Low-redshift lowest-metallicity star-forming galaxies in the SDSS DR14
  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 [OII]3727, [OIII]4959, and [OIII]5007 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 with12+logO/H>7.5. The third, the direct Te
method, was applied for galaxies with detected [OIII]4363 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 [OIII]4363 emission line. In total, our list of selected candidates
for extremely low-metallicity galaxies includes 66 objects.
