PHYSICAL PROPERTIES OF EMISSION-LINE GALAXIES AT z ∼ 2 FROM NEAR-INFRARED SPECTROSCOPY WITH MAGELLAN FIRE

We present results from near-infrared spectroscopy of 26 emission-line galaxies at z similar to 2.2 and z similar to 1.5 obtained with the Folded-port InfraRed Echellette (FIRE) spectrometer on the 6.5 m Magellan Baade telescope. The sample was selected from the WFC3 Infrared Spectroscopic Parallels survey, which uses the near-infrared grism of the Hubble Space Telescope Wide Field Camera 3 (WFC3) to detect emission-line galaxies over 0.3 less than or similar to z less than or similar to 2.3. Our FIRE follow-up spectroscopy (R similar to 5000) over 1.0-2.5 mu m permits detailedmeasurements of the physical properties of the z similar to 2 emission-line galaxies. Dust-corrected star formation rates for the sample range from similar to 5-100 M-circle dot yr(-1) with a mean of 29 M-circle dot yr(-1). We derive a median metallicity for the sample of 12 + log(O/H) = 8.34 or similar to 0.45 Z(circle dot). The estimated stellar masses range from similar to 10(8.5)-10(9.5) M-circle dot, and a clear positive correlation between metallicity and stellar mass is observed. The average ionization parameter measured for the sample, log U approximate to -2.5, is significantly higher than what is found for most star-forming galaxies in the local universe, but similar to the values found for other star-forming galaxies at high redshift. We derive composite spectra from the FIRE sample, from which we measure typical nebular electron densities of similar to 100-400 cm(-3). Based on the location of the galaxies and composite spectra on diagnostic diagrams, we do not find evidence for significant active galactic nucleus activity in the sample. Most of the galaxies, as well as the composites, are offset diagram toward higher [O III]/H beta at a given [N II]/H alpha, in agreement with other observations of z greater than or similar to 1 star-forming galaxies, but composite spectra derived from the sample do not show an appreciable offset from the local star-forming sequence on the [O III]/H beta versus [S II]/H alpha diagram. We infer a high nitrogen-to-oxygen abundance ratio from the composite spectrum, which may contribute to the offset of the high-redshift galaxies from the local star-forming sequence in the [O III]/H beta versus [N II]/H alpha diagram. We speculate that the elevated nitrogen abundance could result from substantial numbers of Wolf-Rayet stars in starbursting galaxies at z similar to 2.