Partially Ionized Gas at Equilibrium Powered by Ultraviolet Irradiation

Abstract We study the formation of a partially ionized zone behind the usual H II zone in photoionized gas for a density intermediate between ordinary H II regions and active galactic nucleus broad-line regions, and without X-rays. While Lyman continuum photons are depleted before reaching this zone, Balmer continuum photons can ionize the n = 2 hydrogen atoms as those are pumped by scattering-trapped Ly α photons. Ly α photons need to be replenished in a steady state, but fast radiative cooling at high gas temperature makes replenishment through electron collisional excitation of n = 2 inefficient. Independent of the temperature, Raman scattering of incident continuum photons on the Lyman-series damping wings injects Ly α photons in the line core and pumps the n = 2 state, which helps realize a partially ionized equilibrium at 5000–7000 K. Since the ratio between the incident radiation flux and gas density is limited by the dynamic effects of radiation pressure, significant ionization fraction is only realized at densities n H ≳ 10 7 cm −3 . A large H α /H β line ratio results from such partially ionized gas, as large Balmer line optical depths enhance the n = 3 population relative to higher n states. Partial ionization also provides the ideal condition for internally forming an intense and very broad Ly α line, which can power strong fluorescent metal emission lines observed from the Weigelt blobs of η Carinae in the Galaxy, and from a candidate young super star cluster in the Cosmic Noon Sunburst galaxy. This phenomenon may also have implications for understanding the spectra of high- z galaxies that show unusually dense ionized gas.