I was reading the Aquarius simulation preprint (Springel et al. 2008) The Aquarius Project: the subhalos of galactic halos as a reference for my internship. I came across the term Gravitational softening length in the Initial conditions section and couldn't get a clear explanation when I searched online.
Can someone explain what it means in this context?
2.2 The Aquarius simulation suite
In Table 1, we provide an overview of the basic numerical parameters of our simulations. This includes a symbolic simulation name, the particle mass in the high-resolution region, the gravitational softening length, the total particle numbers in the high- and low-resolution regions, as well as various characteristic masses and radii for the final halos, and the corresponding particle numbers. Our naming convention is such that we use the tags “Aq-A” to “Aq-F” to refer to simulations of the six Aquarius halos. An additional suffix “1” to “5” denotes the resolution level. “Aq-A-1” is our highest resolution calculation with ∼ 1.5 billion halo particles. We have level 2 simulations of all 6 halos, corresponding to 160 to 224 million particles per halo.
We kept the gravitational softening length fixed in comoving coordinates throughout the evolution of all our halos. The dynamics is then governed by a Hamiltonian and the phase-space density of the discretized particle system should be strictly conserved as a function of time (Springel, 2005), modulo the noise introduced by finite force and time integration errors. Timestepping was carried out with a kick-driftkick leap-frog integrator where the timesteps were based on the local gravitational acceleration, together with a conservatively chosen maximum allowed timestep for all particles