## Sunday, December 6, 2015

### 37 | The Illustris Simulation

In this post, we will explore the Illustris Simulation, a numerical cosmological simulation developed by a collaboration of scientists working to better understand the formation of large-scale structure in the universe. The online tools associated with Illustris allow us to explore the data generated from this simulation.

We will begin by looking closely at an over-dense region. By choosing such a region on the simulation explorer, the program will automatically select nearby subhalos, which are essentially regions with very high relative density contrast.

 a selection within the Illustris Explorer

The simulation explorer allows us to learn more about the nature of these halos. Let's start by generating a histogram of the masses of these halos. To make the data easier to visualize, we will consider the logarithm of the mass:

From this plot, it is clear that the mass distribution seems to be shifted slightly more towards the low-mass halos, meaning that low-mass halos are more numerous. From the data provided, we can also learn about the total stellar mass within each halo. From the data set we collected, we find that about 93% of the halo mass comes from the stars contained within it.

At a large scale, the density of dark matter and gas seem to be quite similar, and we can see similar structural features in spatial plots of each component. The following plots show the density of dark matter and gas, respectively.

Note how both appear to peak in similar regions, and exhibit "filamentary" structure elsewhere. However, it's interesting to note that the dark matter appears to be more closely defined to the filaments, whereas the gas is a bit more diffuse.

On a small scale, however, the gas density appears to be more defined than the dark matter density. Again these plots show dark matter and gas density, respectively--this time at a much smaller scale:

In some of the larger galaxies, we can see that the gas density is actually greatest in the disk of the galaxy and note in the nucleus. This is clearly visible in the image below where white represents the highest gas density. Note how the white appears in a ring around the center of galaxy, but the center of the galaxy is actually less dense.

The explorer also allows us to see that the largest galaxies tend to exist in clusters rather than on their own in the universe. This is evident in the screenshot below, where the most massive galaxies have been circled.

Next, we can look at an animation which shows the development of dark matter and gas temperature over the evolution of our universe. This animation seems to show that structure formation is lead by dark matter, then baryons (gas) tend to develop along these filaments of dark matter. However, the gas is initially not very warm. In fact, researchers refer to the time before the gas heats up as the "Dark Ages." Based on the animation, gas did not begin to warm up until a redshift of $$z \approx 10$$, which corresponds to about 0.5 billion years after the Big Bang.

The model also provides us with the approximate rate of star formation as the universe forms. More specifically, the model predicts this by the rate of increase in stellar mass. There are several periods throughout the evolution of the universe when star formation greatly increases. In particular, between $$z = 1.5$$ and $$z=1.2$$, the rate of star formation is quite fast.

As time progresses, the animation shows large structures collapsing and generating "outbursts" of gas. This suggests that as time passes, large objects break up and this ultimately results in the formation of new, smaller structures throughout the universe. These small structures grow until they too collapse on themselves.

Finally, it is curious why this structure forms in filaments. We can see that the dark matter occurs in filaments throughout the universe. The gravity of this dark matter pulls baryons towards it. As a result of this matter being pulled together, stars and galaxies can form. Consequently, this gravitational attraction from the dark matter generates structures along the filaments.

#### 1 comment:

1. you've gotta watch the Log(M) units. the ratio of the masses is more like 10-20%.