The [O III]λ5007 equivalent width distribution at z ∼2: the redshift evolution of the extreme emission line galaxies

We determine the [O III] λ5007 equivalent width (EW) distribution of 1 . 700 < z < 2 . 274 rest-frame ultraviolet (UV)-selected (M UV < -19) star-forming galaxies in the GOODS North and South fields. We make use of deep HDUV broad-band photometry catalogues for selection and 3D- HST WFC3/IR grism spectra for measurement of line properties. The [O III] λ5007 EW distribution allows us to measure the abundance of extreme emission line galaxies (EELGs) within this population. We model a lognormal distribution to the [O III] λ5007 rest-frame EWs of galaxies in our sample, with location parameter μ= 4.24 ±0.07 and variance parameter σ= 1.33 ±0.06. This EW distribution has a mean [O III] λ5007 EW of 168 ±1 Å. The fractions of z ∼2 rest-UV-selected galaxies with [O III] λ5007 EWs greater than 500, 750, and 1000 Åare measured to be 6 . 8 + 1 . 0 -0 . 9 per cent , 3 . 6 + 0 . 7 -0 . 6 per cent , and 2 . 2 + 0 . 5 -0 . 4 percent , respectively. The EELG fractions do not vary strongly with UV luminosity in the range ( -21.6 < M UV < -19.0) considered in this paper, consistent with findings at higher redshifts. We compare our results to z ∼5 and z ∼7 studies, where candidate EELGs have been discovered through Spitzer /IRAC colours, and we identify rapid evolution with redshift in the fraction of star-forming galaxies observed in an extreme emission line phase (a rise by a factor ∼10 between z ∼2 and z ∼7). This evolution is consistent with an increased incidence of strong bursts in the galaxy population of the reionizationera. While this population makes a sub-dominant contribution of the ionizing emissivity at z _ 2, EELGs are likely to dominate the ionizing output in the reionization era.