We have found a sample of extreme emission-line galaxies ( EELGs ) with strong [ OIII ] \lambda 5007 emission at z \sim 0.5 .
Using broadband photometric selection and requiring small uncertainties in photometry , we searched the 14th Data Release ( DR14 ) of the Sloan Digital Sky Survey ( SDSS ) and found 2658 candidates with strong i -band excess ( i - z \leqslant r - i - 0.7 ) .
We further obtained 649 SDSS spectra of these objects , and visually identified 22 [ OIII ] emitters lying at 0.40 < z < 0.63 .
Having constructed their UV-IR spectral energy distributions ( SED ) we found that they have fairly blue r -W2 and red W1-W4 colors , indicative of strong warm dust emission .
Their rest-frame [ OIII ] \lambda 5007 equivalent widths are mostly 200-600 Å , and their high [ OIII ] \lambda 5007/H \beta ratios put them at the boundary of star-forming galaxies and AGNs on line ratio classification diagrams .
The typical E ( B–V ) and electron temperature of [ OIII ] emitters are \sim 0.1-0.3 mag and \sim 10 ^ { 4 } K , respectively .
The lowest metallicity of our [ OIII ] emitters with S/N _ { [ OIII ] \lambda 4363 } > 3 is 12 + log ( O/H ) = 7.98 ^ { +0.12 } _ { -0.02 } , with a median value of 8.24 ^ { +0.05 } _ { -0.04 } .
Our [ OIII ] emitters exhibit remarkably high line luminosity – 18/22 have L _ { [ OIII ] \lambda 5007 } > 5 \times 10 ^ { 42 } erg s ^ { -1 } and 5/22 have L _ { [ OIII ] \lambda 5007 } > 10 ^ { 43 } erg s ^ { -1 } .
Their estimated volume number density at z \sim 0.5 is \sim 2 \times 10 ^ { -8 } Mpc ^ { -3 } , with L _ { [ OIII ] \lambda 5007 } down to \sim 3 \times 10 ^ { 42 } erg s ^ { -1 } .
The cumulative number fraction distribution of EELGs across different redshifts is indicative of a strong redshift evolution at the bright end of the [ OIII ] luminosity function .