Metallicity, ionization parameter, and pressure variations of HII regions in the TYPHOON spiral galaxies

TL;DR

This study maps metallicity, ionization, and pressure in HII regions of six spiral galaxies, revealing weak azimuthal metallicity variations and strong local correlations, with resolution effects impacting measurements.

Contribution

It provides the first simultaneous, self-consistent maps of metallicity, ionization parameter, and pressure in local spiral galaxies, highlighting the influence of local conditions over spiral structure.

Findings

Weak azimuthal metallicity variations (mean 0.03 dex) observed.

Strong correlation between ionization parameter and local metallicity.

Unresolved observations inflate ionization and pressure diagnostics.

Abstract

We present a spatially-resolved HII region study of the gas-phase metallicity, ionization parameter, and ISM pressure maps of 6 local star-forming and face-on spiral galaxies from the TYPHOON program. Self-consistent metallicity, ionization parameter, and pressure maps are calculated simultaneously through an iterative process to provide useful measures of the local chemical abundance and its relation to localized ISM properties. We constrain the presence of azimuthal variations in metallicity by measuring the residual metallicity offset $\Delta$(O/H) after subtracting the linear fits to the radial metallicity profiles. We however find weak evidence of azimuthal variations in most of the galaxies, with small (mean 0.03 dex) scatter. The galaxies instead reveal that HII regions with enhanced and reduced abundances are found distributed throughout the disk. While the spiral pattern plays a role in organizing the ISM, it alone does not establish the relatively uniform azimuthal variations we observe. Differences in the metal abundances are more likely driven by the strong correlations with the local physical conditions. We find a strong and positive correlation between the ionization parameter and the local abundances as measured by the relative metallicity offset $\Delta$(O/H), indicating a tight relationship between local physical conditions and their localized enrichment of the ISM. Additionally, we demonstrate the impact of unresolved observations on the measured ISM properties by rebinning the datacubes to simulate low-resolution (1 kpc) observations, typical of large IFU surveys. We find that the ionization parameter and ISM pressure diagnostics are impacted by the loss of resolution such that their measured values are larger relative to the measured values on sub-HII region scales.