “normal” matter; the other 99.99% is in the form of dark matter. The Milky Way has more than a hundred times more stars than Dragonfly 44.

Finding a galaxy with the mass of the Milky Way that is almost entirely dark was unexpected. “We have no idea how galaxies like Dragonfly 44 could have formed,” Roberto Abraham, a co-author of the study, said. “The Gemini data show that a relatively large fraction of the stars is in the form of very compact clusters, and that is probably an important clue. But at the moment we’re just guessing.”

“This has big implications for the study of dark matter,” van Dokkum said. “It helps to have objects that are almost entirely made of dark matter so we don’t get confused by stars and all the other things that galaxies have.”

Before the discovery of Dragonfly 44, astronomers could only study dark matter up close in tiny galaxies which tend to be richer in the stuff than larger ones. Dwarf galaxies are stripped of stars and gas (normal matter) through interactions with other larger galaxies or when material is ejected into space in supernovae, leaving behind greater concentrations of the dark stuff. Finding a massive dark galaxy closer to home give astronomers hope that there are more out there.

“The race is on to find massive dark galaxies that are even closer to us than Dragonfly 44, so we can look for feeble signals that may reveal a dark matter particle,” said van Dokkum.