The Lyα forest at redshifts 0.1-1.6: good agreement between a large hydrodynamic simulation and HST spectra

We give a comprehensive statistical description of the Ly alpha absorption from the intergalactic medium in a hydrodynamic simulation at redshifts 0.1-1.6, the range of redshifts covered by spectra of quasi-stellar objects obtained with the Hubble Space Telescope (HST). We use the ENZO code to make a simulation in a 1024(3) cube with side length 76.8Mpc comoving using 75-kpc cells, for a Hubble constant of 71 km s(-1) Mpc(-1). The best prior work, by Dave et al., used a smoothed particle hydrodynamics simulation in a 15.6-Mpc box with an effective resolution of 245 kpc at the cosmic mean density and slightly different cosmological parameters. We use a popular cosmological model and astrophysical parameters that describe the ultraviolet background that photoionizes and heats the gas. At redshift z = 2 this simulation is different from data. Tytler et al. found that the simulated spectra at z = 2 have too little power on large scales, Ly alpha lines are too wide, there is a lack of high column density lines and there is a lack of pixels with low flux. Here we present statistics at z < 1.6, including the flux distribution, the mean flux, the effective opacity and the power and correlation of the flux. We also give statistics of the Ly alpha lines including the linewidth distribution, the column density distribution, the number of lines per unit equivalent width and redshift and the correlation between the linewidth and column density. We find that the mean amount of absorption in the simulated spectra changes smoothly with redshift as DA(z) = 0.0102(1 + z)(2.252). Both the trend and absolute values are close to measurements of HST spectra by Kirkman et al. The column density distribution is the same as data at redshift 1.6, but at lower redshifts the simulation has fewer lines than data, perhaps because of issues making measurements in low signal-to-noise ratio spectra. The lines in data have smaller widths than in the simulation but again the data are not very reliable. We argue that these differences are probably not caused by lack of numerical resolution. Although these possible differences are larger than those that we saw at z = 2, overall, the simulation gives a good description of HST spectra at 0.1 < z < 1.6 given that we did not adjust either the parameters that we input to specify the simulations or the output from the simulation.