

The Illustris project: A large cosmological simulation of galaxy formation - carljoseph
http://www.illustris-project.org/about/

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privong
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...](http://www.nature.com/nature/journal/v509/n7499/full/nature13316.html)

[1] [http://arxiv.org/abs/1405.1418](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](http://dx.doi.org/10.1111/j.1365-2966.2009.15715.x)

~~~
carljoseph
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...](http://www.nature.com/nature/journal/v509/n7499/full/nature13316.html)

~~~
privong
> 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.

~~~
carljoseph
Interesting. Thanks very much for the explanation.

------
typpo
I'm fascinated by the distinct types of structures that gravity and other
forces create at such large scales. The really zoomed-out views of the
universe remind me of neurons in a microscopic view of the brain.

When I was researching this stuff last year, I tried to use the Millenium
Simulation (the world's largest n-body simulation at the time, 2 billion cubic
light-years) to visualize this effect using webgl, but wasn't quite satisfied
with the result [1]. Thanks to Illustris I've found out that one of the
researchers on the project has created several excellent visualizations that
seem to use some of the techniques from the larger simulation. They are worth
taking a look if you're interested in visualization/simulation or how galaxies
form [2].

[1] [http://www.asterank.com/galaxies/](http://www.asterank.com/galaxies/) [2]
[https://www.cfa.harvard.edu/~dnelson/#tab-5](https://www.cfa.harvard.edu/~dnelson/#tab-5)

