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

TL;DR

This paper shows that aperture effects, rather than the diffuse ionized gas, primarily cause differences in emission line ratios between local HII regions and distant star-forming galaxies, impacting metallicity measurements.

Contribution

It demonstrates that aperture effects are the main factor influencing emission line ratios, reducing the perceived impact of DIG on integrated galaxy spectra.

Findings

Aperture effects explain most differences in emission line ratios.

DIG's influence on integrated spectra is less significant than previously thought.

Strong-line metallicity methods remain reliable when aperture effects are properly considered.

Abstract

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 HII regions. Comparing the emission of [SII]6717,31 between pointed observations of HII 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 HII regions. Here we show instead that the effect of DIG on the integrated spectra of star-forming galaxies is lower than assumed in previous work. Indeed, aperture effects on the spectroscopy of nearby HII regions are largely responsible for the observed difference: when spectra of local HII 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.