Galaxy formation in a CDM+Lambda universe .1. Properties of gas and galaxies

A new approach to cosmological hydrodynamics, called ''softened Lagrangian hydrodynamics'' is now supplemented with the dark matter evolution, radiative processes including cooling, heating, and ionization, and evolution of radiation background and a galaxy-formation algorithm. The code is therefore able to follow the evolution of a representative piece of the universe with allowance for all physical processes that play dominant roles in the evolution of large-scale structure on scales where the galaxy formation is thought to occur. A series of simulations of a CDM + Lambda model were performed with 64 h(-1) Mpc box size, 64(3) baryonic quasi-Lagrangian cells, an equal number of dark matter particles and radiation spectrum represented on 150 points from 1 eV to 200 keV. The softening length of simulations is 100 h(-1) kpc. Three production runs were performed: run A without radiative processes and galaxy formation, run B with radiative cooling included, and run C with the complete description of radiative processes and galaxy formation included. The present paper discusses in detail physical properties of gas and galaxies (spatial distributions discussed subsequently). Comparison of cosmic gas properties between these three runs shows that radiative heating by UV emission from galaxies plays a dominant role in heating and ionizing voids, whereas the supernovae thermal energy injection into the intergalactic medium is the dominant mechanism in regulating gravitational collapse, and shock transformation of kinetic to thermal energy is most important in clusters of galaxies. Galaxy formation seems to proceed from collapsing of cold dense gas, cooled by exciting hydrogen lines at 10(4) K. However, hydrogen quickly becomes ionized and heats up to 10(5) K, where helium becomes the dominant cooling agent and galaxy formation proceeds further with cooling by exciting helium lines. The local geometry of a gas element that is gravitationally unstable and cools faster than it collapses is neither spherical nor filamentary but more pancake-like and gravitationally unstable but hot and unable to cool gas is typically in filamentary regions. The places where galaxies are born are remarkably similar in their physical properties (densities, temperatures, local geometry) at z = 3 and z = 0. However, the galaxy age correlates with the density of environment where galaxies are found: old galaxies are primarily found in dense clusters, while young galaxies live on caustics.