Hello, and welcome to Gemini Observatory!
I’m your host, Hannah Blomgren.
Throughout the course of this series, you
will embark journey through the universe,
as we discover some of the most fascinating
research recently conducted at the observatory.
During this installment, we will be discussing
a galaxy without dark matter, and its implications
on galaxy formation.
The Gemini Observatory is an international
consortium of five partner countries and two
identical 8- meter telescopes.
The Frederick C. Gillett Gemini Telescope
is located on Mauna Kea, Hawaii (Gemini North)
and the other is located Cerro Pachón in
central Chile (Gemini South).
Together the twin telescopes provide full
coverage over both hemispheres of the sky.
The telescopes incorporate technologies that
allow large, relatively thin mirrors, under
active control, to collect and focus both
visible and infrared radiation from space.
The cutting edge technologies employed at
Gemini, such as an advanced adaptive optics
system, make Gemini one of the most adept
ground-based telescopes in the world, capable
of redefining the boundaries of astronomical
research.
Gemini North played an instrumental role in
recent research led by Principal Investigator
Pieter van Dokkum of Yale University.
The team was imaging a galaxy called NGC 1052-DF2
(DF2 for short) and realized that it had little
to no dark matter.
In fact, it has 400 times less dark matter
than expected for an object of its size.
Generally, most of the mass present in galaxies
is from dark matter.
It is currently believed that as much as 85%
of the universe is comprised of dark matter,
leaving only 15% as visible material.
This is the first galaxy to ever be observed
without dark matter, which has massive implications
for our understanding of galaxy formation.
Dark matter gets its name because it is invisible
and does not interact with normal particles.
However, it is still detectable because it
interacts with us through gravity.
Researchers can determine how much dark matter
is in a system by looking at the mass of the
galaxy and the gravitational interactions
there.
The mass in a system is determined by the
speed at which the galaxy rotates.
The faster the galaxy rotates, the more mass
there is in the system.
All normal galaxies rotate faster than can
be accounted for by the visible matter that’s
present, which means that there is some undetectable
mass that is influencing galaxy rotation,
which we call dark matter.
Galaxies without dark matter would move much
more slowly.
If these galaxies moved as fast as they are
observed, but without dark matter, they would
actually fly apart without the gravity of
dark matter holding them together.
Researchers looked at the velocities of ten
globular clusters in the galaxy, to determine
whether there was a discrepancy between the
speed and mass of the galaxy.
But their movements can be accounted for entirely
by the mass of the visible material, meaning
there’s no dark matter!
The lack of dark matter in DF2 is actually
a case for its existence.
If dark matter didn’t exist, and it was
just an inherent property of gravity that
caused galaxies to behave the way they do,
then we would expect all galaxies to behave
the same way.
They would all appear to have the same portion
of dark matter.
Because this galaxy has no dark matter, it
is apparent that dark matter is not always
coupled with regular matter, making it a real
and separate entity.
DF2 is an ultra-diffuse galaxy, which means
it is extremely low density.
DF2 has the same volume as our Milky Way Galaxy,
but has only 0.5% the amount of stellar material.
This type of galaxy is actually very common,
however DF2 is unusual in this group for a
number of reasons.
Shany Danieli, a graduate student from Yale
University who is part of the DF2 team, will
explain why.
Dragonfly 2 really caught our eyes.
It was very faint.
It had a component of very smooth, faint light.
But it was all peppered with many compact
objects, which are basically compact and bright
star clusters, that are being called globular
clusters, which is something we never saw
before.
So we were really curious about what is this thing, and how was it formed, and what kind
of dark matter component it has.
The globular clusters that were measured are
much brighter than normal.
They emit almost as much light as those in
the Milky Way.
No other galaxy so far has appeared to be
so lacking in dark matter, in fact, other
ultra-diffuse galaxies seem to have an over-abundance
of dark matter.
For instance, another ultra-diffuse galaxy
was found to be 99% dark matter.
Researchers used the Gemini Multi Object Spectrograph
(GMOS) to take images to assess its structure,
and confirm that the galaxy had no signs of
interactions with other galaxies, which helps
answer questions about the conditions surrounding
its birth.
So we used to think that galaxies are formed
inside halos, so these big blobs of dark matter,
and that dark matter is the most dominant
aspect of any galaxy that we know.
Basically, you know, we do get some upper
estimate for the galactic mass, which is at
least of couple of hundred times lower than
we expected.
So, it doesn't quite speak on this relation
between the stellar mass component of the
galaxy and the halo mass component.
So this should be explained somehow...
NGC1052-DF2 challenges the standard ideas
of how we think galaxies form.
DF2 was first spotted with the Dragonfly Telephoto
Array in Mexico, which was designed to detect
ultra-diffuse galaxies.
The team then used Gemini, Keck, and the Sloan
Digital Sky Survey (SDSS) to further probe
this faint galaxy, before turning to Hubble
for its final observations.
DF2 is about 65 million light years away,
in a collection of galaxies dominated by the
giant elliptical galaxy NGC 1052, and the
growth of this galaxy may have resulted in
DF2’s dark matter deficiency.
Another idea is that some cataclysmic event
swept out all the gas and dark matter.
This is, however, conjecture.
So we are working currently on a big survey,
called the Dragonfly Wide Field Survey, which
is going to cover a very wide area.
We hope to find many of these ultra-diffuse
galaxies and maybe, perhaps, some new ultra-diffuse
galaxies that lack dark matter, to have a
better statistical sample of these objects.
Every galaxy we knew about before has dark
matter, and they all fall in familiar categories
like spiral or elliptical galaxies.
But what would you get if there was no dark
matter at all?
Maybe this is what you would get.
Stay tuned for our next installment!
Thanks for watching and continue to be curious
about the world around us.
