Influence of Gamma-Ray Emission on the Isotopic Composition of Clouds in the Interstellar Medium

We investigate one mechanism of the change in the isotopic composition of cosmologically distant clouds of interstellar gas whose matter was subjected only slightly to star formation processes. According to the standard cosmological model, the isotopic composition of the gas in such clouds was formed at the epoch of Big Bang nucleosynthesis and is determined only by the baryon density in the Universe. The dispersion in the available cloud composition observations exceeds the errors of individual measurements. This may indicate that there are mechanisms of the change in the composition of matter in the Universe after the completion of Big Bang nucleosynthesis. We have calculated the destruction and production rates of light isotopes (D, 3He, 4He) under the influence of photonuclear reactions triggered by the gamma-ray emission from active galactic nuclei (AGNs). We investigate the destruction and production of light elements depending on the spectral characteristics of the gamma-ray emission. We show that in comparison with previous works, taking into account the influence of spectral hardness on the photonuclear reaction rates can increase the characteristic radii of influence of the gamma-ray emission from AGNs by a factor of 2-8. The high gamma-ray luminosities of AGNs observed in recent years increase the previous estimates of the characteristic radii by two orders of magnitude. This may suggest that the influence of the emission from AGNs on the change in the composition of the medium in the immediate neighborhood (the host galaxy) has been underestimated.

💡 Research Summary

The paper addresses the long‑standing puzzle of why the isotopic composition of apparently pristine interstellar clouds shows a scatter larger than observational uncertainties, despite the standard cosmological view that such clouds retain the primordial abundances set by Big‑Bang nucleosynthesis (BBN). The authors propose that high‑energy gamma‑ray emission from active galactic nuclei (AGNs) can induce photon‑nuclear reactions that alter the abundances of the light isotopes deuterium (D), helium‑3 (³He) and helium‑4 (⁴He) after BBN.

Methodology – The study models the AGN gamma‑ray spectrum as a power‑law, (F(E) \propto E^{-\Gamma}), with spectral indices (\Gamma) ranging from 1.5 (hard spectrum) to 3.0 (soft spectrum). Using up‑to‑date photon‑nuclear cross‑section data for reactions such as (\gamma + \mathrm{D} \rightarrow p + n) and (\gamma + {}^{3}\mathrm{He} \rightarrow p + \mathrm{D}), the authors compute reaction rates as a function of distance from the source: \