EMPRESS. VII. Ionizing Spectrum Shapes of Extremely Metal-Poor Galaxies: Uncovering the Origins of Strong HeII and the Impact on Cosmic Reionization

Understand

Strong high-ionization lines such as HeII of young galaxies are puzzling at high and low redshift.

• Although recent studies suggest the existence of non-thermal sources, whether their ionizing spectra can consistently explain multiple major emission lines remains a question.
• Here we derive the general shapes of the ionizing spectra for three local extremely metal-poor galaxies (EMPGs) that show strong HeII$\lambda$4686.
• We parameterize the ionizing spectra composed of a blackbody and power-law radiation mimicking various stellar and non-thermal sources.