The diffuse ionized gas (DIG) contributes to the nebular emission of galaxies, resulting in emission line flux ratios that can be significantly different from those produced by H ii regions. Comparing the emission of [SII]λ6717,31 between pointed observations of H ii regions in nearby galaxies and integrated spectra of more distant galaxies, it has been recently claimed that the DIG can also deeply affect the emission of bright, star-forming galaxies, and that a large correction must be applied to observed line ratios to recover the genuine contribution from H ii regions. Here, we show instead that the e?ect of DIG on the integrated spectra of star-forming galaxies is lower than assumed in previous work. Here we show that, in contrast, aperture effects on the spectroscopy of nearby H ii regions are largely responsible for the observed difference: When spectra of local H ii regions are extracted using large enough apertures while still avoiding the DIG, the observed line ratios are the same as in more distant galaxies. This result is highly relevant for the use of strong-line methods to measure metallicity.

中文翻译：

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恒星形成星系中的扩散电离气体（DIG）：孔径效应对局部 H ii 区域的影响

扩散电离气体 (DIG) 有助于星系的星云发射，导致发射线通量比可能与 H ii 区域产生的明显不同。比较 [SII]λ6717,31 在附近星系中 H ii 区域的定向观测和更遥远星系的综合光谱之间的发射，最近有人声称 DIG 还可以深刻影响明亮的恒星形成星系的发射，并且必须对观察到的线比率进行较大的校正，以恢复 H ii 区域的真正贡献。在这里，我们展示了 DIG 对恒星形成星系的综合光谱的影响低于先前工作中的假设。在这里，我们表明，相比之下，孔径对附近 H ii 区域光谱的影响是观察到差异的主要原因：当使用足够大的孔径提取局部 H ii 区域的光谱同时仍然避免 DIG 时，观察到的线比与更遥远的星系中的相同。该结果与使用强线法测量金属丰度高度相关。