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