Gemini Near-Infrared Spectrograph Observations of a Red Star-forming Galaxy at <i>z</i> = 2.225: Evidence of Shock Ionization Due to a Galactic Wind

Abstract

Recent studies have shown that K-luminous galaxies at 2<z<2.5 have high Halpha line widths and high [NII]/Halpha ratios. If these lines arise from photo-ionization by massive stars in HII regions these results imply that massive, metal rich galaxies exist at high redshift. Here we investigate the ionization mechanism in a galaxy with Ks=19.1 and z=2.225 in the Chandra Deep Field South, using the new Gemini Near Infrared Spectrograph (GNIRS). GNIRS' cross-dispersed mode gives simultaneous access to the entire 1-2.5 micron wavelength range, allowing accurate measurements of line ratios of distant galaxies. From the ratio of Halpha/Hbeta we infer that the line emitting gas is heavily obscured, with E(B-V)=0.8. The reddening is higher than that inferred previously from the UV-optical continuum emission, consistent with findings for nearby star burst galaxies. We find that the galaxy has Seyfert-like line ratios, [NII]/Halpha ~ 0.6 and [OIII]/Hbeta ~ 6, which can be caused by photo-ionization 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 [OIII]/Hbeta ratio remains high out 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 star burst galaxies with strong winds. 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. [ABRIDGED]