The effect of large-scale power on simulated spectra of the Lyα forest

We explore the effects of size of the box that we use for simulations of the intergalactic medium (IGM) at redshift two. We examine six simulations from the hydrodynamic code ENZO using the same cosmological and astrophysical input parameters and cell size, but different box size. We study the CDM distribution and many statistics of the Lyα forest absorption from the IGM. Larger boxes have fewer pixels with significant absorption (flux 0.96), more pixels in longer stretches with little or no absorption, and they have wider Lyα lines. The larger boxes differ only because they include power from long wavelength modes that do not fit inside the periodic conditions of the smaller boxes. The long modes change the density, velocity and temperature fields and these increase in the gas temperature. Small simulations are too cold compared to larger ones. When we deliberately increase the heat we put into the IGM, we can approximate the Lyα forest in a simulation of twice the size. When we double the box size, the difference of most statistics from their value in our largest 76.8 Mpc box is reduced by approximately a factor of two. Most of the statistics converge towards