  We report the results of a systematic near-infrared spectroscopic survey
using the Subaru, VLT and Keck Telescopes of a sample of high redshift
Ultra-luminous Infrared Galaxies (ULIRGs) mainly composed of
submillimeter-selected galaxies. Our observations span the restframe optical
range containing nebular emission lines such as H_beta, [OIII], and [OII],
which are essential for making robust diagnostics of the physical properties of
these ULIRGs. Using the H_alpha/H_beta emission line ratios, we derive internal
extinction estimates for these galaxies similar to those of local
ULIRGs:A_v~2.9+-0.5. Correcting the H_alpha estimates of the star formation
rate for dust extinction using the Balmer decrement, results in rates which are
consistent with those estimated from the far-infrared luminosity. The majority
(>60%) of our sample show spectral features characteristic of AGN, with ~65%
exhibiting broad Balmer emission lines. A proportion of these sources show
relatively low [OIII]/H_beta line ratios, which are similar to those of Narrow
Line Seyfert 1 galaxies suggesting small mass black holes which are rapidly
growing. In the subsample of our survey with both [OIII] and hard X-ray
coverage, at least ~60% show an excess of [OIII] emission, by a factor of 5-10
times, relative to the hard X-ray luminosity compared to the correlation
between these two properties seen in Seyferts and QSOs locally. From our
spectral diagnostics, we propose that the strong [OIII] emission in these
galaxies arises from shocks in dense gaseous regions in this vigorously
star-forming population.
