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