The low-redshift evolution of the Lyman-α forest

The low-redshift evolution of the intergalactic medium is investigated using hydrodynamic cosmological simulations. The assumed cosmological model is a critical density cold dark matter universe. The imposed uniform background of ionizing radiation has the amplitude, shape and redshift evolution as computed from the observed quasar luminosity function by Haardt & Madau. We have analysed simulated Lyman-alpha spectra using Voigt-profile fitting, mimicking the procedure with which quasar spectra are analysed. Our simulations reproduce the observed evolution of the number of Lyman-alpha absorption lines over the whole observed interval of z = 0.5 to 4. In particular, our simulations show that the decrease in the rate of evolution of Lyman-alpha absorption lines at z less than or equal to 2, as observed by the Hubble Space Telescope, can be explained by the steep decline in the photoionizing background resulting from the rapid decline in quasar numbers at low redshift.