H2 Formation in Low-Metallicity Galaxies

Abstract

Abstract A possible formation mechanism of hydrogen molecules on a galactic scale is examined. We are interested especially in the role of hydrogen molecules for the formation and evolution of primordial galaxies. Thus, the 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 the absorption lines of hydrogen molecules in I Zw 18, we obtain the upper limit for the ionization degree in the case where hydrogen molecules can form via the $\mathrm{H}^{-}$-process, although they are generally believed to form on the surface of dust grains. Furthermore, we present a critical ionization degree, above which the $\mathrm{H}^{-}$-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 determining the formation process of hydrogen molecules, future observational facilities can be useful. Thus, we examine the detectability in some wavelengths for metal-deficient galaxies. According to our estimate, the near-infrared line emission of hydrogen molecules is observable at the level of $10\,\mu\mathrm{Jy}$, the free–free radio emission is at the level of mJy, and the far-infrared emission from the dust on which hydrogen molecules form can also be detected at the 10-mJy level with its temperature of 16 K. The near-infrared line and the far-infrared continuum are feasible for ASTRO-F observations.