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 \pm 0.026 is among the lowest ever observed for a SFG .
Its absolute magnitude M _ { g } = - 17.35 mag makes it the brightest galaxy among the known BCDs with 12 + log O/H \la 7.3 .
With its low metallicity , low stellar mass M _ { \star } = 10 ^ { 7.13 } M _ { \odot } and very low mass-to-light ratio M _ { \star } / L _ { g } \sim 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 \sim 100 Gyr ^ { -1 } , indicating very active ongoing star-formation .
Its spectrum shows a strong He ii \lambda 4686 emission line , with a flux \sim 2.4 per cent that of the H \beta emission line .
The most probable source of ionizing radiation for producing such a strong line is fast radiative shocks .
J1234 + 3901 has a ratio O _ { 32 } = [ O iii ] 5007/ [ O ii ] 3727 \sim 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 .