Analyses of QSO absorption lines show that the H I content has evolved over the redshift range z = 5 to z = 0. The 21-cm line measurements of the z = 0 H I content avoid several biases inherent in the absorption-line technique, such as the influence of evolving dust content in the absorbers, and will produce a reliable measure to anchor theories of galaxy evolution. Examples of important questions to be addressed by local H I surveys are: (1) Is there a significant population of gas-rich galaxies or intergalactic clouds that is missing from the census of optically selected galaxies? (2) Is there an adequate reservoir of neutral gas to substantially prolong star formation at its present rate? (3) Are there massive objects of such low H I column density that they can have escaped detection in the ‘unbiased’ H I surveys that have been conducted so far?
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A 1997 study studied this question.
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