How Some Galaxies Defy the Dark Matter-to-Normal Matter Ratio
Exploring the exceptional cases of galaxies that have the wrong amount of dark matter
All About the Dark Matter-to-Normal Matter Ratio
In almost all large-scale cosmic structures, such as galaxies, galaxy clusters, and the cosmic web, the ratio of dark matter to normal matter is consistently 5-to-1.
However, there are a few exceptional cases where this ratio is defied. These cases include small galaxies near large ones, aftermaths of major galaxy collisions, and very low-mass galaxies.
Some galaxies appear to have lost significant amounts of normal matter, leaving only dark matter behind. On the other hand, there are galaxies that are dark matter deficient, or even completely devoid of dark matter.
The Role of Dark Matter in Cosmic Structure Formation
Dark matter plays a crucial role in the formation of cosmic structures, including galaxies. Simulations of the Universe's large-scale structure must take into account both dark matter and normal matter, along with factors such as star-formation and gas infall.
Observations of various phenomena, such as galaxies rapidly orbiting within clusters and gravitational lensing effects, provide strong evidence for the presence of dark matter.
The cosmic microwave background, as well as the cosmic web, also exhibit signatures of dark matter, further confirming its role in the Universe.
Exceptional Galaxies and the Loss of Normal Matter
In some cases, the loss of normal matter from galaxies leads to elevated dark matter ratios. When stars form, energetic winds and radiation can expel normal matter, escaping its gravitational pull.
This loss of normal matter creates galaxies with higher dark matter-to-normal matter ratios. Conversely, interactions between galaxies and the surrounding gas-rich medium can rip out normal matter, leading to dark matter-deficient galaxies.
Tidal forces from larger galaxies and the aftermath of galaxy collisions can also result in the temporary creation of dark matter-deficient or even dark matter-free galaxies.
