Photoionization models for extreme Ly$\alpha$ $\lambda$1216 and HeII $\lambda$1640 ratios in quasar halos, and PopIII vs AGN diagnostics

TL;DR

This paper investigates how various physical conditions in high-redshift quasar halos can produce extremely high Ly-alpha/HeII ratios, revealing new insights into ionization mechanisms and diagnostics for Pop III stars versus AGN.

Contribution

It provides detailed photoionization models showing how low metallicity, ionization parameter, and other factors enhance Ly-alpha emission and offers diagnostic tools to distinguish Pop III stars from AGN.

Findings

Ly-alpha/HeII ratios can reach up to 4.6 under certain conditions.

Collisional excitation significantly boosts Ly-alpha photon production.

UV-optical diagnostics can differentiate Pop III stars from AGN.

Abstract

We explore mechanisms to produce extremely high Ly-alpha/HeII flux ratios, or to enhance the observed number of Ly-alpha photons per incident ionizing photon, in extended AGN-photoionized nebulae at high-redshift. Using photoionization models, we explore the impact of ionization parameter, gas metallicity, ionizing spectrum, electron energy distribution, and cloud viewing angle on the relative fluxes of Ly-alpha, HeII and other lines, and on the observed number of Ly-alpha photons per incident ionizing photon. We find that low ionization parameter, a relatively soft or filtered ionizing spectrum, low gas metallicity, kappa-distributed electron energies, or reflection of Ly-alpha photons by HI can all result in significantly enhanced Ly-alpha relative to other lines (>10%), with log Ly-alpha/HeII reaching values up to 4.6. In the cases of low gas metallicity, reflection by HI, or a hard or filtered ionizing spectrum, the observed number of Ly-alpha photons per incident ionizing photon is itself significantly enhanced above the nominal Case B value of 0.66 due to collisional excitation, reaching values up to 5.3 in our 'extreme case' model. At low gas metallicity (e.g. 0.1 x Solar), the production of Ly-alpha is predominantly via collisional excitation rather than recombination. In addition, we find that collisional excitation of Ly-alpha becomes more efficient if the ionizing continuum is pre-filtered through an optically thin screen of gas closer to the AGN. We also show that Ly-alpha / HeII ratios of the z~3.5 quasars studied by Borisova et al. (2016) are consistent with AGN-photoionization of gas with moderate to low metallicity and/or low ionization parameter, without requiring exotic ionization/excitation mechanisms such as strong line-transfer effects. We also present UV-optical diagnostic diagrams to distinguish between photoionization by Pop III stars and AGN photoionization.