J1234+3901: an extremely metal-deficient compact star-forming dwarf galaxy at redshift 0.133. (arXiv:1812.07917v1 [astro-ph.GA])
<a href="http://arxiv.org/find/astro-ph/1/au:+Izotov_Y/0/1/0/all/0/1">Y. I. Izotov</a> (1), <a href="http://arxiv.org/find/astro-ph/1/au:+Thuan_T/0/1/0/all/0/1">T. X. Thuan</a> (2), <a href="http://arxiv.org/find/astro-ph/1/au:+Guseva_N/0/1/0/all/0/1">N. G. Guseva</a> (1) ((1) Bogolyubov Institute for Theoretical Physics, National Academy of Sciences of Ukraine, Kyiv, Ukraine, (2) Astronomy Department, University of Virginia, Charlottesville, USA)

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.

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.

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