The first molecules in the intergalactic medium and halos of the Dark Ages and Cosmic Dawn

Abstract

We study the formation and destruction of the first molecules at the epochs of the Dark Ages and Cosmic Dawn to evaluate the luminosity of the protogalaxy clumps (halos) in the molecular lines. The cosmological recombination is described using the model of an effective three-level atom, while the chemistry of the molecules is examined using the relevant basic kinetic equations. We then studied the effect of collisional and radiative excitation of molecules on the intensity of molecular emission in both warm and hot halos. Using the Planck data on the reionization of the intergalactic medium at z~6-8, we evaluated the upper limits of the light energy density for four models of thermal light from the first sources that appeared in the Cosmic Dawn epoch. Assuming that in the halos, the light energy density may essentially be even higher, we estimated the impact of the light from the first sources on the formation and destruction of the first molecules. We show that the molecules H2 and HD are destroyed by photodissociation processes shortly before the full reionization in the inter-halo medium, in the medium of both types of halos and for all models of the first light. At the same time, the number density of helium hydride ions, HeH+, shows essentially more complicated dependences on the kinetic temperature of halos and the models of the first light. Furthermore, we estimated the differential brightness temperature of the individual halo in the rotational lines of H2, HD and HeH+ molecules at redshifts corresponding to the Dark Ages and Cosmic Dawn epochs. It does not exceed the microkelvin, but its detection may be an important source of information about the physical processes taking place at the beginning of the formation of the first stars and galaxies at the epochs of the Dark Ages and Cosmic Dawn.

15 pages, 14 figures, 2 tables; accepted for publication in A&A

References in corpus (5)

• Star formation at very low metallicity. I: Chemistry and cooling at low densities
• Effects of primordial chemistry on the cosmic microwave background
• Atomic and molecular gas from the epoch of reionization down to redshift 2
• Global radiation signature from early structure formation
• Spectral Spatial Fluctuations of CMBR: Strategy and Concept of the Experiment

Cited by in corpus (6)

• Signal in the Hyperfine Structure Line of the Ground State of Atomic Hydrogen from the Dark Ages as a Cosmological Test
• Global signal in the redshifted hydrogen 21-cm line from the Dark Ages and Cosmic Dawn: Dependence on the nature of dark matter and modeling of first light
• On the Possibility of Detecting a Global Signal in the Line of the Hyperfine Structure of Hydrogen from the Dark Ages
• Distortion of the CMB spectrum by the first molecules of the Dark Ages
• Impact of the cosmological expansion on spectral energy density of radiation in the intergalactic medium and once more about Olbers' paradox
• Sensitivity of the redshifted 21 cm signal from the Dark Ages to parameters of primordial magnetic fields