Integrated Characteristics of SDSS DR14 Star-Forming Galaxies with Extremely Low Oxygen Abundances

Abstract

Integrated characteristics of a sample of 66 star-forming galaxies with extremely low oxygen abundances from the SDSS Data Release 14 are studied. The oxygen abundances were determined by the direct Te method for 42 galaxies with detected [O III] 436.3 nm emission lines and by the strong-line method for the remaining galaxies. Derived abundances 12 + log (O/H) fall within the 6.97–7.52 range and are, on average, four times lower than the corresponding values for a large comparison sample of compact star-forming galaxies from the SDSS. Stellar masses and Hβ luminosities for both samples were derived from SDSS spectra with a small spectroscopic aperture (2–3 arcsec in diameter). In order to determine their values for the entire galaxy, aperture corrections taking radiation outside the spectroscopic slit into account were introduced. Stellar masses and luminosities in the optical range are 100 times lower than the corresponding values for the galaxies from the comparison sample. At fixed values of luminosity and stellar mass, these galaxies have lower oxygen abundances in the oxygen abundance–luminosity and oxygen abundance–stellar mass diagrams than the galaxies from the main SDSS sample. This offset is likely caused by accretion of unenriched intergalactic gas, which reduces the oxygen abundance in the galactic interstellar medium. The majority of galaxies with extremely low oxygen abundances were detected in the mid-infrared range by the WISE space telescope. Color index W1 – W2, where W1 and W2 are the magnitudes at a wavelength of 3.4 and 4.6 μm, of these galaxies corresponds to the values typical for stellar emission and/or free-free ionized gas emission. The low ultraviolet luminosity, which is the main source of dust heating in star-forming galaxies, thus eliminates the possibility that warm and hot dust is present.