Extreme [OIII] Emitters at z $\sim$ 0.5

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 $ $ 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$_{\rm [OIII]\lambda5007}$ $>$ 5$\times$10$^{42}$ erg $\rm s^{-1}$ and 5/22 have L$_{\rm [OIII]\lambda5007}$ $>$ 10$^{43}$ erg $\rm s^{-1}$. Their estimated volume number density at z $\sim$ 0.5 is $\sim$ 2$\times$10$^{-8}$ Mpc$^{-3}$, with L$_{\rm [OIII]\lambda5007}$ down to $\sim$ 3$\times$10$^{42}$ erg $\rm s^{-1}$. 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.