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

TL;DR

The paper addresses how extremely metal-poor local galaxies produce strong HeII emission that requires hard ionizing photons and develops a generalized ionizing-spectrum model formed by a blackbody plus a non-thermal component to emulate stellar and non-stellar sources. By coupling CLOUDY photoionization calculations with an MCMC framework, it infers seven parameters describing both the ionizing spectrum and the nebular gas while minimizing abundance-systematic biases, and uses this to reproduce about 10 emission lines across three EMPGs. The results show one EMPG with a blackbody-dominated spectrum and two with convex downward shapes beyond the hydrogen edge, implying diverse spectra that are best explained by combinations of stellar emission and ULX/HMXB contributions. Comparisons with stellar population models suggest the diversity arises from stellar age differences, and the analysis indicates that, if z>6 galaxies resemble EMPGs, high-energy photons from non-stellar sources would contribute only weakly to cosmic reionization due to their relatively modest radiation field.

Abstract

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$λ$4686. We parameterize the ionizing spectra composed of a blackbody and power-law radiation mimicking various stellar and non-thermal sources. We use photoionization models for nebulae, and determine seven parameters of the ionizing spectra and nebulae by Markov Chain Monte Carlo methods, carefully avoiding systematics of abundance ratios. We obtain the general shapes of ionizing spectra explaining $\sim 10$ major emission lines within observational errors with smooth connections from observed X-ray and optical continua. We find that an ionizing spectrum of one EMPG has a blackbody-dominated shape, while the others have convex downward shapes at $>13.6$ eV, which indicate a diversity of the ionizing spectrum shapes. We confirm that the convex downward shapes are fundamentally different from ordinary stellar spectrum shapes, and that the spectrum shapes of these galaxies are generally explained by the combination of the stellar and ultra-luminous X-ray sources. Comparisons with stellar synthesis models suggest that the diversity of the spectrum shapes arises from differences in the stellar age. If galaxies at $z\gtrsim 6$ are similar to the EMPGs, high energy ($>54.4$ eV) photons of the non-stellar sources negligibly contribute to cosmic reionization due to relatively weak radiation.