VIRGO: Neskutečné faux pas... a zároveň to nejzajímavější těchto let.
John Baez on Dragonfly 44 and dark matter in general: Dark mysteries
You probably heard the news this week: astronomers found a galaxy that's 98% dark matter.
It's called Dragonfly 44. It's extremely faint, so it doesn't have many stars. But we can use redshifts to see how fast those stars are moving -
over 40 kilometers per second on average. If you do some calculations, you can see this galaxy would fly apart unless there's a lot of invisible
matter providing enough gravity to hold it together. (Or unless something even weirder is happening.)
Something similar is true for most galaxies, including ours. What makes Dragonfly 44 special is that 98 percent of the matter must be invisible.
And this is just in the part where we see stars. If we count the outer edges of the galaxy, the halo, the percentage could rise to 99% or more!
By comparison, the Milky Way is roughly 90% dark matter if you count the halo. We know this pretty well, because we can see a few stars out in
there and measure how fast they're moving.
There are also galaxies like NGC 3379 that may have less than the average amount of dark matter in their halo, though this is debatable.
And most excitingly, sometimes clusters of galaxies collide and stop moving, but their dark matter keeps on going!
We can see this because light from more distant galaxies is bent, not toward the colliding clusters, but toward something else. The most famous
example is the Bullet Cluster, but there are others.
All these discoveries - and more - make dark matter seem more and more like a real thing. So it's more and more frustrating that we don't know
what it is. As I explained a while ago, recent experiments to detect particles of dark matter have failed. So it could be something else, like
black holes about 30 solar masses in size. And intriguingly, the first black hole collision seen by LIGO involved a 35-solar-mass and a 30-solar-
mass black hole. These are too big to have formed from the collapse of a single star. They might be primordial black holes, left over from
the early Universe.
But more on that later.
For more on Dragonfly 44, see:
• Pieter van Dokkum, Roberto Abraham, Jean Brodie, Charlie Conroy, Shany Danieli, Allison Merritt, Lamiya Mowla, Aaron Romanowsky and Jielai Zhang,
A high stellar velocity dispersion and ~100 globular clusters for the ultra diffuse galaxy Dragonfly 44,
http://arxiv.org/abs/1606.06291.
For our failure to find dark matter particles, see this post of mine:
The search for dark matterIn South Dakota, in a town named Lead, there was ...
https://plus.google.com/117663015413546257905/posts/3U53iqtWYXk
For more on dark matter on the outer edges of galaxies, see:
Dark matter halo - Wikipedia, the free encyclopedia
https://en.wikipedia.org/wiki/Dark_matter_halo
For the Milky Way's dark matter halo, see:
• G. Battaglia et al, The radial velocity dispersion profile of the Galactic halo:
constraining the density profile of the dark halo of the Milky Way,
http://arxiv.org/abs/astro-ph/0506102