Using some of the world's largest, and smallest
telescopes, an international team of astronomers
have discovered a massive galaxy that consists
almost entirely of Dark Matter.
Even though it is relatively nearby at 300
million light years from Earth, the galaxy,
named Dragonfly 44, had been missed by astronomers
for decades because it is very dim.
The discovery of this dim object heralds in
a new class of galaxies that have become known
as ‘dark galaxies’ or ‘ultra diffuse
galaxies’ because they contain relatively
few stars, yet have a mass equal to our own
Milky Way with hundreds of billions of stars.
It was only discovered recently, in 2015,
when the Dragonfly Telephoto Array, then a
small collection of 24 commercially available
400mm telephoto lenses, looked towards a region
of sky in the constellation Coma.
But as astronomers looked closer using larger
telescopes with specialized instruments, they
realized this dim, tiny galaxy was more than
meets the eye - literally.
It contained so few stars.
There were so few stars in this galaxy that
there was no way they could all stay together
as a gravitationally bound object - they would
simply fly apart.
So what was keeping the stars in Dragonfly
44 together?
Astronomers had pretty good idea: dark matter.
Observations using the DEIMOS instrument mounted
on the gigantic Keck Telescope on Mauna Kea
in Hawai’i allowed astronomers to determine
the amount of dark matter in the Dragonfly
44 galaxy.
This was accomplished by staring at it for
thirty three and a half hours over a period
of six nights and measuring the motions and
velocities of the few stars that could be
seen in Dragonfly 44.
Additionally, the Gemini Multi-object Spectograph
on the very large 8-meter Gemini North Telescope,
revealed a halo of spherical clusters of stars
around the galaxy’s core, similar to the
halo that surrounds our Milky Way Galaxy.
Armed with this information, the motions of
the stars told the story of how much matter
is in the galaxy.
The moving stars don’t tell you what kind
of matter is there, only that it is there.
The discrepancy between the visible, normal
matter and dark matter was huge.
Both the halo of star clusters seen by Gemini
North and the motions of the stars measured
by Keck all added up to only a tiny fraction
of the mass that must be there inferred by
the looking at the velocities of the stars.
This discrepancy between the amount of visible
and dark matter was the largest anyone had
ever seen.
As its name implies, dark matter is rather
mysterious.
No one knows exactly what it is, we can’t
see it or detect it in any meaningful way.
Because of this, the amount of dark matter
contained in a galaxy is inferred by measuring
things we can see, like the motions of the
stars inside it.
The only way we can detect dark matter is
by looking at the effect it has on things
we can see.
In the case of Dragonfly 44, it’s the stars
moving around in the galaxy we can see that
were being affected by the gravitational pull
of the dark matter inside that we couldn’t.
In the dark Dragonfly galaxy, the stars were
see to move very fast - too fast to account
for just the pull exerted by the visible stars.
Something else - something invisible was accelerating
them and affecting their motions.
Based on the observed motions of its stars
made by Keck, the mass of the Dragonfly 44
Galaxy is estimated to be the same as a trillion
suns – this is very similar to the mass
of our own Milky Way.
Strangely however, when closely examined,
there are nowhere near a trillion stars in
this galaxy.
Only one hundredth of one percent of the material
in this galaxy is in the form of stars, gas
and "normal" matter; the other 99.99 percent
of the mass of this galaxy can’t be seen.
It isn’t there.
Astronomers infer that the rest must be dark
matter.
The Milky Way has more than a hundred times
more stars than Dragonfly 44.
And yet they have the same mass.
Finding a galaxy with the mass of the Milky
Way that is almost entirely dark was unexpected.
Astronomers have no idea how dark galaxies
like Dragonfly 44 could have formed,
The observations from Gemini North also showed
that a relatively large fraction of the stars
that can been seen is in the form of very
compact clusters, and that is probably an
important clue.
Another clue is that the stars in this galaxy
while few, are also very, very old.
It’s possible all star formation ended long
ago and the remaining material was dispersed
by the prevalent dark matter.
But at the moment astronomers are only guessing
- Dragonfly 44 is a galactic enigma and astronomers
would love to find more dark matter galaxies
like it.
This discovery has big implications for the
study of Dark Matter.
Finding objects like this that are made up
of almost entirely Dark Matter helps in our
understanding of what it is because, well,
these galaxies are simpler: we don’t get
confused by stars, gas, black holes, dust
and all the other things that galaxies have.
The only other galaxies like this we’ve
had to study so far were tiny.
This finding opens up a whole new class of
massive objects for further study.
Ultimately what we really want is to learn
what Dark Matter is, it accounts for 27 percent
of the mass and energy of the universe, yet
it doesn’t interact with us in any direct
way.
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.
