Metallicity Gradients in Local Star-Forming Galaxies

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

Starburst-AGN mixing: TYPHOON observations of NGC 1365, NGC 1068, and the effect of spatial resolution on the AGN fraction

We demonstrate a robust method of resolving the star-formation and AGN contributions to emission lines using two very well known AGN systems: NGC 1365, and NGC 1068, using the high spatial resolution data from the TYPHOON/PrISM survey. We expand the previous method of calculating the AGN fraction by using theoretical-based model grids rather than empirical points. The high spatial resolution of the TYPHOON/PrISM observations show evidence of both star formation and AGN activity occurring in the nuclei of the two galaxies. We rebin the data to the lower resolutions, typically found in other integral field spectroscopy surveys such as SAMI, MaNGA, and CALIFA. The results show that when rebinned from the native resolution of TYPHOON (< 200 pc/pixel) to 1 kpc/pixel, the effects include a roughly 3 kpc increase in the radius of measured AGN activity, and a factor of 2 to 7 increase in the detection of low surface brightness features such as shocks. All of this information is critical, because information on certain physical processes may be lost at varying resolutions. We make recommendations for analysing data at current IFU survey resolutions.Comment: 30 pages, 28 figures, accepted for publication by MNRA

The Effects of Diffuse Ionized Gas and Spatial Resolution on Metallicity Gradients: TYPHOON Two-Dimensional Spectrophotometry of M83

We present a systematic study of the diffuse ionized gas (DIG) in M83 and its effects on the measurement of metallicity gradients at varying resolution scales. Using spectrophotometric data cubes of M83 obtained at the 2.5m duPont telescope at Las Campanas Observatory as part of the TYPHOON program, we separate the HII regions from the DIG using the [SII]/H$\alpha$ ratio, HIIphot (HII finding algorithm) and the H$\alpha$ surface brightness. We find that the contribution to the overall H$\alpha$ luminosity is approximately equal for the HII and DIG regions. The data is then rebinned to simulate low-resolution observations at varying resolution scales from 41 pc up to 1005 pc. Metallicity gradients are measured using five different metallicity diagnostics at each resolution. We find that all metallicity diagnostics used are affected by the inclusion of DIG to varying degrees. We discuss the reasons of why the metallicity gradients are significantly affected by DIG using the HII dominance and emission line ratio radial profiles. We find that applying the [SII]/H$\alpha$ cut will provide a closer estimate of the true metallicity gradient up to a resolution of 1005 pc for all metallicity diagnostics used in this study.Comment: 26 pages, 12 figures + Appendix/Supplementary Material, accepted for publication by MNRA

The effects of diffuse ionized gas and spatial resolution on metallicity gradients: TYPHOON two-dimensional spectrophotometry of M83

We present a systematic study of the diffuse ionized gas (DIG) in M83 and its effects on the measurement of metallicity gradients at varying resolution scales. Using spectrophotometric data cubes of M83 obtained at the 2.5m duPont telescope at Las Campanas Observatory as part of the TYPHOON programme, we separate the H II regions from the DIG using the [S II]/H α ratio, HIIPHOT (H II-finding algorithm), and the H α surface brightness. We find that the contribution to the overall H α luminosity is approximately equal for the H II and DIG regions. The data is then rebinned to simulate low-resolution observations at varying resolution scales from 41 pc up to 1005 pc. Metallicity gradients are measured using five different metallicity diagnostics at each resolution. We find that all metallicity diagnostics used are affected by the inclusion of DIG to varying degrees. We discuss the reasons why the metallicity gradients are significantly affected by DIG using the H II dominance and emission line ratio radial profiles. We find that applying the [S II]/H α cut will provide a closer estimate of the true metallicity gradient up to a resolution of 1005 pc for all metallicity diagnostics used in this study.Parts of this research were conducted by the Australian Research Council Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D), through project number CE170100013. BG gratefully acknowledges the support of the Australian Research Council as the recipient of a Future Fellowship (FT140101202)

The Chemical Evolution Carousel of Spiral Galaxies : Azimuthal Variations of Oxygen Abundance in NGC1365

19 pages, 13 figures. Accepted to ApJThe spatial distribution of oxygen in the interstellar medium of galaxies is the key to understanding how efficiently metals that are synthesized in massive stars can be redistributed across a galaxy. We present here a case study in the nearby spiral galaxy NGC1365 using 3D optical data obtained in the TYPHOON Program. We find systematic azimuthal variations of the HII region oxygen abundance imprinted on a negative radial gradient. The 0.2 dex azimuthal variations occur over a wide radial range of 0.3 to 0.7 R25 and peak at the two spiral arms in NGC1365. We show that the azimuthal variations can be explained by two physical processes: gas undergoes localized, sub-kpc scale self-enrichment when orbiting in the inter-arm region, and experiences efficient, kpc scale mixing-induced dilution when spiral density waves pass through. We construct a simple chemical evolution model to quantitatively test this picture and find that our toy model can reproduce the observations. This result suggests that the observed abundance variations in NGC1365 are a snapshot of the dynamical local enrichment of oxygen modulated by spiral-driven, periodic mixing and dilution.Peer reviewedFinal Published versio

