Gemini Near Infrared Spectrograph Observations of a Red Star Forming Galaxy at z=2.225: Evidence for Shock-Ionization due to a Galactic Wind

Recent studies have shown that K-luminous galaxies at 2 < z < 2.5 have high Ha line widths and high [N ii] lambda 6583/H alpha ratios. If these lines arise from photoionization by massive stars in H ii regions, these results imply that massive, metal-rich galaxies exist at high redshift. Here we investigate the ionization mechanism in a galaxy with K-s = 19.1 and z = 2.225 in the Chandra Deep Field-South, using the new Gemini Near-Infrared Spectrograph (GNIRS). GNIRS's cross- dispersed mode gives simultaneous access to the entire 1 - 2.5 mm wavelength range, allowing accurate measurements of line ratios of distant galaxies. From the ratio of H alpha/H beta, we infer that the line-emitting gas is heavily obscured, with E(B - V) = 0.8(-0.2)(+0.3). The reddening is higher than that inferred previously from the UV-optical continuum emission, consistent with findings for nearby starburst galaxies. We find that the galaxy has Seyfert-like line ratios, [N II]/H alpha approximate to 0.6 and [O III] lambda 5007/H beta approximate to 6, which can be caused by photoionization by an active galactic nucleus (AGN) or shock ionization due to a strong galactic wind. Although we cannot exclude the presence of an AGN, the lack of AGN spectral features in the rest-frame ultraviolet, the consistency of radio, X-ray, and rest-frame UV star formation indicators, the fact that the [O III]/H beta ratio remains high out to similar to 10 kpc from the nucleus, and the observed gas kinematics all argue for the wind hypothesis. Similar shock-induced ionization is seen in nearby starburst galaxies with strong winds, such as NGC 1482 and NGC 3079. The evidence for shock ionization implies that measurements of metallicities and dynamical masses of star-forming z > 2 galaxies should be regarded with caution, especially since the existence of strong galactic winds in these objects is well established. Based on Sloan Digital Sky Survey data for nearby galaxies and the limited data available at high redshift, we speculate that the effects of shocks may correlate with dust content. The results presented here demonstrate the importance of measuring the full rest-frame optical spectra of high-redshift galaxies and showcase the potential of GNIRS for such studies.