2017A&A...599A..65G

We present a spectroscopic study of metal-deficient dwarf galaxy candidates, selected from the SDSS DR12. The oxygen abundances were derived using the direct method in galaxies with the electron temperature-sensitive emission line [OIII]λ4363 Å measured with an accuracy better than 30%. The oxygen abundances for the remaining galaxies with larger uncertainties of the [OIII]λ4363 Å line fluxes were calculated using a strong-line semi-empirical method by Izotov and Thuan. The resulting sample consists of 287 low-metallicity candidates with oxygen abundances below 12+logO/H=7.65 including 23 extremely metal-deficient (XMD) candidates with 12+logO/H≤7.35. Ten out of sixteen XMDs known so far (or ∼60%) have been discovered by our team using the direct method. Three XMDs were found in the present study. We study relations between global parameters of low-metallicity galaxies, including absolute optical magnitudes, Hβ luminosities (or equivalently star formation rates), stellar masses, mid-infrared colours, and oxygen abundances. Low-metallicity and XMD galaxies strongly deviate to lower metallicities in L-Z, L(Hβ)-Z and M_*-Z diagrams than in relations obtained for large samples of low-redshift, star-forming galaxies with non-restricted metallicities. These less chemically evolved galaxies with stellar masses ~=10⁶-10⁸M_☉, Hβ luminosities ~=10³⁸-10⁴¹erg/s, SFR~=0.01-1.0M_☉/yr, and sSFR∼50Gyr^–1 have physical conditions which may be characteristic of high-redshift low-mass star-forming galaxies which are still awaiting discovery.