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dark galaxy Dragonfly 44. The image on the left is a wide view of the
galaxy taken with the Gemini North telescope using the Gemini
Multi-Object Spectrograph (GMOS). The close-up on the right is from the
same very deep image, revealing the large, 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 Digital Sky Survey.PNG image
MAUNAKEA, Hawaii — Using the world's most powerful telescopes, an international
team of astronomers has discovered a massive galaxy that consists almost
entirely of Dark Matter. Using the W. M. Keck Observatory and the Gemini North
telescope – both on Maunakea, Hawaii – the team found a galaxy whose mass is
almost entirely Dark Matter. The findings are being published in The
Astrophysical Journal Letters today.
Even though it is relatively nearby, the galaxy, named
Dragonfly 44, had been missed by astronomers for decades because it is very
dim. It was discovered just last year when the Dragonfly Telephoto Array
observed a region of the sky in the constellation Coma. Upon further scrutiny, the team realized the galaxy had to
have more than meets the eye: it has so few stars that it quickly would be
ripped apart unless something was holding it together.
To determine the amount of Dark Matter in Dragonfly 44,
astronomers used the DEIMOS instrument installed on Keck II to measure the
velocities of stars for 33.5 hours over a period of six nights so they could
determine the galaxy’s mass. The team then used the Gemini
Multi-Object Spectrograph (GMOS) on the 8-meter Gemini North telescope on
Maunakea in Hawaii to reveal a halo of spherical clusters of stars around the galaxy’s core, similar to the halo that surrounds
our Milky Way Galaxy.
“Motions of the stars tell you how much matter there is, van
Dokkum said. “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 percent is in the form of dark
matter. The Milky Way has more than a hundred times more stars than Dragonfly
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
“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. The only such galaxies we had to study before were tiny. This
finding opens up a whole new class of massive objects that we can study.
“Ultimately what we really want to learn is what Dark Matter
is,” van Dokkum said. “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.”
Additional co-authors are Shany
Danieli, Allison Merritt, and Lamiya Mowla of Yale, Jean Brodie of the
University of California Observatories, Charlie Conroy of Harvard, Aaron
Romanowsky of San Jose State University, and Jielai Zhang of the
University of Toronto.
W. M. Keck Observatory operates the largest, most scientifically productive
telescopes on Earth. The two, 10-meter optical/infrared telescopes near the
summit of Maunakea on the Island of Hawaii feature a suite of advanced
instruments including imagers, multi-object spectrographs, high-resolution
spectrographs, integral-field spectrographs and world-leading laser guide star
adaptive optics systems.
DEIMOS (DEep Imaging Multi-Object Spetrograph) boasts the largest field of view (16.7
arcmin by 5 arcmin) of any of the Keck Observatory
instruments, and the largest number of pixels (64 Mpix). It is used primarily
in its multi-object mode, obtaining simultaneous spectra of up to 130 galaxies
or stars. Astronomers study fields of distant galaxies with DEIMOS, efficiently
probing the most distant corners of the universe with high sensitivity.
Observatory is a private 501(c) 3 non-profit organization and a scientific
partnership of the California Institute of Technology, the University of
California and NASA.