Constraining the noisefree distribution of halo spin parameters
Abstract
Any measurement made using an Nbody simulation is subject to noise due to the finite number of particles used to sample the dark matter distribution function, and the lack of structure below the simulation resolution. This noise can be particularly significant when attempting to measure intrinsically small quantities, such as halo spin. In this work, we develop a model to describe the effects of particle noise on halo spin parameters. This model is calibrated using Nbody simulations in which the particle noise can be treated as a Poisson process on the underlying dark matter distribution function, and we demonstrate that this calibrated model reproduces measurements of halo spin parameter error distributions previously measured in Nbody convergence studies. Utilizing this model, along with previous measurements of the distribution of halo spin parameters in Nbody simulations, we place constraints on the noisefree distribution of halo spins. We find that the noisefree median spin is 3 per cent lower than that measured directly from the Nbody simulation, corresponding to a shift of approximately 40 times the statistical uncertainty in this measurement arising purely from halo counting statistics. We also show that measurement of the spin of an individual halo to 10 per cent precision requires at least 4 × 10^{4} particles in the halo  for haloes containing 200 particles, the fractional error on spins measured for individual haloes is of order unity. Nbody simulations should be viewed as the results of a statistical experiment applied to a model of dark matter structure formation. When viewed in this way, it is clear that determination of any quantity from such a simulation should be made through forward modelling of the effects of particle noise.
 Publication:

Monthly Notices of the Royal Astronomical Society
 Pub Date:
 November 2017
 DOI:
 10.1093/mnras/stx1804
 arXiv:
 arXiv:1705.01915
 Bibcode:
 2017MNRAS.471.2871B
 Keywords:

 methods: numerical;
 galaxies: haloes;
 dark matter;
 Astrophysics  Astrophysics of Galaxies
 EPrint:
 12 pages, 7 figures, submitted to MNRAS, comments welcome