Observation of QSO absorption line systems is the most sensitive way to probe baryonic material in the whole redshift range 0<z<5. Among absorption line systems, Lyα systems without detected associated metal lines are the most numerous (about 150 per unit redshift at z~2). They have been extensively studied, since they may trace the evolution of the intergalactic medium and the UV background. Indeed the clouds could be an intergalactic population possibly of primordial gas evolving independently of galaxies under the direct influence of the intergalactic medium, the ionizing background, and the gravitational potential of dark matter. They are observed over a large redshift range, and they thus probe the evolution of the baryonic material and the structures formed by dark matter.
As part of my scientific work, we studied the evolution with redshift of the number density of Lyα clouds in a CDM model using numerical simulations including photo-ionization and cooling of the baryonic component.
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