Hydrodynamical simulations of the Ly alpha forest: data comparisons

Numerical hydrodynamical simulations are used to predict the expected absor ption properties of the Ly alpha forest for a variety of cold dark matter ( CDM) dominated cosmological scenarios: CHDM, OCDM, Lambda CDM, SCDM, and tC DM. Synthetic spectra are constructed duplicating the resolution, signal-to -noise ratio and wavelength coverage of several published high resolution s pectra, and their statistical properties are compared on the basis of the f lux distribution of the spectra, the distribution of coefficients in a wave let decomposition of the spectra and the distributions of absorption line p rofile parameters. Agreement between the. measured and predicted cumulative distributions is found at the few to several per cent level. The best-fitt ing models to the flux distribution correspond to normalizations on the sca le of the cosmological Jeans length of 1.3 < <sigma>(J) < 1.7 at z = 3. No single model provides a statistically acceptable match to all the distribut ions. Significantly larger median Doppler parameters are found in the measu red spectra than predicted by all but the lowest normalization models (CHDM and tCDM), which provide poor fits to the flux distributions. The discrepa ncy in Doppler parameters is particularly large for absorption systems opti cally thin at the Ly<alpha> line centre. This may indicate a need to introd uce additional energy injection throughout the intergalactic medium, as may be provided by late He II reionization (z(He II) similar or equal to 3.5) or supernovae-driven winds from young galaxies, and/or a larger baryon frac tion than given by recent determinations of the deuterium abundance within the context of standard Big Bang nucleosynthesis. The models require a hydr ogen ionization rate at redshifts 1.7 < z < 3.8 within a factor of 2 of tha t predicted from quasi-stellar objects (QSOs) alone as, the sources of the UV photoionization background, although with a slower rate of decline with redshift at z > 3.5 than predicted from current QSO counts. Principal syste matic uncertainties in comparing the models with the observations are the s etting of the continuum level of the QSO: spectra and the prevalence of met al absorption lines, particularly at z < 3.