2-Fluid Simulations of Galaxy Formation

Abstract

We investigate the formation of galaxies and larger structure with a simulation modeling two gravitationally coupled fluids representing dark matter and baryons. The baryon gas dynamics are calculated with a smoothed particle hydrodynamics (SPH) method, and the physics modeled includes thermal pressure, shock heating, and radiative cooling. We simulate a 16 Mpc periodic cube with 64(3) particles in each fluid and 10% baryon mass fraction. The mass per baryon particle of 1.1 x 10(8) M. implies an L* galaxy is resolved by similar to 10(3) particles, a factor 10 improvement over previous large-scale studies. Initial perturbations from a cold dark matter spectrum were generated such that a poor cluster of galaxies on a mass scale 10(13.6) M. would form. For computational reasons, the volume was evolved only to a final redshift z = 1. We identify a population of galaxy-like objects by searching for baryon concentrations above a fixed physical density equivalent to 10(6) times the z = 0 mean background value. These objects trace the overall filamentary structure in the dark matter at high z, and a subset drain into the cluster, which forms at the intersection of several filaments. With dissipation reducing their collision cross section, the simulated galaxies retain their identity within the cluster, alleviating the ''overmerger problem'' inherent in collisionless simulations of dark matter halos. The efficiency of galaxy formation is lower in the largest halos, as observed in rich clusters of galaxies. The inferred abundance of galaxies grows rapidly to z = 2 and flattens thereafter. The mass function has a low-mass slope shallower than that of the dark matter hares but steeper than the observed faint end slope of the luminosity function. Merger rates for galaxies are lower by a factor similar to 2 relative to dark matter halos. We confirm, for the first time experimentally, disk formation as a natural consequence of hierarchical clustering in a large-scale cosmological environment. The majority of isolated galaxies exhibit centrifugally supported disks. A power-law relation between cold baryonic mass and maximum rotation velocity is found, M proportional to upsilon(rot)(alpha), with alpha = 2.5 after correcting for differential numerical resolution. The correlation is extremely tight, with rms residual equivalent to 0.15 mag. An assumption of L proportional to M(p) with P > 1 would both reconcile this relation with the observed Tully-Fisher law and produce a luminosity function faint end slope closer to that observed. Both the spatial and velocity distributions of the simulated galaxies are biased with respect to the dark matter. The galaxy population is strongly biased at z = 3, with spatial bias parameter b xi = 2 or 3 for low- and high-mass samples. By z = 1, the high-mass bias disappears and the low-mass sample is actually less clustered than the dark matter. A counts-in-cells analysis indicates that an unphysical degree of merging in the central cluster is likely responsible for the antibias signal in the correlation function. A robust, scale-dependent velocity bias is measured. The ratio of galaxy to dark matter pairwise velocity dispersions on a scale of 1 Mpc is 0.7. The amplitude is only mildly dependent on redshift or mass cutoff and scales with separation as r(O.2).