The SAMI Galaxy Survey: spatially resolved metallicity and ionization mapping

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 ([O II]+[O III])/Hβ (R23) and [O III]/[O II](O32). The maps are created on a spaxel-by-spaxel basis because H II regions are not resolved at the SAMI spatial resolution. We combine the SAMI data with stellar mass, star formation rate (SFR), effective radius (Re), ellipticity, and position angles (PA) from the GAMA survey to analyse 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/Re. Only two galaxies show radial gradients in the ionization parameter. We find that the ionization parameter has no significant correlation with either SFR, sSFR (specific SFR), or metallicity. For several individual galaxies, we find the 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 that have not considered the effects of a varying ionization parameter distribution

The SAMI Galaxy Survey: Data Release Two with absorption-line physics value-added products

We present the second major release of data from the Sydney – Australian Astronomical Observatory Multi-Object Integral Field Spectrograph (SAMI) Galaxy Survey. Data Release Two includes data for 1559 galaxies, about 50 per cent of the full survey. Galaxies included have a redshift range 0.004 <z< 0.113 and a large stellar mass range 7.5 < log (M/M) < 11.6. The core data for each galaxy consist of two primary spectral cubes covering the blue and red optical wavelength ranges. For each primary cube, we also provide three spatially binned spectral cubes and a set of standardized aperture spectra. For each core data product, we provide a set of value-added data products. This includes all emission line value-added products from Data Release One, expanded to the larger sample. In addition, we include stellar kinematic and stellar population value-added products derived from absorption line measurements. The data are provided online through Australian Astronomical Optics’ Data Central. We illustrate the potential of this release by presenting the distribution of ∼350 000 stellar velocity dispersion measurements from individual spaxels as a function of R/Re, divided in four galaxy mass bins. In the highest stellar mass bin [log (M/M) > 11], the velocity dispersion strongly increases towards the centre, whereas below log (M/M) < 10 we find no evidence for a clear increase in the central velocity dispersion. This suggests a transition mass around log (M/M) ∼ 10 for galaxies with or without a dispersion-dominated bulge.NS acknowledges support of a University of Sydney Postdoctoral Research Fellowship. JvdS is funded under Bland-Hawthorn’s ARC Laureate Fellowship (FL140100278). SMC acknowledges the support of an Australian Research Council Future Fellowship (FT100100457). BG is the recipient of an Australian Research Council Future Fellowship (FT140101202). MSO acknowledges the funding support from the Australian Research Council through a Future Fellowship (FT140100255). Support for AMM is provided by NASA through Hubble Fellowship grant #HST-HF2- 51377 awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract NAS5-26555. CFe gratefully acknowledges funding provided by the Australian Research Council’s Discovery Projects (grants DP150104329 and DP170100603). SB acknowledges the funding support from the Australian Research Council through a Future Fellowship (FT140101166). TMB is supported by an Australian Government Research Training Program Scholarship. MLPG acknowledges the funding received from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No 707693

SAMI Galaxy Survey: Spatially resolved metallicity and ionization mapping

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 ([O II]+[O III])/Hβ (R_(23)) and [O III]/[O II](O32). The maps are created on a spaxel-by-spaxel basis because H II 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 analyse 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 the ionization parameter. We find that the ionization parameter has no significant correlation with either SFR, sSFR (specific SFR), or metallicity. For several individual galaxies, we find the 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 that have not considered the effects of a varying ionization parameter distribution

The SAMI Galaxy Survey: Data Release Two with absorption-line physics value-added products

Instrumentatio

The SAMI Galaxy Survey: The third and final data release

We have entered a new era where integral-field spectroscopic surveys of galaxies are sufficiently large to adequately sample large-scale structure over a cosmologically significant volume. This was the primary design goal of the SAMI Galaxy Survey. Here, in Data Release 3, we release data for the full sample of 3068 unique galaxies observed. This includes the SAMI cluster sample of 888 unique galaxies for the first time. For each galaxy, there are two primary spectral cubes covering the blue (370-570 nm) and red (630-740 nm) optical wavelength ranges at spectral resolving power of R = 1808 and 4304, respectively. For each primary cube, we also provide three spatially binned spectral cubes and a set of standardized aperture spectra. For each galaxy, we include complete 2D maps from parametrized fitting to the emission-line and absorption-line spectral data. These maps provide information on the gas ionization and kinematics, stellar kinematics and populations, and more. All data are available online through Australian Astronomical Optics Data Central