We construct a set of composite spectral energy distributions (SEDs) from ZFOURGE galaxies in 2.5<z<4.0 and SNR<20 (and removing those with evidence of active galactic nuclei). Included are two composite SEDs comprised of galaxies with particularly strong nebular emission features. The SEDs of these galaxies are consistent with young, blue, vigorously star-forming galaxies, analogous to those believed to play a role in the epoch of reionization. In contrast to the local universe, these extreme emission line galaxies dominate the bluest galaxies as measured by UV slope.
Fitting the stellar continuum in several ways, we find these galaxies have [OIII]+Hβ emission equivalent widths of around 800Å in the rest-frame. This is consistent with many other studies of these extreme emitters.
Additionally, the composite SEDs bring to light an overdensity of galaxies in CDFS at z=3.47, which includes 5 spectroscopically confirmed members (green). The below figure is colored by the overdensity relative to the mean density in the field. This is determined by calculating the 7th nearest neighbor metric for all galaxies with 3.42<z_phot<3.56. One sigma overdensity contours are in yellow, while five sigma overdensity contours are in orange.
For comparison, the largest one sigma overdensities in CDFS, COSMOS, and UDS encapsulate ~50, ~30, and ~10 galaxies, respectively. Those with strong emission are shown in purple in the lower right panel.