The SAMI Galaxy Survey: Spatially Resolved Metallicity and Ionization Mapping

TL;DR

This study maps gas-phase metallicity and ionization parameters in 25 spiral galaxies, revealing weak metallicity gradients and highlighting the importance of considering ionization variations in metallicity diagnostics.

Contribution

First simultaneous mapping of metallicity and ionization parameters in a large galaxy sample, accounting for their interdependence and spatial variations.

Findings

Metallicity gradients range from -0.03 to -0.20 dex/R_e.

Ionization parameter shows no strong correlation with SFR or metallicity.

Ionization parameter varies within a range of 7.0 to 7.8 in log scale.

Abstract

We present gas-phase metallicity and ionization parameter maps of 25 star-forming face-on spiral galaxies from the SAMI Galaxy Survey Data Release 1. Self-consistent metallicity and ionization parameter maps are calculated simultaneously through an iterative process to account for the interdependence of the strong emission line diagnostics involving ([OII]+[OIII])/H$\beta$ (R23) and [OIII]/[OII] (O32). The maps are created on a spaxel-by-spaxel basis because HII regions are not resolved at the SAMI spatial resolution. We combine the SAMI data with stellar mass, star formation rate (SFR), effective radius (R$_e$), ellipticity, and position angles (PA) from the GAMA survey to analyze their relation to the metallicity and ionization parameter. We find a weak trend of steepening metallicity gradient with galaxy stellar mass, with values ranging from -0.03 to -0.20 dex/R$_e$. Only two galaxies show radial gradients in ionization parameter. We find that the ionization parameter has no significant correlation with either SFR, sSFR (specific star formation rate), or metallicity. For several individual galaxies we find structure in the ionization parameter maps suggestive of spiral arm features. We find a typical ionization parameter range of $7.0 < \log(q) < 7.8$ for our galaxy sample with no significant overall structure. An ionization parameter range of this magnitude is large enough to caution the use of metallicity diagnostics which have not considered the effects of a varying ionization parameter distribution.