Hello Space Fans and welcome to another edition
of Space Fan News.
In this episode, the case for proving that
there is no such thing as dark matter just
got a lot harder.
After announcing the discovery of an ultra-diffuse
galaxy made up of 99.99 percent dark matter
called Dragonfly 44, the same team of astronomers
have now found another galaxy with almost
NO dark matter.
A while back, around 2015 or so, astronomers
had announced the discovery of a faint galaxy
that had almost no stars but contained a lot
of dark matter, in fact, almost the whole
thing was made up of the stuff.
This galaxy was so strange that astronomers
have been looking for more like it: faint,
diffuse and has roughly the same size and
mass as our galaxy.
They named the galaxy Dragonfly 44 after the
telescope that observed it and you can learn
much more about it in the video I made about
it and posted a couple of weeks ago.
Well this week astronomers looking at a galaxy
first recorded back in 2000 and has the catalogue
name NGC1052-DF2 have announced that they
think they’ve found another ultra-diffuse
galaxy that appears to have no dark matter
in it.
So to be clear: the same team of astronomers
have found an example of an ultra-diffuse
galaxy that’s almost entirely dark matter
to one that doesn’t seem to have any.
Now I know just what you’re thinking, ‘How
in the hell can astronomers possibly know
if a galaxy has 99.99%, 50%, 10% or no % of
dark matter in it?’
It all comes down to how galaxies rotate.
Galaxies with lots of dark matter have stars
that travel quickly around the galaxy’s
core while galaxies that don’t have that
much travel slower.
Think of it this way: if our Sun were to suddenly
get four times more massive, the Earth would
have to move twice as fast to stay in its
orbit.
Same thing here, if a galaxy is rotating fast,
it must have lots of dark matter in it or
else all the stars would fall in toward the
center.
Adding up all the stars we can see, and include
the gas, dust and any supermassive black hole
that might be there, still isn’t enough
to explain the rates at which these stars
are travelling.
Something else we can’t see is adding mass
to the galaxy and causing the stars to rotate
fast.
Nobody knows what dark matter is, but this
is one way astronomers can find out if it’s
there: by looking at things we can see, like
stars going around a galaxy.
Dark matter is a crappy term: we could just
as well call it transparent matter or WTF
matter or unknown matter or clear matter but
the point is, we can’t see it directly but
astronomers think it is an actual thing that
galaxies have and not just some misunderstanding
about how gravity works.
Well this discovery of a galaxy with almost
no dark matter strengthens the case that dark
matter might actually be a thing, a particle
or something tangible associated with large
scale structures in the universe and galaxies.
How?
Well given what I just said before, if a galaxy
is spinning fast, like in Dragonfly 44, it
must have a lot of dark matter and if it is
spinning slowly, like in the galaxy studied
in this research, then there must not be much
dark matter.
The stars in this galaxy, or more accurately
the globular clusters observed in this galaxy
(because they are brighter than stars) are
moving so slowly that astronomers aren’t
even sure the galaxy is rotating at all!
Remember what I said ultra-diffuse galaxies
were: very faint, very spread out and fluffy
(about the size of our Milky Way) and don’t
have many stars.
So these ultra-diffuse galaxies are really
teaching us a lot about dark matter, and as
you can imagine, astronomers are very eager
to find more of them.
If you go down the road of saying dark matter
doesn’t exist and there is something we
need to modify about gravity to explain how
galaxies rotate, well now you have a problem
because NOW you have to explain this galaxy,
which isn’t rotating at all.
The laws of physics you’d have to come up
with to explain fast rotating galaxies now
suddenly doesn’t apply to this one, so now
what?
Here is a galaxy that is rotating exactly
as you’d expect it to if there weren’t
dark matter in it.
This discovery that the ultra-diffuse galaxy
NGC1052-DF2 is barely rotating, if at all,
highly supports the idea that dark matter
is a thing that galaxies have: they either
have dark matter or they don’t.
And if it’s a thing galaxies can have, a
property of the galaxy itself, then there’s
probably a particle associated with that thing.
So now it just got a lot harder to explain
away dark matter.
Or rather clear matter, I think I like that
term a lot better.
Sounds like a very refreshing beverage.
Maybe WTF Matter….
That’s it for this episode Space Fans.
Please let me know what you think about the
new SFN format and give me your theories on
what you think Dark Matter is in the comments
below.
I want to thank all Deep Astronomy Patreon
Patrons who are keeping the lights on in SFN
HQ, thank you all for watching and as always,
Keep Looking Up!
