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.
