Discovery of Extreme [O III]+Hβ Emitting Galaxies Tracing an Overdensity at z ∼ 3.5 in CDF-South

Using deep multi-wavelength photometry of galaxies from zfourge, we group galaxies at 2.5 < z < 4.0 by the shape of their spectral energy distributions (SEDs). We identify a population of galaxies with excess emission in the K s-band, which corresponds to [O III]+Hβ emission at 2.95 < z < 3.65. This population includes 78% of the bluest galaxies with UV slopes steeper than β = -2. We de-redshift and scale this photometry to build two composite SEDs, enabling us to measure equivalent widths of these extreme [O III]+Hβ emission line galaxies (EELGs) at z ∼ 3.5. We identify 60 galaxies that comprise a composite SED with a [O III]+Hβ rest-frame equivalent width of 803 ±228 Å and another 218 galaxies in a composite SED with an equivalent width of 230 ±90 Å. These EELGs are analogous to the "green peas" found in the SDSS and are thought to be undergoing their first burst of star formation due to their blue colors (β < -1.6), young ages (log(age yr-1) ∼ 7.2), and low dust attenuation values. Their strong nebular emission lines and compact sizes (typically ∼1.4 kpc) are consistent with the properties of the star-forming galaxies possibly responsible for reionizing the universe at z > 6. Many of the EELGs also exhibit Lyα emission. Additionally, we find that many of these sources are clustered in an overdensity in the Chandra Deep Field South, with five spectroscopically confirmed members at z = 3.474 ± 0.004. The spatial distribution and photometric redshifts of the zfourge population further confirm the overdensity highlighted by the EELGs.