EMPRESS. VII. Ionizing Spectrum Shapes of Extremely Metal-poor Galaxies: Uncovering the Origins of Strong He ii and the Impact on Cosmic Reionization
EMPRESS. VII. Ionizing Spectrum Shapes of Extremely Metal-poor Galaxies: Uncovering the Origins of Strong He ii and the Impact on Cosmic Reionization
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DOI：
10.3847/1538-4357/ac602d
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发表时间：
2022-01
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影响因子：
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通讯作者：
Hiroya Umeda;M. Ouchi;K. Nakajima;Y. Isobe;S. Aoyama;Y. Harikane;Y. Ono;A. Matsumoto
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文献类型：
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作者：
Hiroya Umeda;M. Ouchi;K. Nakajima;Y. Isobe;S. Aoyama;Y. Harikane;Y. Ono;A. Matsumoto
年轻星系的诸如He ii等强高电离线在高红移和低红移时都令人困惑。尽管近期研究表明存在非热辐射源，但它们的电离光谱能否一致地解释多条主要发射线仍是一个问题。在此，我们推导了三个显示出强He ii λ4686的本地极贫金属星系（EMPG）的电离光谱的一般形状。我们将由黑体和幂律辐射组成的电离光谱参数化，以模拟各种恒星和非热辐射源。我们对星云使用光致电离模型，并通过马尔可夫链蒙特卡罗方法确定电离光谱和星云的七个参数，小心地避免丰度比的系统误差。我们得到了在观测误差范围内能解释约10条主要发射线的电离光谱的一般形状，并且与观测到的X射线和光学连续谱平滑连接。我们发现一个EMPG的电离光谱具有以黑体为主的形状，而其他的在>13.6 eV时具有向下凸的形状，这表明电离光谱形状具有多样性。我们确认向下凸的形状与普通恒星光谱形状有根本不同，并且这些星系的光谱形状一般可由恒星和超亮X射线源的组合来解释。与恒星合成模型的比较表明，光谱形状的多样性源于恒星年龄的差异。如果在z≳6的星系与EMPG相似，由于相对较弱的辐射，非恒星源的高能（>54.4 eV）光子对宇宙再电离的贡献可忽略不计。
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 nonthermal 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 ii λ4686. We parameterize the ionizing spectra composed of a blackbody and power-law radiation mimicking various stellar and nonthermal 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 ultraluminous 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 nonstellar sources negligibly contribute to cosmic reionization due to relatively weak radiation.