Interpreting the kinematics of the extended gas in distant radiogalaxies from 8<formula notation="TeX">$-$</formula>10 m telescope spectra

The nature of the extreme kinematics in the extended gas of distant radio galaxies (<formula notation="TeX">$z> 0.7$</formula>) is still an open question. With the advent of the 8<formula notation="TeX">$-$</formula>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<formula notation="TeX">$\alpha$</formula> and [OII]<formula notation="TeX">$\lambda$</formula>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<formula notation="TeX">$\alpha$</formula> 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</it> 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 <formula notation="TeX">$FWHM$</formula> (<formula notation="TeX">$\geq900$</formula> km s<formula notation="TeX">$^{-1}$</formula>), low ionization level [OIII]<formula notation="TeX">$\lambda5007/{\rm H}\beta \sim 2-4$</formula> and weak <formula notation="TeX">$\rm HeII\lambda 4686/H\beta\leq 0.07$</formula>, together with a narrow component (<formula notation="TeX">$\leq$</formula> few hundred km s<formula notation="TeX">$^{-1}$</formula>) with higher ionization level and strong HeII emission (<formula notation="TeX">$\rm HeII/H\beta\sim0.5$</formula>).