REST-FRAME OPTICAL SPECTRA OF THREE STRONGLY LENSED GALAXIES AT z ∼ 2

We present Keck II NIRSPEC rest-frame optical spectra for three recently discovered lensed galaxies: the Cosmic Horseshoe (z = 2.38), the Clone (z = 2.00), and SDSS J090122.37+181432.3 (z = 2.26). The boost in signal-to-noise ratio (S/N) from gravitational lensing provides an unusually detailed view of the physical conditions in these objects. A full complement of high S/N rest-frame optical emission lines is measured, spanning from rest frame 3600 to 6800 angstrom, including robust detections of fainter lines such as H gamma, [S II]lambda 6717,6732, and in one instance [Ne III]lambda 3869. SDSS J090122.37+181432.3 shows evidence for active galactic nucleus activity, and therefore we focus our analysis on star-forming regions in the Cosmic Horseshoe and the Clone. For these two objects, we estimate a wide range of physical properties. Current lensing models for the Cosmic Horseshoe and the Clone allow us to correct the measured Ha luminosity and calculated star formation rate. Metallicities have been estimated with a variety of indicators, which span a range of values of 12 + log(O/H) = 8.3-8.8, between similar to 0.4 and similar to 1.5 of the solar oxygen abundance. Dynamical masses were computed from the Ha velocity dispersions and measured half-light radii of the reconstructed sources. A comparison of the Balmer lines enabled measurement of dust reddening coefficients. Variations in the line ratios between the different lensed images are also observed, indicating that the spectra are probing different regions of the lensed galaxies. In all respects, the lensed objects appear fairly typical of ultraviolet-selected star-forming galaxies at z similar to 2. The Clone occupies a position on the emission-line diagnostic diagram of [O III]/H beta versus [N II]/H alpha that is offset from the locations of z similar to 0 galaxies. Our new NIRSPEC measurements may provide quantitative insights into why high-redshift objects display such properties. From the [S II] line ratio, high electron densities (similar to 1000 cm(-3)) are inferred compared to local galaxies, and [O III]/[O II] line ratios indicate higher ionization parameters compared to the local population. Building on previous similar results at z similar to 2, these measurements provide further evidence (at high S/N) that star-forming regions are significantly different in high-redshift galaxies, compared to their local counterparts.