Blog Post 37, ILLUSTRIS Simulation

The Illustis Simulation is a computer simulation of an entire universe using numerical calculations.  This project simulates most if not all relevant physics to high accuracy and results in a statistically accurate model of the universe in which one can study the distributions of gas, heat, dark matter, strs, etc.

First we will examine a random overdense region for halo data.  We will then plot this data in a histogram and interpret it.

This plot shows \(log(M)\) of the galactic halos on the x axis, and how many of these occurred in our sample location.  The histogram shows a clear trend in halo size, in that low mass halos are much more common than high mass halos.  Remember, we are using a log scale, so a halo in the 14-14.5 bin is 10,000 times more massive than one in the 10-10.5 bin.
Quickly analyzing the full dataset for our selected halos, we find that on average, 15% of the halo mass is stellar mass.  That implies that the rest is primarily dark matter.  Cool!

Using the simulation, we can make observations about the structure of a universe like our own.

Gas and Dark Matter Densities:

(Gas density (left) and Dark Matter density (right))

On a large scale, Gas and Dark Matter densities seem to correlate with one another, with the gas density being more spread out than the dark matter.  On a small scale (see below) this same trend is visible, but with the gas being very poorly defined compared to the stark definition of the dark matter filamentary structure.  This is likely due to baryonic interaction of the gas matter with itself, causing a counter-force to simply gravitation.  This greater degree of randomness could serve as a viable explanation for the greater dispersion of normal matter.

(Gas Density and Dark Matter density of a single cluster)

In a single galaxy (see below), the Gas is highly concentrated at the nucleus, while the dark matter is more dispersed in the halo.

(Gas density and dark matter density above the stellar material of the galaxy)

The most massive galaxies tend to be found in clusters, not in the background field.

Gas Temperature Evolution:
The following observations are based upon a video derived from the simulation.  This video can be found at: http://www.illustris-project.org/movies/illustris_movie_cube_sub_frame.mp4

Stars first begin to form win the early universe along the dark matter filaments as shown by the increase in gas temperature.  This starts slowly, then accelerates, reaching maximum star formation rate around redshift 1.5 to 1.0, where massive amounts of stars form.  The first stars begin to form at about 1 billion years after the Big Bang (Redshift 5.75), although they do not widely populate the universe until about 2.5 billion years after the Big Bang (Redshift: 2.75).  This marks the end fate "Dark Ages."  In the simulation, structure formation usually occurs through parts of very large structures collapsing due to gravity, breaking the very large structures into smaller more compact ones.  However, over time, these smaller structures tend to consume their neighbors, forming larger high density structures throughout the universe.  These structures form along filaments because gravity from dark matter is strongest there.  This gravity will pull both dark matter and gas together to form structures.

References:
http://www.illustris-project.org/explorer/
http://www.illustris-project.org/movies/illustris_movie_cube_sub_frame.mp4