MOSEL: Strong [Oiii] 5007 Å Emitting Galaxies at (3 < z < 4) from the ZFOURGE Survey

To understand how strong emission-line galaxies (SELGs) contribute to the overall growth of galaxies and star formation history of the universe, we target SELGs from the ZFOURGE imaging survey that have blended Hβ+[O iii] rest-frame equivalent widths of >230 Å and 2.5 < $z_{phot}$ < 4.0. Using Keck/MOSFIRE, we measure 49 redshifts for galaxies brighter than K _s = 25 mag as part of our Multi-Object Spectroscopic Emission Line (MOSEL) survey. Our spectroscopic success rate is ∼53% and $z_{phot}$ uncertainty is ${\sigma }_z$ = [Δz/(1+z)] = 0.0135. We confirm 31 ELGs at $3<z_{spec}<3.8$, and show that SELGs have spectroscopic rest-frame [O iii]5007 Å equivalent widths of 100–500 Å and tend to be lower-mass systems [$log\left(M_{\star }/M_{\odot }\right)$ ∼ 8.2–9.6] compared with more typical star-forming galaxies. The SELGs lie ∼0.9 dex above the star-forming main sequence at z ∼ 3.5 and have high inferred gas fractions of $f_{gas}$ ≳ 60%, i.e., the inferred gas masses can easily fuel a starburst to double stellar masses within ∼10–100 Myr. Combined with recent results using ZFOURGE, our analysis indicates that (1) strong [O iii]5007 Å emission signals an early episode of intense stellar growth in low-mass [$M_{\star }<0.1M^{\star }$] galaxies and (2) many, if not most, galaxies at z > 3 go through this starburst phase. If true, low-mass galaxies with strong [O iii]5007 Å emission (EW_rest > 200 Å) may be an increasingly important source of ionizing UV radiation at z > 3.