EVOLUTION OF THE STELLAR MASS–GAS-PHASE METALLICITY RELATION OVER SEVEN GIGAYEARS

Abstract

Galaxy evolution can be understood by measuring characteristic properties of the diffuse, ionized gas within galaxies. The gas-phase metallicity (Z), or the chemical content, is one measurement that can tell us about the cumulative star formation in a galaxy while taking into account galactic gas flows. In my Honors thesis, I analyzed ∼4,000 galaxy spectra at redshift (z) ≈ 1 from the DEEP2 Galaxy Redshift Survey to derive metallicities for galaxies. Unlike previous studies, my analyses utilized the electron temperature (Te) method to determine more robust measurements of gas temperature and metallicity. The Te method uses the [Oiii]λ4363 emission line as a direct Te probe. However, because of its weak signal, I used a stacking approach to combine the spectra of hundreds of individual galaxies of similar stellar masses (M) to increase the detection of [Oiii]λ4363. I constructed six robust stellar mass composites: four with [Oiii]λ4363 detections and two with robust non-detections of [Oiii]λ4363. With derived average metallicities for different stellar mass composite spectra, I then constructed a M–Z relation that is ∼0.1 dex lower in metallicity at a given stellar mass than the relation for local galaxies (Andrews & Martini 2013). This result indicates that chemical enrichment in galaxies has increased by ∼0.1 dex in the past seven gigayears of evolution.