Galaxy Tracers and Velocity Bias

This paper examines several methods of tracing galaxies in N-body simulations and the effects of these methods on the derived galaxy statistics. Special attention is paid to the phenomenon of velocity bias, the idea that the velocities of galaxies may be systematically different from that of the mass distribution. Using two simulations with identical initial conditions, one following a single dark matter particle fluid and the other following two particle fluids of dark matter and baryons, both collisionless and collisional methods of tracing galaxies are compared to one another and against a set of idealized criteria. None of the collisionless methods prove satisfactory, including an elaborate scheme developed here to circumvent previously known problems. The main problem is that galactic overdensities are both secularly and impulsively disrupted while orbiting in cluster potentials. With dissipation, the baryonic tracers have much higher density contrasts and much smaller cross sections, allowing them to remain distinct within the cluster potential. The question remains whether the incomplete physical model, especially the expected conversion from a collisional gas to a collisionless stellar fluid, introduces systematic biases. Statistical measures determined from simulations can vary significantly based solely on the galaxy tracing method utilized. The two-point correlation function differs most on cluster scales (less than 1 Mpc), with generally good agreement on larger scales (except for one systematically biased method). Pairwise velocity dispersions show less uniformity on all scales addressed here. All tracing methods show a velocity bias to varying degrees, but the predictions are not firm: either the tracing method is not robust, or the statistical significance has not been demonstrated. Though theoretical arguments suggest that a mild velocity bias should exist, simulation results are not yet conclusive.XS