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For context, this project just published a paper in Nature on this simulation [0] and a preprint is available [1]. One of the reasons this simulation is big news is its use of a new numerical technique (a "moving mesh"[2]) for the hydrodynamics which purportedly handles fluid instabilities with much greater accuracy. One effect of this is that cold gas flowing onto galaxies interacts with the hot gas halo around galaxies, which prevents that cold gas from going directly to the centers of galaxies. This results in larger simulated galaxies, which match better with observations.

[0] http://www.nature.com/nature/journal/v509/n7499/full/nature1...

[1] http://arxiv.org/abs/1405.1418

[2] Previous work used either a fixed mesh with adaptive refinement for higher-resolution or a "smoothed particle hydrodynamics" scheme where particles are used to simulate fluid flows. Both schemes have advantages and disadvantages, but the claim is that this moving mesh code ("Arepo") does the best job of treating fluid instabilities. The paper describing the new code is at: http://dx.doi.org/10.1111/j.1365-2966.2009.15715.x

1 comments

The Nature paper mentions that past simulations were unable to create the current population of disk galaxies (e.g. Andromeda, Milky Way).

They say "The culprit was an angular momentum deficit leading to too high central concentrations, overly massive bulges and unrealistic rotation curves." [0]

Any idea why these older sims wouldn't create the correct angular momentum in late-type galaxies?

[0] http://www.nature.com/nature/journal/v509/n7499/full/nature1...

> Any idea why these older sims wouldn't create the correct angular momentum in late-type galaxies?

My understanding from seeing talks this group has given is the culprit was the same effect I mentioned. If the fluid instabilities aren't properly handled, gas flowing onto galaxies stays cold and flows into the centers of galaxies [0], which results in less angular momentum because much of the material is at small radii. Correctly computing the fluid behavior results in gas not plunging directly to the centers of galaxies and instead building up at somewhat larger radii. For a given rotation speed, a larger galaxy will have higher angular momentum, so that's the sense in which their simulation improves on things.

[0] - This behavior has been called "cold flows" or "cold mode accretion" and was seen in simulations, but hasn't had direct observational support.

Interesting. Thanks very much for the explanation.