Why am I playing Summly?
> there are more things than stars in galaxies were these included in the simulation
From the paper:
The galaxies in our study consist of dark matter halos and rotationally supported disks of stars. The parameters describing each component are independent and the models are constructed in a manner similar to the approach described in previous works (Hernquist 1993; Springel 2000; Springel et al. 2005).
2.2. Dark Halo
We model the dark matter mass distribution with a Hernquist (1990) profile:
rho_dm = M_dm/ (2*Pi) * a / (r * (r + a)³)
which has a cumulative mass distribution M (< r) = M_dm * r² / (r + a)² , where a is the radial scale length and M_dm is the total halo mass here set to 9.5 * 10¹¹ M_solar.
In the past, models of disk galaxies run in isolation and used to study the properties of spiral arms employed only a few million particles to sample both the stellar disk and the dark matter halo. In such experiments, randomly-placed particles produce fluctuations in the halo potential. Even if the disk is initially featureless, the Poisson noise owing to such discretization of the mass in N-body experiments is inevitably swing amplified, producing trailing multi-armed spiral patterns in the disk (Toomre 1977; Fujii et al. 2011; Sellwood 2012).
In order to suppress the development of artificial features in all the N-body experiments that follow, we set up a live disk of stars embedded in a rigid dark matter potential. We employ simulations with a sufficiently large number of particles in the disk, i.e. 100 million, so that the disks are essentially featureless when evolved without any perturbers acting on them. These simulations serve as “controls,” making it possible to identify the response of the disk to imposed perturbations. In this manner, we will be able to separate the sources responsible for exciting features in the disk from the stars which react to the perturbations, unlike previous experiments in which the stars themselves acted as perturbers, complicating the interpretation of the experiments, as emphasized by Toomre (1990).
Looking forward to gas + full stellar evolution simulations for added beauty.