J2229+2725: an extremely low-metallicity dwarf compact star-forming galaxy with an exceptionally high [O iii]λ5007/[O ii]λ3727 flux ratio of 53

Using the Large Binocular Telescope (LBT)/Multi-Object Dual Spectrograph (MODS), we have obtained optical spectroscopy of one of the most metal-poor dwarf star-forming galaxies (SFG) in the local Universe, J2229+2725. This galaxy with a redshift z=0.0762 was selected from the Data Release 16 (DR16) of the Sloan Digital Sky Survey (SDSS). Its properties derived from the LBT observations are most extreme among SFGs in several ways. Its oxygen abundance 12+logO/H=7.085+/-0.031 is among the lowest ever observed for a SFG. With its very low metallicity, an absolute magnitude Mg=-16.39 mag, a low stellar mass Mstar=9.1x10^6 Msun and a very low mass-to-light ratio Mstar/Lg~0.0166 (in solar units), J2229+2725 deviates strongly from the luminosity-metallicity relation defined by the bulk of the SFGs in the SDSS. J2229$+$2725 has a very high specific star-formation rate sSFR~75 Gyr^-1, indicating very active ongoing star formation. Three other features of J2229+2725 are most striking, being the most extreme among lowest-metallicity SFGs: 1) a ratio O32=I([OIII]5007)/I([OII]3727)~53, 2) an equivalent width of the Hbeta emission line EW(Hbeta) of 577A, and 3) an electron number density of ~1000 cm^-3. These properties imply that the starburst in J2229+2725 is very young. Using the extremely high O32 in J2229+2725, we have improved the strong-line calibration for the determination of oxygen abundances in the most metal-deficient galaxies, in the range 12 + logO/H<7.3.