Emission-Line Properties of 3CR Radio Galaxies. III. Origins and Implications of the Velocity Fields

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Abstract
We present the results of an analysis of the large-scale velocity fields of the ionized gas associated with powerful radio galaxies. Long-slit spectra of 52 objects provide a sample of resolved velocities that span a wide range of redshifts and radio and emission-line luminosities. Line widths reaching 1000 km s-1 and resolved velocity fields with amplitudes of up 1500 km s-1 are found on scales from 10 to 100 kpc in the environments of radio galaxies at redshifts greater than 0.5. The global velocities and FWHM are of comparable amplitudes in the FR II sources, while the FR I sources have FWHM values that are larger than their resolved velocity fields. We find evidence for systematically larger line widths and velocity field amplitudes at z > 0.6. Several of the largest amplitude systems contain two galaxies with small projected separations. All of the greater than 1000 km s-1 systems occur in objects at z > 0.6 and all have comparable radio and [O II] sizes. There is a weak correlation of off-nuclear line widths and velocity field with the ratio of the radio and emission-line sizes, but it is of low statistical significance and there is a very large dispersion. The change in properties at redshifts above z ~ 0.6 could reflect a difference in environments of the host galaxies, with the hosts inhabiting higher density regions with increasing redshift (e.g., Hill & Lilly). The mass of ionized gas and the apparent enclosed dynamical mass are correlated and both increase steeply with redshift and/or radio power. The origin of the velocities remains uncertain. The data do not require jet-gas interactions to explain the kinematics and superficially are slightly more consistent with gravitational origins for the bulk of the kinematics. If the line width reflects the underlying gravitational potential, the observed FWHM traces the velocity dispersion of the host galaxy or its surrounding group or cluster. The highest velocities seen then point to interesting environments for intermediate- and high-redshift radio galaxies. Turbulent interactions with the expanding radio source as the origin of the kinematics are certainly not ruled out. In the jet interaction scenario, the maximum velocities seen in the nebula can be used to constrain the density of the preshock gas to be roughly ne > 0.6 cm-3.
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