The [OIII]$\lambda 5007$ equivalent width distribution at z $\sim 2$: The redshift evolution of the extreme emission line galaxies

Abstract

We determine the [OIII]$\lambda 5007$ equivalent width (EW) distribution of $1.700<\mathrm{z}<2.274$ rest-frame UV-selected (M${}_{\mathrm{UV}}<-19$) star-forming galaxies in the GOODS North and South fields. We make use of deep HDUV broadband photometry catalogues for selection and 3D-HST WFC3/IR grism spectra for measurement of line properties. The [OIII]$\lambda 5007$ EW distribution allows us to measure the abundance of extreme emission line galaxies (EELGs) within this population. We model a log-normal distribution to the [OIII]$\lambda 5007$ rest-frame equivalent widths of galaxies in our sample, with location parameter $\mu =4.24\pm 0.07$ and variance parameter $\sigma =1.33\pm 0.06$. This EW distribution has a mean [OIII]$\lambda 5007$ EW of 168$\pm 1\mathring{A}$. The fractions of $\mathrm{z}\sim 2$ rest-UV-selected galaxies with [OIII]$\lambda 5007$ EWs greater than $500,750$ and $1000\mathring{A}$ are measured to be $6.8_{-0.9}^{+1.0}\%$, $3.6_{-0.6}^{+0.7}\%$, and $2.2_{-0.4}^{+0.5}\%$ respectively. The EELG fractions do not vary strongly with UV luminosity in the range ($-21.6<M_{\mathrm{UV}}<-19.0$) considered in this paper, consistent with findings at higher redshifts. We compare our results to $\mathrm{z}\sim 5$ and $\mathrm{z}\sim 7$ 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 $\sim 10$ between $\mathrm{z}\sim 2$ and $\mathrm{z}\sim 7$). This evolution is consistent with an increased incidence of strong bursts in the galaxy population of the reionisation era. While this population makes a sub-dominant contribution of the ionising emissivity at $\mathrm{z}\simeq 2$, EELGs are likely to dominate the ionising output in the reionisation era.

Submitted to MNRAS. 13 pages, 6 figures