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Earlier work has raised the potential importance of nebular emission in thederivation of the physical characteristics of high redshift Lyman breakgalaxies. Within certain redshift ranges, and especially at z ~ 6-7, such linesmay be strong enough to reduce estimates of the stellar masses and ages ofgalaxies compared those derived assuming broad-band photometry representsstellar light alone. To test this hypothesis at the highest redshifts wheresuch lines can be probed with ground-based facilities, we examine thenear-infrared spectra of a representative sample of 20 3.0 < z < 3.8 Lymanbreak galaxies using the newly-commissioned MOSFIRE near-infrared spectrographat the Keck I telescope. We use this data to derive the rest-frame equivalentwidths (EW) of [O III] emission and show that these are comparable to estimatesderived using the SED fitting technique introduced for sources of knownredshift by Stark et al (2013). Although our current sample is modest, its [OIII] EW distribution is consistent with that inferred for H\alpha\ based on SEDfitting of Stark et al's larger sample of 3.8 < z < 5 galaxies. For a subset ofsurvey galaxies, we use the combination of optical and near-infraredspectroscopy to quantify kinematics of outflows in z ~ 3.5 star-forminggalaxies, and discuss the implications for reionization measurements. Thetrends we uncover underline the dangers of relying purely on broad-bandphotometry to estimate the physical properties of high redshift galaxies andemphasize the important role of diagnostic spectroscopy.

Contamination of Broad-Band Photometry by Nebular Emission in High Redshift Galaxies: Investigations with Keck's MOSFIRE Near-Infrared Spectrograph