Drag in SPH
In collaboration with Frazer Pearce (University of Nottingham) and Hugh Couchman(McMaster University), I have revealed the behavior of drag on resolution-limited objects in smooth particle hydrodynamic (SPH) simulations. SPH is a popular method for treating the baryonic component in cosmological simulations.
In the popular hierarchical model of structure formation, galaxies and clusters of galaxies are built by the amalgamation of smaller objects which are, in turn, built up by accreting even smaller objects. In any simulation of the universe, then, the first objects to form will be at the resolution limit. The rate that objects accrete will be determined in part by the drag on the clumps as they fall into the halos of larger objects.
My work shows a strong numerical effect at low velocities which causes the accretion of `remora' particles onto the clump. This accreted shell dramatically increases the effective cross section of the clump. Furthermore, at all velocities the change in smoothing radii of particles with changing density (a universal feature of SPH which is considered `a good thing') leads to excessive deceleration in the outer parts of halos.
The drag on accreting clumps has ramifications for cosmology on the whole since the standard cold dark matter (CDM) model has suffered in recent years from attacks due to its apparent inability to successfully predict the observed number of galaxy and group-sized objects in clusters. First it produced too few which was finally attributed to poor resolution in the simulations. Nowadays CDM simulations produce too many objects leading to conjecture that dark matter is self interacting or mixed with some warm component. Rarely is the contribution of the baryonic component considered. The baryons are ignored because the dark matter really is the dominant form of matter in the universe. But this is not necessarily true on smaller scales since gas is able to radiate away energy and consequently contract to higher densities such as occurs during galaxy formation. It is in this regime that the work has great relevance.
The results will appear in the November 1 volume of the Astrophysical Journal.
Figure 1 Flow of particles about resolution limited clumps in smoothed particle hydrodynamics (SPH) simulations. The flow is shown in the rest frame of the clump which is moving from left to right at a) Mach 1/3, b) Mach 1, and c) Mach 2. The inner and outer circles indicate the smoothing radii of the clump and medium particles respectively. In the Mach 1/3 simulation, a ring of particles is accreted.