The initial conditions use several cosmological boxes, run at different resolution levels and cosmological parameters. This variety of initial conditions unveils any bias introduced by box-size, resolution or cosmology:
• Box size: 10 Mpc/h - 80 Mpc/h
• Mass of DM particle: ~10^4 Msun (effective # of DM particles: 2048^3-16384^3 )
• Starting redshift: z=150
• Cosmological parameters according to WMAP7 and PLANCK
• Physical spatial resolution: ~10 pc at z=10
From the numbers quoted above it becomes clear that one cannot simulate the whole box at this resolution. I use the well known zooming technique in cosmological simulations to resimulate selected objects in the computational volume by sampling the density field with variable mass resolution in a selected lagrangian region of a large cosmological box (Klypin et al. 2001). The selection of objects is done using a cluster sampling approach. This yields a random and homogeneous sample with a range of halo masses between 10^9 – 10^12 Msun at z=5.
The simulations are performed using the ART code. ART (Kravtsov, Klypin, & Khokhlov 1997; Ceverino & Klypin 2009) follows the evolution of a gravitating N-body system and the Eulerian gas dynamics using an adaptive mesh refinement approach. Beyond gravity and hydrodynamics, the code incorporates many of the physical processes relevant for galaxy formation as state-of-the-art subgrid models: gas cooling by atomic hydrogen and helium, metal and molecular hydrogen cooling, and photoionization heating by an UV background with partial self-shielding, star formation and feedback from supernovae, stellar winds and radiation.
Cooling and heating rates are tabulated for a given gas density, temperature, metallicity and UV background based on the CLOUDY code (version 96b4), assuming a slab of thickness 1 kpc. A uniform UV flux based on the redshift-dependent Haardt & Madau (1996) model is assumed, except at gas densities higher than 0.1 cm^−3, where a substantially suppressed UV background is used in order to mimic the partial self-shielding of dense gas. This allows the dense gas to cool down to temperatures of around 300 K. The assumed equation of state is that of an ideal mono-atomic gas. Artificial fragmentation on the cell size is prevented by introducing a pressure floor, which ensures that the Jeans scale is resolved by at least seven cells. Star formation is assumed to occur at densities above a threshold of 1 cm^−3 and at temperatures below 104 K. The code implements a stochastic star formation model that yields the empirical Kennicutt–Schmidt law.
Supernova and stellar winds are modeled as thermal stellar feedback, in which the combined energy from stellar winds and supernova explosions is released as a constant heating rate over 40 Myr following star formation, the typical age of the lightest star that explodes as a core-collapsed supernova. The heating rate due to feedback may or may not overcome the cooling rate, depending on the gas conditions in the star-forming regions. We also include the effect of runaway stars by assigning a velocity kick of around 10 km/s to 30 per cent of the newly formed stellar particles. The code also includes the later effects of Type-Ia supernova and stellar mass-loss, and it follows the metal enrichment of the ISM. In addition to thermal feedback, the runs use radiative feedback. In short, this model adds a non-thermal pressure, radiation pressure, to the total gas pressure in regions where ionising photons from massive stars are produced and trapped, assuming a single scattering limit. New models that take into account the physics of the first galaxies will also be developed. These included primordial non-equilibrium cooling or feedback from population III stars.
SCIENTIFIC GOALS:
What is the star formation efficiency in the first galaxies?
How are gas, stars and metals distributed within galaxies?
Morphological transformations at early times
Properties of galaxy outflows
Star formation history at different galaxy scales
What is the escape fraction of ionising photons?
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