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 ($N_{\mathrm{H}} > 5 \times 10^{21} \mathrm{cm^{-2}}$), where cosmic rays remain the primary ionising agent. At low column densities, the deabsorbed diffuse X-ray field produces ionisation rate of $ζ_X \sim 10^{-18}\mathrm{s^{-1}}$ at $N_{\mathrm H} \sim 10^{19}\mathrm{cm^{-2}}$, remaining below the lower end of Galactic cosmic ray ionisation rate ($\sim 10^{-17} \mathrm{s^{-1}}$). The X-ray contribution decreases rapidly with increasing shielding, falling to $ζ_{X}\sim10^{-19}\mathrm{s^{-1}}$ by $N_{\rm H}\sim10^{21}\mathrm{cm^{-2}}$ and to $ζ_X \sim5\times10^{-21}\mathrm{s^{-1}}$ by $N_{\rm H}\sim10^{22}\mathrm{cm^{-2}}$. 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.