We all know what a galaxy looks
like, right? It’s a huge
collection of stars and other
matter that’s shaped like a
spiral or an ellipse, and if
you’re an astronomy fan you
probably know that most of the
mass is from invisible,
mysterious material called dark
matter. Well, NASA’s Hubble
Space Telescope just took an
image of a galaxy that is none
of those things. For the first
time, astronomers have strong
evidence for a galaxy not having
a significant amount of dark
matter. Most astronomers
currently believe that dark
matter plays a fundamental role
in our universe and the
formation of galaxies. This is
because galaxies seem to have a
lot more mass than what we can
account for based on just the
stars we see. How much mass is
in a system is determined by
measuring the speed at which
galaxies rotate or individual
stars in a galaxy move. Without
the gravity from that mass, a
galaxy would fly apart if
everything in it is moving as
quickly as we observe it moving.
Decades of research have led
astronomers to the extraordinary
conclusion that 85% of the mass
in our universe is invisible in
all wavelengths of radiation,
and that it’s composed of matter
that does not contain protons or
neutrons or any type of particle
we’ve detected before, and that
this invisible material is all
around us, passing through us
without interacting with regular
matter except by gravity. For
some, that’s a tough pill to
swallow, and a minority of
astronomers wonder if maybe we
just don’t completely understand
how gravity works. If that were
the case, and it was an inherent
property of gravity that causes
galaxies to move the way they
do, then we could expect all
galaxies to behave the same way.
In other words, they would all
seem to have about the same
portion of “dark matter.” But
with the galaxy in this Hubble
image, astronomers looked at the
velocities of ten globular
clusters in the galaxy, each a
spherical collection of hundreds
of thousands of stars, and
calculated that their movements
can be accounted for entirely by
the mass of the visible material
in this system. That means this
galaxy has little to no dark
matter. Strangely, this absence
of dark matter actually provides
evidence that dark matter is
real. It shows that dark matter
isn’t always coupled with
regular matter – that it’s
something separate. You can have
regular matter without dark
matter. This galaxy is really
weird even beyond the dark
matter thing. You may have
noticed you can see straight
through it. That’s because this
galaxy is what’s called an
“ultra diffuse galaxy,” which as
the name implies, is extremely
low density. This galaxy is
about the same volume as our own
Milky Way galaxy, but only has
about 0.5% the amount of stars.
Though astronomers have known
about ultra diffuse galaxies
since the early 1980s, they can
be difficult to find since
they’re so faint. A team of
astronomers is using an array of
telephoto lenses called
Dragonfly to seek out these
ghostly-looking objects. They
obtained observations from
Dragonfly, the Sloan Digital Sky
Survey, the Gemini Observatory,
and the Keck Observatory, then
requested time on the Hubble
Space Telescope to take a closer
look at this unusual galaxy.
Having images and data from
multiple sources allowed the
team to determine that this
galaxy does not have a
significant amount of dark
matter. This was definitely
surprising to find. No other
galaxies so far have appeared to
be so lacking in dark matter. In
fact, other ultra-diffuse
galaxies seem to have an
overabundance of dark matter.
The same team who studied this
galaxy discovered a different
ultra diffuse galaxy in 2016
that they calculated was 99.9%
dark matter. Yet another weird
thing about this galaxy – the
globular clusters used to
measure the galaxy’s rotation
are way brighter than normal
globular clusters. The
researchers have written a
different paper that focuses on
just these oddball collections
of stars. So, this is a very
strange galaxy in several ways.
Astronomers will be looking at
Hubble observations of other
ultra diffuse galaxies to see if
there are any other examples of
galaxies with unusually low or
high amounts of dark matter.
With more samples, astronomers
will be able to better
understand the nature of dark
matter, the formation and
evolution of galaxies, and the
overall structure of our
universe.
www.nasa.gov/hubble
@NASAHubble
