Can diffuse X-rays be important in driving photoionisation in molecular clouds?
4 affiliations
- 1Max Planck Institute for Extraterrestrial Physics
- 2Physics Department, Technion – Israel Institute of Technology
- 3Faculty of Physics, University of Duisburg-Essen
- 4INAF–Osservatorio Astrofisico di Arcetri
Abstract
Context. 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. Aims. Our goal is to quantify the contribution of the large-scale diffuse X-ray radiation field to ionisation in molecular clouds in the absence of strong local X-ray sources, such as young stars, and to assess its significance relative to Galactic cosmic rays. Methods. We used measurements of diffuse X-ray emission from the first eROSITA all-sky survey to estimate the depth-dependent X-ray ionisation rate in molecular clouds across different Galactic environments. We considered 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. Results. Diffuse Galactic X-rays are unlikely to dominate the ionisation balance in well-shielded molecular gas (NH > 5 × 1021cm−2), where cosmic rays remain the primary ionising agent. At low column densities, the deabsorbed diffuse X-ray field produces an ionisation rate of ζX ∼ 10−18s−1 at NH ∼ 1019cm−2, remaining below the lower end of the Galactic cosmic ray ionisation rate (∼10−17s−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
@article{e2026diffuse,
title = {{Can diffuse X-rays be important in driving photoionisation in molecular clouds?}},
author = {Makarenko E. I. and Ivlev A. V. and Bialy S. and Zheng X. and Gaches B. A. L. and Caselli P. and Padovani M. and Yeung M. C. H.},
journal = {Astronomy \& Astrophysics},
year = {2026},
month = oct,
doi = {10.1051/0004-6361/202662444},
url = {https://doi.org/10.1051/0004-6361/202662444},
note = {CC BY},
}
RIS
TY - JOUR
TI - Can diffuse X-rays be important in driving photoionisation in molecular clouds?
AU - Makarenko E. I.
AU - Ivlev A. V.
AU - Bialy S.
AU - Zheng X.
AU - Gaches B. A. L.
AU - Caselli P.
AU - Padovani M.
AU - Yeung M. C. H.
PY - 2026
DA - 2026/10/27
JO - Astronomy & Astrophysics
DO - 10.1051/0004-6361/202662444
UR - https://doi.org/10.1051/0004-6361/202662444
AB - Context. 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. Aims. Our goal is to quantify the contribution of the large-scale diffuse X-ray radiation field to ionisation in molecular clouds in the absence of strong local X-ray sources, such as young stars, and to assess its significance relative to Galactic cosmic rays. Methods. We used measurements of diffuse X-ray emission from the first eROSITA all-sky survey to estimate the depth-dependent X-ray ionisation rate in molecular clouds across different Galactic environments. We considered 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. Results. Diffuse Galactic X-rays are unlikely to dominate the ionisation balance in well-shielded molecular gas (NH > 5 × 1021cm−2), where cosmic rays remain the primary ionising agent. At low column densities, the deabsorbed diffuse X-ray field produces an ionisation rate of ζX ∼ 10−18s−1 at NH ∼ 1019cm−2, remaining below the lower end of the Galactic cosmic ray ionisation rate (∼10−17s−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.
SN - 0004-6361
SN - 1432-0746
ER -