Shape of Dark Matter Halos & Visible Matter Angular Momentum

[Ranku]

What is the shape of a typical dark matter halo around a galaxy? Does it orbit a galaxy like visible matter? It is said that dark matter does not lose angular momentum, necessary for compactification. How then does visible matter lose angular momentum?

 

[fresh_42]

Here is a recent article about this question:

popular version:
https://scitechdaily.com/astrophysi...f-how-dark-matter-is-distributed-in-galaxies/

scientific version:
https://www.aanda.org/articles/aa/abs/2020/10/aa39190-20/aa39190-20.html

The principle of maximum entropy explains the cores observed in the mass distribution of dwarf galaxies
Jorge Sánchez Almeida, Ignacio Trujillo, and Angel Ricardo Plastino
https://doi.org/10.1051/0004-6361/202039190

abstract:

Cold dark matter (CDM) simulations predict a central cusp in the mass distribution of galaxies. This prediction is in stark contrast with observations of dwarf galaxies that show a central plateau or “core” in their density distribution. The proposed solutions to this core-cusp problem can be classified into two types. One invokes feedback mechanisms produced by the baryonic component of the galaxies and the other assumes that the properties of the dark matter particle depart from the CDM hypothesis. Here we propose an alternative yet complementary explanation. We argue that cores are unavoidable in the self-gravitating systems of maximum entropy that result from non-extensive statistical mechanics. Their structure follows from the Tsallis entropy, which is attributed to systems with long-range interactions. Strikingly, the mass density profiles predicted by such thermodynamic equilibrium match the observed cores without any adjustment or tuning. Thus, the principle of maximum Tsallis entropy explains the presence of cores in dwarf galaxies.

 

[anorlunda]

Think of an ice skater, how they spin faster when they pull in their arms. A halo can't collapse into a disk or a ring, or a star or a black hole without getting rid of some energy. Normal matter gets rid of energy via radiation. Since dark matter is dark, it doesn't radiate. So while the normal matter in many galaxies collapsed into a disk shape, the dark matter can't.

The following article (one of my favorites) talks about the relationships between mass, momentum and energy, and why we see those different shapes in the sky.

The Potato Radius: a Lower Minimum Size for Dwarf Planets

 

[DrStupid]

How then does visible matter lose angular momentum?

On large scales it doesn't. In case of a protostellar disc angular momentum is just transferred from the inner to the outer region. In the result the center collapses to a star and the outer region expands.