Dark matter distribution in galaxy formation simulations

All of the light emitted from galaxies accounts for only ~15% of its total mass. The rest of the mass belongs to the dark matter halo in which baryonic matter (gas + stars) lies. Dark matter earns its name because it does not interact via the electromagnetic force meaning we cannot "see" it. We can, however, infer its presence by studying the motions of galaxies and stars in the potential wells of dark matter halos.

Although baryons take up a small fraction of a galaxy's mass, their interactions are complicated and computationally expensive. For this reason, astronomers often use dark-matter-only simulations of galaxies and cosmological volumes to study galaxy formation and cosmological structure.  I spent the summer of 2013 working with Andrea Maccio in Heidelberg, Germany comparing the different dark matter properties in simulations with baryons to those without in the MaGICC galaxy simulation suite. Later, we repeated our analysis on the Numerical Investigation of a Hundred Astrophysical Objects (NIHAO) simulation suite (see Figure 1 for a sample of the simulated galaxies). 

One of the advantages of the NIHAO project is that it provides a varied sample of simulated galaxies, each with a dark-matter-only counterpart.  Therefore, we were able to take a statistical approach to understanding the impact of baryons (referred to as "hydro" simulations) on properties of the dark matter halo. Some of the main results of this research are summarized below.  For more detail, check out this work on the arXiv!

 
  Figure 1:  Face-on (upper panels) and edge-on (lower panels) views of four selected “test” galaxies from the NIHAO simulation suite arranged from left to right by increasing mass. Galaxies have been processed through the Monte Carlo radiative transfer code sunrise. Images are 50 kpc on a side..

Figure 1: Face-on (upper panels) and edge-on (lower panels) views of four selected “test” galaxies from the NIHAO simulation suite arranged from left to right by increasing mass. Galaxies have been processed through the Monte Carlo radiative transfer code sunrise. Images are 50 kpc on a side..


Dark Matter Halo Shape

Dark matter halos aren't perfectly spherical. Some halos have cigar-like tendencies, while others strive for the shape of a pancake.  Sort of.  Observations indicate that dark matter halos are quite close to being spherical, with an average ratio of their minor to-major-axes, c/a ~ 0.8. However, dark-matter-only simulations have historically underpredicted this value.   

Using the NIHAO simulations, we saw that including baryons resulted in rounder dark matter halos (see Figure 2).  At nearly all radii, there is a notable difference in the triaxial shape of galaxies simulated with baryons and those without. There is a strong mass dependence to the difference between the inner halo shape from the DM-only simulations and hydro simulations. At low masses (< 10^11M⊙) the dark matter halo tends to retain its original triaxial shape, while at higher masses (≈ 10^12M⊙) the inner halo becomes more spherical with an average minor to major axis ratio (c/a) of 0.8 . This brings numerical predictions into good agreement with estimates of the inner halo shape in our own Galaxy. The mass dependence of the variation of the halo shape is related to the increase of star formation efficiency with halo mass, which raises the contribution of stars and gas to the overall potential.

 
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Figure 2:  Ratio of minor-to-major axes (c/a) as a function of radius for high mass (top) and intermediate mass (bottom) galaxies. DM-only simulations are depicted in black, while hydro (NIHAO) simulations are depicted in red. The shaded region represents the 1σ scatter from galaxy to galaxy in the respective mass bins. 


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Figure 3: The dark matter particles velocity distribution for a collection of intermediate mass galaxies. The top panels show the global velocity distribution, while the bottom panels show local measurements taken at the solar position ( 7 kpc < r < 9 kpc).  DM-only simulations are depicted in black, while hydro (NIHAO) simulations are depicted in red.

Dark Matter velocity profiles

The velocity distribution of the dark matter particles within the virial radius in the hydro simulations is still well represented by a Maxwellian distribution, and it is similar to the DM-only case at all mass scales (see Figure 3). When we restrict our analysis to the the solar neighborhood (7 kpc < r < 9 kpc) we find that in the hydro simulations the velocity distribution functional form strongly depends on the halo mass.  For high mass galaxies (M ∼ 10^11M⊙) the velocity distribution becomes progressively more symmetric and the velocity peak moves towards lower values w.r.t the dark-matter-only case. We tentatively ascribe this trend to the different reaction of the DM distribution as function of increasing halo mass from few 10^9 M⊙ to M> 10^12 M⊙. To better study this effect, we isolated five galaxies that for stellar and total mass resemble our own Milky Way. For these galaxies the maximum of the velocity distribution in the hydro simulations moves to higher velocities w.r.t the dark-matter-only case, due to an overall halo contraction in these galaxies, and we find very little correlation with the galaxy morphology. In our Milky Way analogs the velocity distribution is well fitted by a Gaussian. We stress that the lack of high velocity particles has important consequences for the interpretation and comparison of Dark Matter direct detection experiments.