Primordial Molecular Emission in Population III Galaxies

We consider the formation of molecules in primordial prestellar clumps and evaluate the line luminosities to assess detectability by next-generation observational facilities. If the initial H₂ fraction is sufficiently high, HD becomes an important coolant in the clumps. The luminosity from such HD cooling clumps is lower than that from H₂ cooling ones because of the lower temperature (<100 \,K). As for Li reactions, we include the three-body LiH formation approximately. The Li molecular fraction remains very low (<10⁻³) throughout the evolution, owing to the high dissociative reaction rate of LiH + H → Li + H₂. LiH does not become an important coolant in any density range. The luminous emission lines from the prestellar cores include H₂ rovibrational lines [1–0 Q(1), 1–0 O(3), 1–0 O(5)] and pure rotational lines [0–0 S(3), 0–0 S(4), 0–0 S(5)]. The next-generation facilities SPICA and JWST are able to detect H₂ emission in a large pre-galactic cloud that forms metal-free stars at a high rate of ~ 10³ \,M_ \,yr⁻¹ at a redshift of $z < 10$. We also derive an analytical expression for the luminosity that reproduces the numerical results.