Thermal and Resonant Emission of Dark Ages Halos in the Rotational Lines of HeH$^{+}$

TL;DR

This paper investigates the thermal emission and resonant scattering of CMB radiation by dark ages halos in the rotational lines of HeH$^{+}$, revealing the dominant collisional processes and the time-dependent luminosity peaks.

Contribution

It provides the first detailed analysis of HeH$^{+}$ emission in dark ages halos, including excitation mechanisms and spectral flux predictions.

Findings

Collisional excitation by hydrogen dominates in halos.

HeH$^{+}$ line luminosities show peak-like time dependence.

Both thermal emission and resonant scattering are significant in dark ages halos.

Abstract

We analyse the thermal emission and resonant scattering of CMB radiation from the dark ages halos in the rotational lines of the helium hydride ion (HeH$^{+}$), one of the first chemical compounds in the Universe. Evaluating the optical depth, thermal and resonant brightness temperatures and spectral fluxes of dark ages halos is based on computing the cross-sections and rate coefficients of excitation/de-excitation of the lowest five rotational states of HeH$^{+}$ by inelastic collisions with atomic hydrogen. It was shown that in the Dark Ages the collisional excitation/de-excitation by atoms of neutral hydrogen and electrons are competitive, whereas in the denser regions, e.g., in the halos, the contribution of collisions with atomic hydrogen is dominant. We demonstrate the peak-like time-dependence of halo luminosities in HeH$^{+}$ lines.