For nearly all large-scale objects, dark matter is a vital part of the story.
We see:
- galaxies inside galaxy clusters rapidly zipping around,
- fast-orbiting stars and gas around spiral galaxies, even toward their outskirts,
- large-magnitude gravitational lensing around galaxies and galaxy clusters,
- mass separated from X-ray emitting gas during cluster collisions,
- plus dark matter’s signature in the cosmic web,
- and cosmic microwave background.
They point toward one universal dark matter-to-normal matter ratio: 5-to-1.
However, some galaxies are exceptional, defying that ratio.
The smallest, lowest mass galaxies display enhanced dark matter ratios.
When stars form, energetic winds and radiation expel normal matter, escaping its gravitational pull.
The loss of normal matter creates elevated dark matter ratios.
However, dark matter deficient galaxies also arise.
Galaxies interact, both gravitationally and by speeding through a gas-rich medium.
These interactions rip out normal matter, where star-formation ensues.
However, dark matter stays its original course, creating normal matter-only stellar clumps.
Small galaxies can also experience tidal forces from larger ones.
Matter gets torn apart from the outside-in, primarily removing dark matter first.
This temporarily creates dark matter-deficient, or even dark matter-free, galaxies.
Ironically, finding examples of galaxies without dark matter helps prove dark matter’s existence.
Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words.