SDSS-IV MaNGA: The Impact of Diffuse Ionized Gas on Emission-line Ratios, Interpretation of Diagnostic Diagrams, and Gas Metallicity Measurements

TL;DR

This study investigates how diffuse ionized gas (DIG) affects emission-line measurements, diagnostic diagrams, and metallicity estimates in star-forming galaxies, revealing biases and the importance of evolved stars as ionization sources.

Contribution

It demonstrates the impact of DIG on emission-line diagnostics and metallicity measurements, highlighting the need to account for DIG in galaxy analyses.

Findings

DIG enhances certain emission line ratios at fixed metallicity.

DIG contamination shifts HII regions in diagnostic diagrams towards LI(N)ER-like regions.

Metallicity estimates using N2O2 are least biased by DIG effects.

Abstract

Diffuse Ionized Gas (DIG) is prevalent in star-forming galaxies. Using a sample of 365 nearly face-on star-forming galaxies observed by MaNGA, we demonstrate how DIG in star-forming galaxies impacts the measurements of emission line ratios, hence the interpretation of diagnostic diagrams and gas-phase metallicity measurements. At fixed metallicity, DIG-dominated low H\alpha\ surface brightness regions display enhanced [SII]/H\alpha, [NII]/H\alpha, [OII]/H\beta, and [OI]/H\alpha. The gradients in these line ratios are determined by metallicity gradients and H\alpha\ surface brightness. In line ratio diagnostic diagrams, contamination by DIG moves HII regions towards composite or LI(N)ER-like regions. A harder ionizing spectrum is needed to explain DIG line ratios. Leaky HII region models can only shift line ratios slightly relative to HII region models, and thus fail to explain the composite/LI(N)ER line ratios displayed by DIG. Our result favors ionization by evolved stars as a major ionization source for DIG with LI(N)ER-like emission. DIG can significantly bias the measurement of gas metallicity and metallicity gradients derived using strong-line methods. Metallicities derived using N2O2 are optimal because they exhibit the smallest bias and error. Using O3N2, R23, N2=[NII]/H\alpha, and N2S2H\alpha\ (Dopita et al. 2016) to derive metallicities introduces bias in the derived metallicity gradients as large as the gradient itself. The strong-line method of Blanc et al. (2015; IZI hereafter) cannot be applied to DIG to get an accurate metallicity because it currently contains only HII region models which fail to describe the DIG.