Search for high column density systems with gamma ray bursts

We investigate the possibility to search for metal-poor high column density (> 10^{23} cm^{-2}) clouds at high redshift (z) by using gamma ray burst (GRB) afterglows. Such clouds could be related to primeval galaxies which 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 (H2) rather than hydrogen atoms should be searched for. Then we show that infrared H2 lines are detectable for metal-poor (< 0.01 solar metallicity) high column density (log NH [cm^{-2}] > 23.5) systems at high-z without suffering dust extinction. Optical properties of dust in infrared could also be constrained by observations of high column density systems. Some possible scenarios of producing high column density systems are finally discussed in the context of galaxy evolution.