Metallicity Gradients in Local Star-Forming Galaxies

Abstract

The gas-phase metallicity is one of the most fundamental properties of a galaxy. Measuring the gas-phase metallicity distribution allows us to gauge the age of the gas and hence how the galaxy has formed over time. By combining the gas-phase metallicity with galaxy evolution simulations, we can gain insight into dynamical processes which may have affected its formation process in the past.

In this thesis, we use spatially resolved spectra from the SAMI galaxy survey and TYPHOON survey to explore the gas-phase metallicity of galaxies as a function of stellar mass, environment and star-formation rate. We also investigate the systematic errors and reliability of measuring gas-phase metallicity through popular strong emission line diagnostics.

In the second chapter of this thesis, we present gas-phase metallicity and ionization parameter maps of 25 star-forming face-on spiral galaxies from the SAMI galaxy survey. We measure the metallicity gradients of each galaxy and find a weak mass dependence of the metallicity gradients ranging from -0.20 to -0.03 dex/Re. No significant trends were found in the ionization parameter distribution with the ionization parameter typically ranging between 7.0 < log(q) < 7.8. Ionization parameter variations of this magnitude may lead to systematic deviations of up to 0.3 dex when using the O3N2 metallicity diagnostic.

It is known that metallicity gradients are significantly flattened when undergoing merger activities. This is due to the mixing of gas through tidal forces as well as inflows of pristine gas. In the third chapter, we compare the metallicity gradients of galaxies in isolated environments to those in denser environments and find no significant trends with any of the three environment density metrics tested (fifth nearest neighbour, number of galaxies within a cylinder and the average Gaussian ellipsoid density parameter).

In the fourth chapter, we discuss the reliability of measuring gas-phase metallicity using strong emission line diagnostics and analyse the large systematic differences between them. Using 13 popular strong emission line diagnostics, we provide a method for converting metallicity gradients derived from different metallicity diagnostics, allowing for the comparison of metallicity gradients between different galaxy surveys and redshift ranges.

In the fifth chapter, we use the highly spatially-resolved TYPHOON data of M83 to explore the consequences of determining metallicity gradients from relatively low spatially-resolved data. We find that the emission of the diffuse ionized gas (DIG) significantly flattens metallicity gradients measured using the R23 and N2O2 metallicity diagnostics, has a small flattening effect on the N2HA and O3N2 metallicity diagnostics and significantly steepens the N2S2HA metallicity diagnostic at the kiloparsec resolution of typical multiplexing surveys.

The works within this thesis aim to inform the readers of the caveats and precautions that need to be taken when measuring the gas-phase metallicity. We hope that these articles provide an initial framework, off which improvements can be made to the way we measure and interpret the gas-phase metallicity distribution. Show more