  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]
