Low-redshift lowest-metallicity star-forming galaxies in the SDSS DR14

Abstract

We present a sample of low-redshift (z < 0.133) candidates for extremely low-metallicity star-forming galaxies with oxygen abundances 12 + logO/H < 7.4 selected from the Data Release 14 (DR14) of the Sloan Digital Sky Survey (SDSS). Three methods are used to derive their oxygen abundances. Among these methods two are based on strong [O II]λ3727Å, [O III]λ4959Å, and [O III]λ5007Å emission lines, which we call strong-line and semi-empirical methods. These were applied for all galaxies. We have developed one of these methods, the strong-line method, in this paper. This method is specifically focused on the accurate determination of metallicity in extremely low-metallicity galaxies and may not be used at higher metallicities with 12 + logO/H > ∼ 7.5. The third, the direct T e method, was applied for galaxies with detected [O III] λ4363 emission lines. All three methods give consistent abundances and can be used in combination or separately for selection of lowest-metallicity candidates. However, the strong-line method is preferable for spectra with a poorly detected or undetected [O III]λ4363 emission line. In total, our list of selected candidates for extremely low-metallicity galaxies includes 66 objects.

J2229+2725: an extremely low-metallicity dwarf compact star-forming galaxy with an exceptionally high [OIII]5007/[OII]3727 flux ratio of 53

“…The dependences of 12 + logO/H on various combinations of oxygen line intensities are illustrated in Fig. 8, where we basically show the data of Izotov et al (2019b), adding the two most recently discovered XMD galaxies, J1631+4426 by Kojima et al (2020) and J2229+2725 discussed here. The oxygen abundances of all galaxies in Fig.…”

“…Many authors, including most recently e.g. Izotov et al (2019b), have developed a so-called "strong-line method" with the use of strong oxygen lines, which is calibrated using high-quality observations of the most metalpoor galaxies with a well-detected [O iii]λ4363 emission line.…”

“…To minimise the data scatter, Izotov et al (2019b) have proposed to use instead the diagram log(R23 -a0O32) -(12 + logO/H), with a0 = 0.080 (Fig. 8b).…”

“…The galaxy was selected from the SDSS Data Release 16 (DR16) data base (Ahumada et al 2020) as a candidate to be an extremely low-metallicity object, using criteria such as the simultaneous presence of a low [O iii]λ5007/Hβ ratio of 3 and of a weak or undetected [N ii]λ6584 emission line with a flux 1/100 that of the Hα emission line (e.g. Izotov et al 2019b). We also selected J2229+2725 because of its extremely high O32 value, defined as the ratio I([O iii]λ5007)/I([O ii]λ3727).…”

Untitled

“…çâåçäíûå âåëè÷èíû íà äëèíàõ âîëí 3.4, 4.6 è 12 ìêì èç êàòàëîãà WISE, FUV -çâåçäíàÿ âåëè÷èíà íà äëèíå âîëíû 0.152 ìêì èç êàòàëîãà GALEX, g -çâåçäíàÿ âåëè÷èíà íà äëèíå âîëíû 0.4825 ìêì èç êàòàëîãà SDSS. Òî÷êàìè ïðåäñòàâëåíû: íà ôðàãìåíòàõ à è â -ãàëàêòèêè èç 14-ãî âûïóñêà êàòàëîãà SDSS[15], èìåþùèå ýêâèâàëåíòíóþ øèðèíó EW ( ) H W3) ïðèâåäåíû ïîëîaeåíèÿ ãàëàêòèê, çàðåãèñòðèðîâàííûõ íà äëèíàõ âîëí 0.152, 0.4825, 3.4 è 12 ìêì. Âèäíî, ÷òî ãàëàêòèêè, èìåþùèå ëèáî îäíîâðåìåííî ìàëûå çíà÷åíèÿ L( ) H çâåçäîîáðàçîâàíèÿ ÿâëÿþòñÿ îñíîâíûì èñòî÷íèêîì íàãðåâà ãîðÿ ÷åé ïûëè.…”