21 cm signal from dark-age collapsing halos with a detailed molecular cooling treatment

Context. To understand the formation of the first stars, a detailed description of the thermal and chemical processes in collapsing gas clouds is essential. Molecular cooling, particularly via H2, plays a significant role in triggering thermal instabilities that lead to star formation. The 21 cm hydrogen line serves as a potential probe of the first collapsing structures during the dark ages of the early Universe and it is affected by the gas temperature evolution. Aims. We aim to investigate the molecular cooling in the gas halos prior to the formation of the first stars, with a particular focus on how the H2 cooling affects the gas temperature. Additionally, we explore the sensitivity of the 21 cm hydrogen line to these cooling processes during the collapse of the first overdense regions. Results. We introduce the <ns0:monospace>CHEMFAST</ns0:monospace> code, which tracks the evolution of chemical abundances and computes the 21 cm neutral hydrogen signal in collapsing halos. Our results show that molecular cooling significantly affects the gas temperature inside collapsing clouds of mass ranging from 106 to 109 M⊙, influencing the 21 cm signal. The signal exhibits an emission feature that is distinct from the one predicted in simpler expansion models. Conclusions. The 21 cm brightness temperature inside collapsing clouds displays an emission feature driven by molecular cooling, closely mirroring the gas temperature evolution. This makes the dark-age 21 cm signal a promising probe for studying the thermal processes and structure formation in the early Universe.