Extreme [OIII] Emitters at z $\sim$ 0.5 (1804.11309v1)
Abstract: 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.