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We present optical-line gas metallicity diagnostics established by thecombination of local SDSS galaxies and the largest compilation of extremelymetal-poor galaxies (EMPGs) including new EMPGs identified by the SubaruEMPRESS survey. A total of 103 EMPGs are included that cover a large parameterspace of magnitude (Mi=-19 to -7) and H-beta equivalent width (10-600 Ang),i.e., wide ranges of stellar mass and star-formation rate. Using reliablemetallicity measurements of the direct method for these galaxies, we derive therelationships between strong optical-line ratios and gas-phase metallicity overthe range of 12+log(O/H)=~6.9-8.9 corresponding to 0.02-2 solar metallicityZsun. We confirm that R23-index, ([OIII]+[OII])/H-beta, is the most accuratemetallicity indicator with the metallicity uncertainty of 0.14 dex over therange among various popular metallicity indicators. The other metallicityindicators show large scatters in the metal-poor range (<0.1 Zsun). It isexplained by our CLOUDY photoionization modeling that, unlike R23-index, theother metallicity indicators do not use a sum of singly and doubly ionizedlines and cannot trace both low and high ionization gas. We find that theaccuracy of the metallicity indicators is significantly improved, if one usesH-beta equivalent width measurements that tightly correlate with ionizationstates. In this work, we also present the relation of physical properties withUV-continuum slope beta and ionization production rate xi_ion derived withGALEX data for the EMPGs, and provide local anchors of galaxy propertiestogether with the optical-line metallicity indicators that are available in theform of ASCII table and useful for forthcoming JWST spectroscopic studies.

EMPRESS. V. Metallicity Diagnostics of Galaxies over 12+log(O/H)=~6.9-8.9 Established by a Local Galaxy Census: Preparing for JWST Spectroscopy