Diffuse ionized gas and its effects on nebular metallicity estimates of star-forming galaxies

• Published in: arXiv.org
• Main Authors: N Vale Asari, Couto, G S, R Cid Fernandes, Stasińska, G, de Amorim, A L, Ruschel-Dutra, D, Werle, A, Florido, T Z
• Format: Paper Journal Article
• Language: English
• Published: Ithaca Cornell University Library, arXiv.org 02.09.2019
• Subjects: Abundance; Apertures; Emission; Galaxies; Mapping; Metallicity; Physics - Astrophysics of Galaxies; Sky surveys (astronomy); Star & galaxy formation; Star formation rate; Stellar mass
• ISSN: 2331-8422
• DOI: 10.48550/arxiv.1907.08635

We investigate the impact of the diffuse ionized gas (DIG) on abundance determinations in star-forming (SF) galaxies. The DIG is characterised using the H\(\alpha\) equivalent width (\(W_{\text{H}\alpha}\)). From a set of 1,409 SF galaxies from the Mapping Nearby Galaxies at APO (MaNGA) survey, we calculate the fractional contribution of the DIG to several emission lines using high-\(S/N\) data from SF spaxels (instead of using noisy emission-lines in DIG-dominated spaxels). Our method is applicable to spectra with observed \(W_{\text{H}\alpha} \gtrsim 10\) angstroms (which are not dominated by DIG emission). Since the DIG contribution depends on galactocentric distance, we provide DIG-correction formulae for both entire galaxies and single aperture spectra. Applying those to a sample of \(\,> 90,000\) SF galaxies from the Sloan Digital Sky Survey, we find the following. (1) The effect of the DIG on strong-line abundances depends on the index used. It is negligible for the ([O III]/H\(\beta\))/([N II]/H\(\alpha\)) index, but reaches \(\sim 0.1\) dex at the high-metallicity end for [N II]/H\(\alpha\). (2) This result is based on the \(\sim\)kpc MaNGA resolution, so the real effect of the DIG is likely greater. (3) We revisit the mass-metallicity-star formation rate (SFR) relation by correcting for the DIG contribution in both abundances and SFR. The effect of DIG removal is more prominent at higher stellar masses. Using the [N II]/H\(\alpha\) index, O/H increases with SFR at high stellar mass, contrary to previous claims.