The present-day mass-metallicity relation for galaxies using a new electron-temperature method

We present a study of electron temperatures (Te) and gas-phase oxygen abundances (Z_Te) for galaxies in the local Universe (z<0.25). Our sample comprises spectra from a total of 264 emission-line systems, ranging from individual HII regions to whole galaxies, and including 23 composite HII regions in predominantly 'main-sequence' galaxies from the MaNGA survey. We utilise 130 of these systems with directly measurable T(OII) to derive a new metallicity-dependent T(OIII)-T(OII) relation that provides a better representation of our varied data set than existing relations from the literature. Importantly, we find that all the T(OIII)-T(OII) relations considered here intrinsically under-estimate Z_Te at low O++/O+ by up to 0.6 dex, and provide an empirical correction based on strong-emission lines to account for this bias when using our relation. Our new Te method therefore allows accurate metallicities (1sigma = 0.08 dex) to be derived for any low-redshift system with an [OIII]4363 line detection, regardless of its physical size or ionisation state. This new Te method is then used to form the mass - metallicity relation (MZR) for a set of 118 local star-forming galaxies that is not strongly biased to starbursts. Our new MZR is in very good agreement with those formed from direct measurements of metal recombination lines and blue supergiantabsorption lines, in contrast to most other Te-based and strong-line-based MZRs. Our new Te method therefore provides an accurate and precise way of obtaining metallicities for a large and diverse range of star-forming systems in the local Universe.