SPH P3MG Simulations of the Lyman-alpha Forest

Our understanding of the Lyman-$\alpha$ forest has received a great boost with the advent of the Keck Telescope and large 3D hydrodynamical simulations. We present new simulations using the SPH technique with a P^3MG (Particle-Particle Particle-MultiGrid) non-periodic gravity solver. Our method employs a high resolution (1 kpc) inner volume, essential for capturing the complex gas physics, a larger low resolution volume, essential for correct larger scale tidal fields and a self-consistently applied, uniform tidal field to model the influence of ultra long waves. We include a photoionizing UV flux and relevant atomic cooling processes. We use constrained field realisations to probe a selection of environments and construct a statistical sample representative of the wider universe. We generate artificial Lyman-$\alpha$ spectra and fit Voigt profiles. We examine the importance of (1) the photoionizing flux level and history, (2) tidal environment and (3) differing cosmologies, including CDM and CDM+{$\Lambda$}. With an appropriate choice for the UV flux, we find that the data is fit quite well if the rms density contrast is $\sim$ 1 at $z\sim 3$ on galaxy scales.