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Published online by Cambridge University Press: 25 September 2002
The observability of hydrogen molecules in absorption was investigated for infrared lines against bright infrared sources.The absorption efficiency of the hydrogen molecules becomes comparable with or larger than that of the dust grains in the metal-poor condition expected in the early Universe. If we can use bright infrared continuum sources behind the molecular gas clouds, the absorption measurement of the hydrogen molecules will be an important technique to explore the primordial gas clouds that are contracting into first-generation objects.