Keck Spectroscopy of 3 <z <7 Faint Lyman Break Galaxies: The Importance of Nebular Emission in Understanding the Specific Star Formation Rate and Stellar Mass Density

The physical properties inferred from the spectral energy distributions (SEDs) of z > 3 galaxies have been influential in shaping our understanding of early galaxy formation and the role galaxies may play in cosmic reionization. Of particular importance is the stellar mass density at early times, which represents the integral of earlier star formation. An important puzzle arising from the measurements so far reported is that the specific star formation rates (sSFRs) evolve far less rapidly than expected in most theoretical models. Yet the observations underpinning these results remain very uncertain, owing in part to the possible contamination of rest-optical broadband light from strong nebular emission lines. To quantify the contribution of nebular emission to broadband fluxes, we investigate the SEDs of 92 spectroscopically confirmed galaxies in the redshift range 3.8 <z <5.0 chosen because the Hα line lies within the Spitzer/IRAC 3.6 μm filter. We demonstrate that the 3.6 μm flux is systematically in excess of that expected from stellar continuum alone, which we derive by fitting the SED with population synthesis models. No such excess is seen in a control sample of spectroscopically confirmed galaxies with 3.1 <z <3.6 in which there is no nebular contamination in the IRAC filters. From the distribution of our 3.6 μm flux excesses, we derive an Hα equivalent width distribution and consider the implications for both the derived stellar masses and the sSFR evolution. The mean rest-frame Hα equivalent width we infer at 3.8 <z <5.0 (270 Å) indicates that nebular emission contributes at least 30% of the 3.6 μm flux and, by implication, nebular emission is likely to have a much greater impact for galaxies with z ~= 6-7 where both warm IRAC filters are contaminated. Via our empirically derived equivalent width distribution, we correct the available stellar mass densities and show that the sSFR evolves more rapidly at z > 4 than previously thought, supporting up to a 5× increase between z ~= 2 and 7. Such a trend is much closer to theoretical expectations. Given our findings, we discuss the prospects for verifying quantitatively the nebular emission line strengths prior to the launch of the James Webb Space Telescope.