Interpreting the kinematics of the extended gas in distant radiogalaxies from 8-10 m telescope spectra

The nature of the extreme kinematics in the extended gas of distant radio galaxies (z > 0.7) is still an open question. With the advent of the 8-10 m telescope generation and the development of NIR arrays we are in the position for the first time to develop a more detailed study by using lines other than Ly alpha and [OII]lambda 3727 depending on redshift. In this paper we review the main sources of uncertainty in the interpretation of the emission line kinematics: the presence of several kinematic components, Ly alpha absorption by neutral gas/dust and the contribution of scattered light to some of the lines. As an example, several kinematic components can produce apparent, false rotation curves. We propose methods to solve these uncertainties. We propose to extend the methods applied to low redshift radio galaxies to investigate the nature of the kinematics in distant radio galaxies: by means of the spectral decomposition of the strong optical emission lines (redshifted into the NIR) we can isolate the different kinematic components and study the emission line ratios for the individual components. If shocks are responsible for the extreme kinematics, we should be able to isolate a kinematic component (the shocked gas) with large FWHM (greater than or equal to 900 km s(-1)), low ionization level [OIII]lambda 5007/H beta similar to 2 - 4 and weak HeII lambda 4686/H beta less than or equal to 0.07, together with a narrow component (less than or equal to few hundred km s(-1)) with higher ionization level and strong HeII emission (HeII/H beta similar to 0.5).