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The Diffuse Ionized Gas (DIG) contributes to the nebular emission ofgalaxies, resulting in emission line flux ratios that can be significantlydifferent from those produced by HII regions. Comparing the emission of[SII]6717,31 between pointed observations of HII regions in nearby galaxies andintegrated spectra of more distant galaxies, it has been recently claimed thatthe 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 recoverthe genuine contribution from HII regions. Here we show instead that the effectof DIG on the integrated spectra of star-forming galaxies is lower than assumedin previous work. Indeed, aperture effects on the spectroscopy of nearby HIIregions are largely responsible for the observed difference: when spectra oflocal HII regions are extracted using large enough apertures while stillavoiding the DIG, the observed line ratios are the same as in more distantgalaxies. This result is highly relevant for the use of strong-line methods tomeasure metallicity.

The diffuse ionized gas (DIG) in star-forming galaxies: the influence of aperture effects on local HII regions