Search for high column density systems with gamma ray bursts

We investigate the possibility of searching for metal-poor high column density (greater than or similar to 10(23) cm(-2)) clouds at high redshift (z) by using gamma ray burst afterglows. Such clouds could be related to primeval galaxies that may cause a burst of star formation. We show that a large part of hydrogen is in molecular form in such a high column density environment. Therefore, hydrogen molecules (H-2) rather than hydrogen atoms should be searched for. Then we show that infrared H-2 lines are detectable for metal-poor (less than or similar to 0.01 solar metallicity) high column density (log N-H [cm(-2)] greater than or similar to 23.5) systems at high-z without suffering dust extinction. The optical properties of dust in infrared could also be constrained by observations of high column density systems. Some possible scenarios for producing high column density systems are finally discussed in the context of galaxy evolution.