Why is the dark matter halo spherical?

https://www.reddit.com/r/askscience/comments/ptsno7/why_is_the_dark_matter_halo_spherical/

created by Formyedification on 23/09/2021 at 10:54 UTC

1289 upvotes, 9 top-level comments (showing 9)

Dilettante scientist here, so be gentle. As I understand it, galaxies are disc shaped due to the conservation of angular momentum, but the all the data I’ve seen about dark matter says that it is a roughly spherical distribution around the galaxy. Are there any theories as to why this is? Or is this evidence that dark matter doesn’t even interact with itself?

Comments

Comment by jarry1250 at 23/09/2021 at 12:26 UTC

216 upvotes, 2 direct replies

Accorrding to this paper: https://iopscience.iop.org/article/10.1088/0004-637X/808/1/40

"In the paradigm of cosmological structure formation gravitationally bound halos form at the peaks of the primordial density field as dark matter and baryons [i.e. regular matter] undergo nonlinear gravitational collapse. Dark matter, because it is collisionless and dissipationless, conserves its orbital energy, remaining in an extended dispersion-supported profile with overlapping inward- and outward-moving orbits. In contrast, gas collides, shocks, mixes, and eventually dissipates energy via radiative cooling, causing it to collapse to the minimum of a halo's potential well and seed the formation of stars and galaxies."

So the answer to your question is "we think yes", but...

"Within this paradigm, debate persists about the most physically meaningful ways to describe the physical extent of a halo, the rate of cosmic accretion into a halo, and the amount of mass growth within a halo, including how these compare for dark matter versus baryons. These are important questions because measurements of cosmic accretion and mass growth depend sensitively on how and where one measures them. Thus, understanding the evolution of halos requires a detailed understanding of the relevant physical scales across cosmic time, including the physical meaning (if any) of a choice for a halo's virial boundary/edge. Furthermore, because cosmic accretion into a halo feeds the growth of the galaxy inside, understanding the physics of all of these scales is necessary for developing a physical picture of galaxy evolution in a cosmological context."

i.e. we still don't fully understand.

Comment by WazWaz at 23/09/2021 at 12:52 UTC

258 upvotes, 3 direct replies

A cloud of matter that *does* collide with itself eventually settles to a disc with the same (net) angular momentum, be it planetary rings, stellar formation discs, or galaxies. That's just a simple result of probability of collisions. In the *absence* of such collisions, a spherical distribution would be the default assumption.

Comment by Tyrannosapien at 23/09/2021 at 13:49 UTC

23 upvotes, 0 direct replies

The other answers here are correct. I will just add that an important reason that baryonic matter collides is due to that matter interacting via the electromagnetic force. The matter itself may be charged, and even if neutral it will be subject to formation of chemical bonds of atoms' and molecules' electron shells. Baryonic matter will also absorb and emit photons in various spectra, all of which tends to introduce friction and impede inertia and bring that matter closer together.

Due to the absence of any evidence of direct dark matter interaction with normal matter, it's reasonable to assume it doesn't interact via the electromagnetic force, leaving only gravity as a relevant force. Common sense suggests gravity "brings things closer together" because that is an effect we see in normal matter, but in matter coming together we're really seeing gravity + electromagnetic forces interacting. Gravity alone won't bring dark matter together, but will just preserve its inertia as orbital motion. Since it will preserve that inertia without any other interaction bias, and if we assume the original dark matter velocity was randomly distributed, then you end up with a sphere-ish cloud.

The dark matter in a galaxy should also be interacting with the baryonic matter that has assumed its preferred disk shape. I don't know whether we assume that will eventually bias that galaxy's dark matter away from a perfectish sphere. Possibly the large ratio of dark matter to baryonic matter makes the effect of the baryonic matter in a galaxy too low to ^(matter).

Comment by Arkoden_Xae at 23/09/2021 at 22:19 UTC

3 upvotes, 0 direct replies

Galaxies are disk shaped due to the resistance of physical interaction that occurred during their formation. Particles going against the prevailing direction of flow or orbit would colide and either begin to orbit in the same direction or fall into the centre of mass. Anything traveling the opposite direction or on a plane that does not line up with the ecretion disk or plane of the galaxy would meet physical resistance and gradually fall in line.

Dark matter does not interact with the strong or weak nuclear force, so kinetic interactions do not influence it, it also has no electromagnetic properties, so it only loosely gathers around bodies of mass in a more globular cloud due to gravity being the only force known to interact with it. The angular momentum is all that keeps it from falling directly into the centre of mass and feeding the black hole.

Comment by Candy_Slut at 24/09/2021 at 13:44 UTC

3 upvotes, 0 direct replies

It looks like others here have answered the question as to why DM halos are spherical but this is a great opportunity to share what I learned yesterday about cases where DM halos are NOT spherically symmetric! Simulations of barred spiral galaxies show that a stellar bar can be massive enough to have a significant torque on the DM halo. The dark matter actually forms a barred structure itself. The dark matter bar ends up being an important part of why the stellar bar gets destroyed as it evolves. In other words, the DM bar could be an important piece of the puzzle of how barred spirals become regular spirals. And here's a cool fact: If the halo spin is retrograde (opposite direction as the stars and gas), the DM bar forms perpendicular to the stellar bar! I thought this was totally amazing and new so I wanted to share :)

Here are some references:

https://arxiv.org/pdf/2105.04698.pdf[1][2]

1: https://arxiv.org/pdf/2105.04698.pdf

2: https://arxiv.org/pdf/2105.04698.pdf

https://arxiv.org/pdf/1811.00033.pdf[3][4]

3: https://arxiv.org/pdf/1811.00033.pdf

4: https://arxiv.org/pdf/1811.00033.pdf

Comment by [deleted] at 23/09/2021 at 14:03 UTC

1 upvotes, 0 direct replies

[removed]

Comment by [deleted] at 23/09/2021 at 14:55 UTC

1 upvotes, 1 direct replies