powerful radio jets [e.g., @carilli97; @pentericci00], indicating that they are the host galaxies of some of the most powerful active galactic nuclei. Their black hole masses fall near the upper end of the mass function of supermassive black holes in nearby galaxies [@nesvadba11a], and scale with the mass of their host galaxies in a fairly similar way as nearby galaxies which fall onto the local black-hole bulge mass relationship, suggesting they must be near the end of their active formation period. @drouart14 argued that the black holes of HzRGs will outgrow the plausible mass range for supermassive black holes even in very massive galaxies, if their growth continues for more than a few $10^7$ yrs, further highlighting that we are observing these sources at an outstanding moment of their evolution. The same is suggested by their high stellar masses, which exceed the amounts of remaining molecular gas by factors of 10 or more [@emonts14], limiting their potential future growth in stellar mass.

HzRGs are often surrounded by extended nebulosities of warm ionized gas [e.g., @villar03; @nesvadba08] with sizes of up to about 60 kpc, and irregular gas kinematics, with velocity offsets and line FWHMs of up to 1000 km s$^{-1}$, 