Spatially resolved direct method metallicity in a high-redshift analogue local galaxy: temperature structure impact on metallicity gradients

Abstract

We investigate how HII region temperature structure assumptions affect "direct-method" spatially-resolved metallicity observations using multispecies auroral lines in a galaxy from the SAMI Galaxy Survey. SAMI609396B, at redshift $z=0.018$, is a low-mass galaxy in a minor merger with intense star formation, analogous to conditions at high redshifts. We use three methods to derive direct metallicities and compare with strong-line diagnostics. The spatial metallicity trends show significant differences among the three direct methods. Our first method is based on the commonly used electron temperature $T_e$([OIII]) from the [OIII]$\lambda$4363 auroral line and a traditional $T_e$([OII]) -- $T_e$([OIII]) calibration. The second method applies a recent empirical correction to the O${}^{+}$ abundance from the [OIII]/[OII] strong-line ratio. The third method infers the $T_e$([OII]) from the [SII]$\lambda \lambda$4069,76 auroral lines. The first method favours a positive metallicity gradient along SAMI609396B, whereas the second and third methods yield flattened gradients. Strong-line diagnostics produce mostly flat gradients, albeit with unquantified contamination from shocked regions. We conclude that overlooked assumptions about the internal temperature structure of HII regions in the direct method can lead to large discrepancies in metallicity gradient studies. Our detailed analysis of SAMI609396B underlines that high-accuracy metallicity gradient measurements require a wide array of emission lines and improved spatial resolutions in order to properly constrain excitation sources, physical conditions, and temperature structures of the emitting gas. Integral-field spectroscopic studies with future facilities such as JWST/NIRSpec and ground-based ELTs will be crucial in minimising systematic effects on measured gradients in distant galaxies.

21 pages, 14 figures, Accepted for Publication in MNRAS

References in corpus (27)

• The Host Galaxies and Classification of Active Galactic Nuclei
• Overview of the SDSS-IV MaNGA Survey: Mapping Nearby Galaxies at Apache Point Observatory
• Improving the full spectrum fitting method: accurate convolution with Gauss-Hermite functions
• The MAPPINGS III Library of Fast Radiative Shock Models
• Evolutionary Stellar Population Synthesis with MILES. Part I: The Base Models and a New Line Index System
• PyNeb: a new tool for analyzing emission lines. I. Code description and validation of results
• Understanding Galaxy Evolution through Emission Lines
• The SAMI Galaxy Survey: instrument specification and target selection
• The Mass-Metallicity and the Fundamental Metallicity Relation revisited on a fully Te-based abundance scale for galaxies
• New fully empirical calibrations of strong-line metallicity indicators in star forming galaxies
• SDSS-IV MaNGA: The Impact of Diffuse Ionized Gas on Emission-line Ratios, Interpretation of Diagnostic Diagrams, and Gas Metallicity Measurements
• Metallicity gradients in local field star-forming galaxies: Insights on inflows, outflows, and the coevolution of gas, stars and metals
• The SAMI Galaxy Survey: Shocks and Outflows in a normal star-forming galaxy
• The SAMI Galaxy Survey: Early Data Release
• The SAMI Galaxy Survey: Spatially Resolving the Main Sequence of Star Formation
• The SAMI Galaxy Survey: Cubism and covariance, putting round pegs into square holes
• The KLEVER Survey: Spatially resolved metallicity maps and gradients in a sample of 1.2 < z < 2.5 lensed galaxies
• Starburst-AGN mixing: II. Optically-selected active galaxies
• The SAMI Galaxy Survey: Data Release One with Emission-line Physics Value-Added Products
• Dissecting Galaxies: Spatial and Spectral Separation of Emission Excited by Star Formation and AGN Activity
• The Effects of Diffuse Ionized Gas and Spatial Resolution on Metallicity Gradients: TYPHOON Two-Dimensional Spectrophotometry of M83
• Dissecting Galaxies: Separating Star Formation, Shock Excitation and AGN Activity in the Central Region of NGC 613
• Metallicity gradients in small and nearby spiral galaxies
• The SAMI Galaxy Survey: energy sources of the turbulent velocity dispersion in spatially-resolved local star-forming galaxies
• SDSS IV MaNGA: Deep observations of extra-planar, diffuse ionized gas around late-type galaxies from stacked IFU spectra
• The Te[N II]-Te[O III] temperature relation in H II regions and the reliability of strong-line methods
• Metal-poor dwarf galaxies in the SIGRID galaxy sample. I. HII region observations and chemical abundances