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

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