paper

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

arXiv:1812.07917 · doi:10.1093/mnras/sty3472

Abstract

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.

arXiv comment9 pages, 4 figures, accepted for publication in Monthly Notices of the Royal Astronomical Society

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