A hydrodynamic approach to cosmology: Texture-seeded cold dark matter and hot dark matter cosmogonies

We present hydrodynamical simulations of galaxy formation in a texture-seeded cosmology, investigating both hot dark matter (HDM) and cold dark matter (CDM) dominated Ω = 1 universes. Texture induces a scale-invariant spectrum of non-Gaussian density fluctuations. The simulations include both gravitational and hydrodynamical physics with a detailed treatment of collisional and radiative thermal processes, and use a cooling criterion to estimate galaxy formation. Background radiation fields and Zel'dovich-Sunyaev fluctuations are explicitly computed. The linear power spectrum for texture-seeded HDM is similar to the Harrison-Zel'dovich (HZ) HDM spectrum. However, nonlinear structure formation begins much earlier in the texture model. As in the HDM + HZ case, neutrino free streaming erases short-wavelength fluctuations, so most of the objects identified in the simulations were cluster-sized. The correlation function of these "clusters" is consistent with the observed (r/ 17/h^-1 Mpc)^-1.8 for objects of the same space density. Far fewer galaxy-sized objects are made than are observed. Galaxies would have to form as a result of secondary processes, such as the fragmentation or explosion of large objects. The clusters are powerful X-ray sources and may be detectable through their Sunyaev-Zel'dovich microwave distortions. The linear power spectrum for texture-seeded CDM is again similar to the CDM + HZ spectrum. Again, galaxy formation begins much earlier. By z = 10, more than 1% of the baryons have already collapsed and cooled to form "galaxies," far in excess of the fraction in a CDM + HZ simulation using identical numerical methods. The early structure formation heats the intergalactic medium. This model is better suited than any other model so far investigated to avoid violation of the Gunn-Peterson limits. The derived galaxy mass function is well fitted by the observed Schechter luminosity function for a baryonic M/L of 3 and total M/L of 60 in galaxies. Textures in regions of low gas density ("voids") do not form galaxies. They are relatively anticorrelated (compared with all dark matter) and are attractive sites for the numerous observed Lyman-alpha systems. Clusters in the texture-seeded model are richer than in the HZ model, with a corresponding higher X-ray emission, perhaps higher than is observed. Observations of X-ray clusters will be an important test of the model. In both HDM and CDM texture scenarios, the "galaxies" and "clusters" are significantly more strongly correlated than the dark matter, due, we believe, to physical bias processes. The slope of the correlation function in both cases is consistent with observation. In contrast to Gaussian models, peaks in the dark matter density distribution are less correlated than average.