Hydrogen Molecules in the Dark Ages Halos: Thermal Emission versus Resonant Scattering

Abstract

Abstract The emission from dark ages halos in the lines of transitions between the lowest rotational levels of hydrogen and hydrogen deuteride molecules is analyzed. It is assumed that molecules are excited by the cosmic microwave background (CMB) and collisions with hydrogen atoms. The physical parameters of halos and the number density of molecules are precalculated assuming that halos are homogeneous top-hat spheres formed from the cosmological density perturbations in the four-component universe with post-Planck cosmological parameters. The differential brightness temperatures and differential spectral fluxes in the rotational lines of H2–HD molecules are computed for two phenomena: thermal luminescence and resonant scattering of CMB radiation. The results show that the expected maximal values of differential brightness temperature of warm halos (T K ∼ 200–800 K) are at the level of nanokelvins, are comparable for both phenomena, and are below the sensitivity of modern submillimeter radio telescopes. For hot halos (T K ∼ 2000–5000 K) the thermal emission of H2-ortho molecules dominates and the differential brightness temperatures are predicted to be of a few microkelvins at the frequencies 300–600 GHz, which could be detectable with next-generation telescopes.