Volume 599, March 2017
|Number of page(s)||12|
|Published online||01 March 2017|
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
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
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.
Key words: galaxies: abundances / galaxies: irregular / galaxies: evolution / galaxies: formation / galaxies: ISM / ISM: abundances
© ESO, 2017
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