tokens: 2.1. Observations 2.1.1. Spectroscopic Data This study makes use of data obtained with the Multi-Object Spectrometer for InfraRed Exploration (MOSFIRE; McLean et al. 2010 ) on Keck I on Maunakea in Hawaii. The observations over COSMOS were carried out in six pointings with a field of view. The observations were conducted on 2013 December 24–25 and 2014 February 10–13. Galaxies were observed in eight masks in the K band, which covers , and can be used to measure H α and [N ii ] emission lines for galaxies at . Two H -band masks were also included in the observations. The H -band coverage is , covering and [O iii ]. For this work, we limit the analysis to the H α emission line data in the K band. Further details on the H -band masks can be found in Nanayakkara et al. ( 2016 ). The total exposure time was 2 hr for each K -band mask. A slit width was used, yielding a spectral resolution of R = 3610. At z = 2.2, the median redshift of the sample of galaxies in this study, this corresponds to ∼26 km s −1 per pixel. The seeing conditions were , with a median of . We used a standard two-position dither pattern (ABBA). Before and after science target exposures, we measured the spectrum of an A0V-type standard star in 07 slits to be used for telluric corrections and standard stars to be used for flux calibration in a slit of width 3′′ to minimize slit loss. Each individual mask also contained a star for monitoring purposes, such as measuring the seeing conditions. The raw data were reduced using a slightly modified version of the publicly available 2015A data reduction pipeline 8 8 The modified version is available at https://github.com/themiyan/MosfireDRP_Themiyan . developed by the MOSFIRE instrument team, resulting in 2D spectra that were background subtracted, rectified, and wavelength-calibrated to vacuum wavelengths, with a typical residual of (Nanayakkara et al. 2016 ). To make up for the lack of sky lines at the red end of the K band, we used both night-sky lines and a neon arc lamp for wavelength calibration. Based on the standard star, we applied a telluric correction and flux calibration to the 2D spectra, similar to the procedure used by Steidel et al. ( 2014 ), and using our own custom IDL routines. We subsequently scaled the flux values to agree with the photometric K s -band magnitudes from the FourStar (Persson et al. 2013 ) Galaxy Evolution Survey (ZFOURGE; Straatman et al. 2016 ), resulting in a final median uncertainty of 0.08 mag (see also Nanayakkara et al. 2016 ). In Figure 1 we show two example spectra at z = 2.175 and z = 2.063, with strong H α emission at observed frame and , respectively. Other lines are visible in the spectrum as well, most notably [N ii ] and [S ii ] . Figure 1. Two example Keck MOSFIRE spectra (inverted gray scale) at z = 2.175 and z = 2.063, with H α clearly visible at (top) and (bottom). Other lines are visible as well, most notably [N ii ] and [S ii ] . 2.1.2. Continuum Subtraction From each 2D
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