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| Format: | Preprint |
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2024
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| Online Access: | https://arxiv.org/abs/2410.09753 |
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| _version_ | 1866917972650491904 |
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| author | Dai, Liang |
| author_facet | Dai, Liang |
| contents | We study the formation of a partially ionized zone behind the usual H II zone in photo-ionized gas for gas density intermediate between ordinary H II regions and AGN BLRs 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. Independently 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 $n_{\rm H} \gtrsim 10^7\,{\rm cm}^{-3}$. 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. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2410_09753 |
| institution | arXiv |
| publishDate | 2024 |
| record_format | arxiv |
| spellingShingle | Partially Ionized Gas at Equilibrium Powered by Ultraviolet Irradiation Dai, Liang Astrophysics of Galaxies Cosmology and Nongalactic Astrophysics We study the formation of a partially ionized zone behind the usual H II zone in photo-ionized gas for gas density intermediate between ordinary H II regions and AGN BLRs 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. Independently 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 $n_{\rm H} \gtrsim 10^7\,{\rm cm}^{-3}$. 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. |
| title | Partially Ionized Gas at Equilibrium Powered by Ultraviolet Irradiation |
| topic | Astrophysics of Galaxies Cosmology and Nongalactic Astrophysics |
| url | https://arxiv.org/abs/2410.09753 |