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 $<$ 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$5007 equivalent widths are mostly 200-600 \AA, and their high
[OIII]$\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$_{\rm [OIII]\lambda4363}$ $>$ 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.
