Non-Voigt Ly alpha absorption line profiles

Recent numerical simulations have lead to a paradigm shift in our understan ding of the intergalactic medium and the loss of a physical justification f or Voigt profile fitting of the Ly alpha forest. Many individual lines seen in simulated spectra have significant departures from the Voigt profile, y et could be well fitted by a blend of two or more such lines. We discuss th e expected effect on the line profiles due to ongoing gravitational structu re formation and Hubble expansion. We develop a method to detect departures from Voigt profiles of the absorption lines in a statistical way and apply this method to simulated Ly alpha forest spectra, confirming that the prof iles seen do statistically differ from Voigt profiles.