J1234+3901: an extremely metal-deficient compact star-forming dwarf galaxy at redshift 0.133

We have obtained optical spectroscopy of one of the most metal-poor dwarf star-forming galaxies (SFG) in the local Universe, J1234+3901, with the Large Binocular Telescope (LBT)/Multi-Object Dual Spectrograph (MODS). This blue compact dwarf (BCD) galaxy with a redshift z=0.133 was selected from the Data Release 14 (DR14) of the Sloan Digital Sky Survey (SDSS). Its properties are extreme in many ways. Its oxygen abundance 12 + log O/H = 7.035+/-0.026 is among the lowest ever observed for a SFG. Its absolute magnitude Mg = -17.35 mag makes it the brightest galaxy among the known BCDs with 12 + log O/H < 7.3. With its low metallicity, low stellar mass M* = 10^7.13 Msun and very low mass-to-light ratio M*/Lg ~ 0.01 (in solar units), it deviates strongly from the mass-metallicity and luminosity-metallicity relations defined by the bulk of the SFGs in SDSS DR14. J1234+3901 has a very high specific star-formation rate sSFR ~ 100 Gyr^-1, indicating very active ongoing star-formation. Its spectrum shows a strong HeII 4686 emission line, with a flux ~ 2.4 per cent that of the Hbeta emission line. The most probable source of ionizing radiation for producing such a strong line is fast radiative shocks. J1234+3901 has a ratio O32 = [OIII]5007/[OII]3727 ~ 15, the highest among the lowest-metallicity SFGs, and is thus likely leaking Lyman continuum radiation. It is a good candidate for being a young dwarf galaxy, with a large fraction of its stars formed recently. As such, it is probably one of the best local counterparts of dwarf primeval galaxies responsible for the reionization of the early Universe.