Extreme [O iii] Emitters at z ∼ 0.5

Abstract

Abstract We have found a sample of extreme emission-line galaxies (EELGs) with strong [O iii]λ5007 emission at z ∼ 0.5. Using broadband photometric selection and requiring small uncertainties in photometry, we searched the 14th Data Release of the Sloan Digital Sky Survey (SDSS) and found 2658 candidates with strong i-band excess (i – z ≤ r – i – 0.7). We further obtained 649 SDSS spectra of these objects, and visually identified 22 [O iii] emitters lying at 0.40 < z < 0.63. Having constructed their ultraviolet–infrared spectral energy distributions, we found that they have fairly blue r – W2 and red W1 – W4 colors, indicative of strong, warm dust emission. Their rest-frame [O iii]λ5007 equivalent widths are mostly 200–600 Å, and their high [O iii]λ5007/Hβ ratios put them at the boundary of star-forming galaxies and active galactic nuclei on line ratio classification diagrams. The typical E(B–V) and electron temperature of [O iii] emitters are ∼0.1–0.3 mag and ∼104 K, respectively. The lowest metallicity of our [O iii] emitters with S/N[O iii]λ4363 > 3 is 12 + log(O/H) = , with a median value of . Our [O iii] emitters exhibit remarkably high line luminosity—18/22 have L [O iii]λ5007 > 5 × 1042 erg s−1 and 5/22 have L [O iii]λ5007 > 1043 erg s−1. Their estimated volume number density at z ∼ 0.5 is ∼2 × 10−8 Mpc−3, with L [O iii]λ5007 down to ∼3 × 1042 erg s−1. The cumulative number distribution of EELGs across different redshifts is indicative of a strong redshift evolution at the bright end of the [O iii] luminosity function.