Integrated characteristics of SDSS DR14 star-forming galaxies with extremely low oxygen abundances

1Izotova, IY, 2Izotov, YI
1Taras Shevchenko National University of Kyiv, Kyiv, Ukraine
2Bogolyubov Institute for Theoretical Physics of the National Academy of Sciences of Ukraine, Kyiv, Ukraine
Kinemat. fiz. nebesnyh tel (Online) 2020, 36(2):3-19
Start Page: Extragalactic Astronomy
Language: Ukrainian

We study the integrated characteristics of the sample of 66 star-forming galaxies with extremely low oxygen abundances from the SDSS Data Release 14. Oxygen abundances in 42 studied galaxies with detected [O III] λ 436.3 nm emission line are derived by the direct Temethod. Oxygen abundances for remaining galaxies are derived by the strong-line method. Derived abundances 12 + log(O/H) are in the range 6.97...7.52 and on average are 4 times lower compared to the value for the large comparison sample of compact star-forming galaxies from the SDSS. Stellar masses and Hβ luminosities for both samples are derived from SDSS spectra with small spectroscopic aperture (2 or 3 arcsec in diameter). For the determination of their values for the entire galaxy we introduce aperture corrections, which take into account radiation outside the spectroscopic slit. Stellar masses and luminosities in the optical range of the galaxies with extremely low metallicities are by 100 times lower than the respective values for the galaxies from the comparison sample. Galaxies with extremely low metallicities have lower oxygen abundances on the diagrams oxygen abundance — luminosity and oxygen abundance — stellar mass compared to relations for the main SDSS sample at fixed the luminosity and the stellar mass. Possible cause of this offset is an accretion of unenriched intergalactic gas, which results in reduction of the oxygen abundance in the galaxy interstellar medium. Most of the galaxies with extremely low oxygen abundances were detected in the mid-infrared range with the space telescope WISE at wavelengths 3.4 m and 4.6 m. Colour index W1 – W2, where W1 and W2 are magnitudes at wavelengths 3.4 and 4.6 m, in these galaxies corresponds to the values for emission by stars and/or free-free ionized gaz emission precluding the presence of warm and hot dust due to the low luminosity of ultraviolet radiation, which is the main source of dust heating in star-forming galaxies.

Keywords: chemical composition, dwarf star-forming galaxies, H II regions, infrared radiation, interstellar dust

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