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

Abstract

Strong high-ionization lines such as He ii 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 He iiλ4686. 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 ∼ 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 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.

Diagnostics for PopIII galaxies and direct collapse black holes in the early universe

“…It should be noted that various studies have found evidence for significant He nebular emission in populations of metal poor, dwarf star forming galaxies (e.g. Schaerer et al 2019;Umeda et al 2022). The observed equivalent width is about 1 Å, similar to what expected by our models in low metallicity regimes (especially if considering high ionisation parameters, which are indeed typically observed in low metallicity star forming galaxies).…”

“…However, the He /H ratio observed in such low metallicity galaxies is often significantly higher (∼ 0.01) than predicted by our models. This is an issue that is faced also by other photoionisation models using stellar population templates (Senchyna et al 2017(Senchyna et al , 2020Chevallard et al 2018;Umeda et al 2022). Possible explanations that have been proposed include the contribution of X-ray binaries (e.g.…”

Elevated Hot Gas and High-mass X-Ray Binary Emission in Low-metallicity Galaxies: Implications for Nebular Ionization and Intergalactic Medium Heating in the Early Universe

“…The recently established connection between the L X (HMXB)-SFR relation and metallicity has led to a surge in interest in HMXBs as potentially important sources of ionizing radiation in low-metallicity galaxies, and several observations have found enhanced X-ray emission in galaxies selected by extreme ionization signatures, including Lyα and He II emitters, green peas, and Lyman-continuum galaxies (see, e.g., Brorby & Kaaret 2017;Bluem et al 2019;Svoboda et al 2019;Bayliss et al 2020;Dittenber et al 2020;Gross et al 2021). Notably, X-ray emission (particularly from HMXBs) has been implicated as a potential solution to the long-standing problem of how nebular He II is produced in low-metallicity star-forming galaxies (see, e.g., Pakull & Angebault 1986;Schaerer 1996;Shirazi & Brinchmann 2012;Jaskot & Oey 2013;Crowther et al 2016;Senchyna et al 2019;Berg et al 2021;Olivier et al 2021, Umeda et al 2022, for discussions of He II production in galaxies); however, uncertainties in the extrapolations of the intrinsic spectral energy distribution (SED) into the EUV (near the He II ionization energy of 54 eV) has resulted in the connection between He II and X-ray emission being a topic of active debate (e.g., Schaerer et al 2019;Saxena et al 2020;Senchyna et al 2020;Kehrig et al 2021;Rickards Vaught et al 2021;Simmonds et al 2021).…”

EMPRESS. VIII. A New Determination of Primordial He Abundance with Extremely Metal-Poor Galaxies: A Suggestion of the Lepton Asymmetry and Implications for the Hubble Tension

“…Nakajima et al in prep. Xu et al 2021;Umeda et al 2022), we have launched an extended project, EMPRESS 3D (PI: M. Ouchi) that perform optical integral-field spectroscopy (IFS) and Y -band spectroscopy for ∼ 30 EMPGs. The EMPRESS 3D project will provide deep optical spectra via the IFS data cube and weak emission lines including Heiλ10830 at the EMPG luminousity peaks.…”