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