Can diffuse X-rays be important in driving photoionisation in molecular clouds?
Abstract
The ionisation balance in molecular clouds is regulated by several ionising sources, including cosmic rays, X-rays, and ultraviolet radiation. Their relative importance depends on the local physical conditions and on the shielding column density. We compute the contribution of the large-scale diffuse X-ray radiation field to the ionisation in molecular clouds in the absence of strong local X-ray sources, such as young stars. Our goal is to quantify its significance relative to the Galactic cosmic rays. Using measurements of diffuse X-ray emission from the first eROSITA all-sky survey, we estimate the depth-dependent X-ray ionisation rate in molecular clouds across different Galactic environments. We consider both the observed diffuse field and a deabsorbed field obtained by correcting for foreground Galactic attenuation using HI4PI line-of-sight column densities in a pixel-by-pixel framework. Diffuse Galactic X-rays are unlikely to dominate the ionisation balance in well-shielded molecular gas (NH > 5 × 1021 cm−2), where cosmic rays remain the primary ionising agent. At low column densities, the deabsorbed diffuse X-ray field produces ionisation rate of ζX ∼ 10−18s−1 at NH ∼ 1019cm−2, remaining below the lower end of Galactic cosmic ray ionisation rate (∼ 10−17 s−1). The X-ray contribution decreases rapidly with increasing shielding, falling to ζX∼10−19s−1 by NH∼1021cm−2 and to ζX ∼5×10−21s−1 by NH∼1022cm−2. Diffuse X-ray may therefore contribute to the thermal and chemical structure of low-extinction material and should be considered when interpreting ionisation tracers in diffuse or cloud-envelope environments.
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BibTeX
@misc{makarenko2026diffuse,
title = {{Can diffuse X-rays be important in driving photoionisation in molecular clouds?}},
author = {E. I. Makarenko and A. V. Ivlev and S. Bialy and X. Zheng and B. A. L. Gaches and P. Caselli and M. Padovani and M. C. H. Yeung},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.02648},
eprint = {2609.02648},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.02648},
note = {CC BY-NC-ND 4.0},
}
RIS
TY - UNPB
TI - Can diffuse X-rays be important in driving photoionisation in molecular clouds?
AU - E. I. Makarenko
AU - A. V. Ivlev
AU - S. Bialy
AU - X. Zheng
AU - B. A. L. Gaches
AU - P. Caselli
AU - M. Padovani
AU - M. C. H. Yeung
PY - 2026
DA - 2026/09/02
PB - arXiv
DO - 10.48550/arxiv.2609.02648
UR - https://arxiv.org/abs/2609.02648
AB - The ionisation balance in molecular clouds is regulated by several ionising sources, including cosmic rays, X-rays, and ultraviolet radiation. Their relative importance depends on the local physical conditions and on the shielding column density. We compute the contribution of the large-scale diffuse X-ray radiation field to the ionisation in molecular clouds in the absence of strong local X-ray sources, such as young stars. Our goal is to quantify its significance relative to the Galactic cosmic rays. Using measurements of diffuse X-ray emission from the first eROSITA all-sky survey, we estimate the depth-dependent X-ray ionisation rate in molecular clouds across different Galactic environments. We consider both the observed diffuse field and a deabsorbed field obtained by correcting for foreground Galactic attenuation using HI4PI line-of-sight column densities in a pixel-by-pixel framework. Diffuse Galactic X-rays are unlikely to dominate the ionisation balance in well-shielded molecular gas (N_H > 5 × 10^21 cm^−2), where cosmic rays remain the primary ionising agent. At low column densities, the deabsorbed diffuse X-ray field produces ionisation rate of ζ_X ∼ 10^−18s^−1 at N_H ∼ 10^19cm^−2, remaining below the lower end of Galactic cosmic ray ionisation rate (∼ 10^−17 s^−1). The X-ray contribution decreases rapidly with increasing shielding, falling to ζ_X∼10^−19s^−1 by N_H∼10^21cm^−2 and to ζ_X ∼5×10^−21s^−1 by N_H∼10^22cm^−2. Diffuse X-ray may therefore contribute to the thermal and chemical structure of low-extinction material and should be considered when interpreting ionisation tracers in diffuse or cloud-envelope environments.
ER -