Cold Dark Matter Cosmogony with Hydrodynamics and Galaxy Formation: Galaxy Properties at Redshift Zero
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Abstract
We have supplemented our code, which computes the evolution of the physical state of a representative piece of the universe, to include not only the dynamics of dark matter (with a standard PM code) and the hydrodynamics of the gaseous component (including detailed collisional and radiative processes), but also galaxy formation on a heuristic but plausible basis. If, within a cell, the gas is Jeans-unstable, collapsing and cooling rapidly, it is transformed to galaxy subunits, which are then followed with a collisionless code. We study two representative boxes with sizes L = (80, 8) h-1 Mpc, in both cases utilizing a mesh of 2003 cells containing 2003 dark matter particles and having nominal resolutions of (400, 40) h-1 kpc, respectively, with true resolution approximately 2.5 times worse. We adopt the standard cold dark matter (CDM) perturbation spectrum with an amplitude of σ8 ≡ = (δM/M)rms,8 = 0.77, a compromise between the COBE normalization σ8 = 1.05 and that indicated by the small-scale velocity dispersion (perhaps σ8 = 0.45). We find a mass function which is similar to that observed. There is a strong correlation between galactic age and environment. Identifying the oldest fraction with elliptical and 50 galaxies, we find a density morphology relation of the same type as is observed as well as a correlation between gas mass/total mass ratio and morphology that is similar to observations. In addition, we find that low-mass galaxies contain relatively more dark matter than giants. We present analytic fits to our derived results for "bias," the dependence of ρgal/ <ρgal> on ρtot/<ρtot>. Spatial structures resemble quantitatively those seen in redshift surveys, with galaxies concentrated in clusters and on filaments (or sheets) which surround quite empty voids. The void probability statistics indicate that this model is consistent with magnitude-limited real data. The small-scale velocity field is too large compared with the observed velocity correlation function. The distribution of proper velocities fits an exponential (not a Maxwellian) P(υ) 2 ∝ υ2e-υ/σexp with σexp = 225 km s-1. For galaxies separated by 1 h-1 Mpc we find a one-dimensional velocity dispersion of 670 km s-1 (490 km s-1 for the most massive subset) compared with 340±40 km s-1 as measured by Davis & Peebles (1983). Adoption of the COBE normalization causes the problem to become worse; the CDM prediction is then approximately (920±160) km s-1. If we look at the genus curves of all our galaxies, they fit the random-phase expectation, but a magnitude-limited sample emphasizes the older, more massive galaxies which have collected at a vertices. This explains the observed "meatball shift" of the genus curves found in observed samples.
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