Tests for the minihalo model of the Lyman alpha forest

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Abstract
According to the minihalo model for the Ly α forest, the absorption lines arise from photoionized gas gravitationally confined in dark matter haloes of low velocity-dispersion. Possible tests for this hypothesis are analysed. The predicted column density distribution is mostly dependent on the density profile of the haloes, and is in basic agreement with the observations. At low column densities, the lines should be probing the infalling, non-virialized regions of the clouds, and they should deviate from Voigt profiles. The Gunn-Peterson absorption should also fluctuate with wavelength, and it may thus be difficult to distinguish from a superposition of many weak absorption lines. We give an approximate estimate of the relation between the Gunn-Peterson optical depth and the observed average decrement produced by the absorption lines in the Ly α forest. The model makes a specific prediction for the size of the clouds, which can be tested by comparing the absorption lines in the multiple images of a gravitationally lensed quasar. The predicted difference in the equivalent widths of corresponding lines in known gravitational lenses is small, and consistent with the results of Smette et al.; observations with higher signal-to-noise ratio could reveal the differences predicted by the model. Finally, we mention the effects that can produce an evolution of the number density of absorption lines with redshift.

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