Extreme [O iii] Emitters at z ∼ 0.5

Abstract

We have found a sample of extreme emission-line galaxies (EELGs) with strong [O iii ] λ 5007 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 ] λ 5007 equivalent widths are mostly 200–600 Å, and their high [O iii ] λ 5007/H β 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 ∼10 4 K, respectively. The lowest metallicity of our [O iii ] emitters with S/N [O iii ] λ 4363 > 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 ] λ 5007 > 5 × 10 42 erg s −1 and 5/22 have L [O iii ] λ 5007 > 10 43 erg s −1 . Their estimated volume number density at z  ∼ 0.5 is ∼2 × 10 −8 Mpc −3 , with L [O iii ] λ 5007 down to ∼3 × 10 42 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.