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With high resolution spectra of quasars, it is possible to consider the physical conditions of the gaseous structures that produce absorption. However, it is challenging to separate the various effects that ``shape'' the spectral features in the different chemical transitions. The absorption profiles observed for the different chemical transitions are determined by a combination of the spatial distribution of material along the line of sight, its bulk kinematics, temperature, metallicity, and abundance pattern. The ionization structure is influenced by gas densities and by the UV radiation field, which is a combination of the extragalactic background radiation due to the accumulated effect of quasars and stellar photons escaped from galaxies (and corrected for absorption by the intergalactic medium).

The shape of an absorption line can be modeled with a Voigt profile, which is a combination of the natural, quantum mechanical Lorentzian broadening and the Gaussian broadening caused by the thermal and turbulent motions in the gas. Several Voigt profiles can be blended together to form an overall complex absorption feature (see Figure 4). The ``width'' of a single Voigt profile is characterized by the Doppler parameter, b (expressed in velocity units and related to the Gaussian sigma by b = 21/2 sigma). Physically, the Doppler parameter is the sum of thermal and turbulent components, b2tot = 2kT/m + b2turb, where T is the temperature of the gas, and m is the mass of an atom.