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

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
² Main Astronomical Observatory, Ukrainian National Academy of Sciences, Zabolotnoho 27, Kyiv 03680, Ukraine
e-mail: guseva@mao.kiev.ua
³ Institut für Astrophysik, Göttingen Universität, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
⁴ Astronomy Department, King Abdulaziz University, PO Box 80203, Jeddah 21589, Saudi Arabia

Received: 24 June 2016
Accepted: 2 November 2016

Abstract

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]λ4363 Å measured with an accuracy better than 30%. The oxygen abundances for the remaining galaxies with larger uncertainties of the [O iii]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 + log O/H = 7.65 including 23 extremely metal-deficient (XMD) candidates with 12 + log O/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 ≈10³⁸–10⁴¹ erg s^-1, SFR ≈ 0.01–1.0 M_⊙ yr^-1, and sSFR ~ 50 Gyr^-1 have physical conditions which may be characteristic of high-redshift low-mass star-forming galaxies which are still awaiting discovery.