NGC 628 in SIGNALS: Explaining the Abundance-Ionization Correlation in HII Regions

TL;DR

This study analyzes over 1500 HII regions in NGC 628 to understand the relationship between oxygen abundance and ionization, revealing an anti-correlation influenced by dust and star formation, with implications for modeling assumptions.

Contribution

It provides a detailed empirical analysis of the abundance-ionization relationship in HII regions using extensive observational data and photoionization models, highlighting the complexity and scatter in this correlation.

Findings

Anti-correlation between abundance and ionization parameter observed

Secondary effects of dust extinction and star formation rate surface density identified

Concerns raised about modeling assumptions affecting observed correlations

Abstract

The variations of oxygen abundance and ionization parameter in HII regions are usually thought to be the dominant factors that produced variations seen in observed emission line spectra. However, if and how these two quantities are physically related is hotly debated in the literature. Using emission line data of NGC 628 observed with SITELLE as part of the Star-formation, Ionized Gas, and Nebular Abundances Legacy Survey (SIGNALS), we use a suite of photoionization models to constrain the abundance and ionization parameters for over 1500 HII regions throughout its disk. We measure an anti-correlation between these two properties, consistent with expectations, although with considerable scatter. Secondary trends with dust extinction and star formation rate surface density potentially explain the large scatter observed. We raise concerns throughout regarding various modeling assumptions and their impact on the observed correlations presented in the literature.