Optical spectroscopy of the extremely metal-deficient star-forming galaxy HSC J1631+4426: a test of the strong-line method

Abstract: Recently, Kojima and co-authors have reported a record low oxygen abundance, 12+logO/H=6.90+/-0.03 in the low-mass star-forming galaxy HSC J1631+4426. This exceptionally low oxygen abundance was obtained by the direct method, using the [OIII]4363 emission line. However, using the strong-line method by Izotov et al. (2019b), these authors have derived a significantly higher metallicity 12+logO/H=7.175+/-0.005. To clarify the situation, we have obtained new observations of HSC J1631+4426 with the Large Binocular Telescope (LBT)/Multi-Object Dual Spectrograph (MODS). We have derived a higher oxygen abundance, 12+logO/H=7.14+/-0.03, using the direct method, a value similar to the oxygen abundance obtained by the strong-line method. Thus, HSC J1631+4426 has a metallicity close to that of the well known blue compact dwarf galaxy IZw18.