Searching for metal-deficient emission- line galaxy candidates: the final sample of the SDSS DR12 galaxies

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 [O III] lambda 4363 angstrom measured with an accuracy better than 30%. The oxygen abundances for the remaining galaxies with larger uncertainties of the [O III] angstrom 4363 line fluxes were calculated using a strong-line semi-empirical method by Izotov and Thuan. The resulting sample consists of 287 lowmetallicity candidates with oxygen abundances below 12 + log O/H = 7.65 including 23 extremely metal-deficient (XMD) candidates with 12 + log O/H less than or similar to 7.35. Ten out of sixteen XMDs known so far (or similar to 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, H beta 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(H beta)- 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 approximate to 10(6)- 10(8) M-circle dot, H beta luminosities approximate to 10(38)- 10(41) erg s(-1), SFR approximate to 0.01-1.0 M-circle dot yr (-1), and sS FR similar to 50 Gyr (-1) have physical conditions which may be characteristic of high-redshift low-mass star-forming galaxies which are still awaiting discovery.