A novel approach to correcting Te-based mass-metallicity relations

Abstract

Deriving oxygen abundances from the electron temperature (hereafter the Te method) is the gold standard for extragalactic metallicity studies. However, unresolved temperature fluctuations within individual H ii regions and across different H ii regions throughout a galaxy can bias metallicity estimates low, with a magnitude that depends on the underlying and typically unknown temperature distribution. Using a toy model, we confirm that computing Te-based metallicities using the temperature derived from the [O iii] λ4363/λ5007 or [O ii] λλ7320, 7330/[O ii] λλ3727 ratio ('ratio temperature', Tratio) results in an underprediction of metallicity when temperature fluctuations are present. In contrast, using the unobservable 'line temperatures' (Tline) that provide the mean electron and ion density-weighted emissivity yields an accurate metallicity estimate. To correct this bias in low-mass galaxies, we demonstrate an example calibration of a relation between Tratio and Tline based on a high-resolution (4.5 pc) ramses-rtz simulation of a dwarf galaxy that self-consistently models the formation of multiple H ii regions and ion temperature distribution in a galactic context. Applying this correction to the low-mass end of the mass-metallicity relation shifts its normalization up by 0.18 dex on average and flattens its slope from 0.87 to 0.58, highlighting the need for future studies to account for, and correct, this bias.