Zfourge: Extreme 5007 Å Emission May Be a Common Early-lifetime Phase for Star-forming Galaxies at z > 2.5

Using the Prospector spectral energy distribution (SED) fitting code, we analyze the properties of 19 extreme emission-line galaxies (EELGs) identified in the bluest composite SED in the zfourge survey at 2.5 ≤ z ≤ 4. Prospector includes a physical model for nebular emission and returns probability distributions for stellar mass, stellar metallicity, dust attenuation, and nonparametric star formation history (SFH). The EELGs show evidence of a starburst in the most recent 50 Myr, with the median EELG having a specific star formation rate (sSFR) of 4.6 Gyr^-1 and forming 15% of its mass in this short time. For a sample of more typical star-forming galaxies (SFGs) at the same redshifts, the median SFG has an sSFR of 1.1 Gyr^-1 and forms only 4% of its mass in the last 50 Myr. We find that virtually all of our EELGs have rising SFHs, while most of our SFGs do not. From our analysis, we hypothesize that many, if not most, SFGs at z ≥ 2.5 undergo an extreme Hβ+[O iii] emission-line phase early in their lifetimes. In a companion paper, we obtain spectroscopic confirmation of the EELGs as part of our MOSEL survey. In the future, explorations of uncertainties in modeling the UV slope for galaxies at z > 2 are needed to better constrain their properties, e.g., stellar metallicities.