Rigorous Theory for Secondary Cosmic-Ray Ionization

The energy spectrum of electrons produced in molecular gas by interstellar cosmic rays (CRs) is rigorously calculated as a function of gas column density $N$ traversed by the CRs. This allows us to accurately compute the local value of the secondary ionization rate of molecular hydrogen, $\zeta_{\rm sec}(N)$, as a function of the local primary ionization rate, $\zeta_p(N)$. The ratio $\zeta_{\rm sec}/\zeta_p$ increases monotonically with $N$, and can considerably exceed the value of $\approx0.67$ commonly adopted in the literature. For sufficiently soft interstellar spectra, the dependence $\zeta_{\rm sec}/\zeta_p$ versus $N$ is practically insensitive to their particular shape and thus is a general characteristic of the secondary CR ionization in dense gas.