H2 Formation in Low Metallicity Galaxies

A possible formation mechanism of hydrogen molecules on a galactic scale is examined. We are interested especially in the role of hydrogen molecules for formation and evolution of primordial galaxies. Then, formation process of hydrogen molecules in a very low-metallicity galaxy (I Zw 18; the most typical metal-deficient galaxy) is studied. Adopting a recent observational result of absorption lines of hydrogen molecule in I Zw 18, we obtain the upper limit for the ionization degree in the case where hydrogen molecules can form via the H$^{-}$-process, although they are generally believed to form on the surface of dust grains. Furthermore, we present a critical ionization degree, above which H$^{-}$-process can be dominant over the formation process on the surface of grains. Interestingly, this critical ionization degree is comparable to the upper limit of the ionization degree for I Zw 18. For determination of the formation process of hydrogen molecules, future observational facilities can be useful. Thus, we examine the detectability in some wavelengths for the metal deficient galaxies. According to our estimate, near-infrared line emission of hydrogen molecule is observable in the level of 10 $μ$Jy, the free-free radio emission is in the level of mJy, and the far-infrared emission from the dust on which hydrogen molecules form can also be detected with 10-mJy level with its temperature of 16 K. The near-infrared line and the far-infrared continuum are feasible for ASTRO-F observations.

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