On $N$-body simulations of globular cluster stream

TL;DR

This paper shows that common N-body simulation resolutions can produce artificial density variations in stellar streams, which can mimic signals from dark matter subhalos, and suggests much higher resolutions are needed for accurate modeling.

Contribution

The study demonstrates that typical N-body simulation resolutions cause spurious stream perturbations comparable to those from dark matter subhalos, highlighting the need for higher resolution to accurately interpret observations.

Findings

Simulations with standard resolutions produce artificial density fluctuations.

Higher resolutions (~100 M_sun) are required to reduce numerical artifacts.

Streams are sensitive to dark matter structures down to very low masses.

Abstract

Stellar tidal streams are sensitive tracers of the properties of the gravitational potential in which they orbit and detailed observations of their density structure can be used to place stringent constraints on fluctuations in the potential caused by, e.g., the expected populations of dark matter subhalos in the standard cold dark matter paradigm (CDM). Simulations of the evolution of stellar streams in live $N$-body halos without low-mass dark-matter subhalos, however, indicate that streams exhibit significant perturbations on small scales even in the absence of substructure. Here we demonstrate, using high-resolution $N$-body simulations combined with sophisticated semi-analytic and simple analytic models, that the mass resolutions of $10^4$--$10^5\,\rm{M}_{\odot}$ commonly used to perform such simulations cause spurious stream density variations with a similar magnitude on large scales as those expected from a CDM-like subhalo population and an order of magnitude larger on small, yet observable, scales. We estimate that mass resolutions of $\approx100\,\rm{M}_{\odot}$ ($\approx1\,\rm{M}_{\odot}$) are necessary for spurious, numerical density variations to be well below the CDM subhalo expectation on large (small) scales. That streams are sensitive to a simulation's particle mass down to such small masses indicates that streams are sensitive to dark matter clustering down to these low masses if a significant fraction of the dark matter is clustered or concentrated in this way, for example, in MACHO models with masses of $10$--$100\,\rm{M}_{\odot}$.