Witnessing the transformation of a quasar host galaxy at z=1.6
  A significant minority of high redshift radio galaxy (HzRG) candidates show
extremely red broad band colours and remain undetected in emission lines after
optical `discovery' spectroscopy. In this paper we present deep GTC optical
imaging and spectroscopy of one such radio galaxy, 5C 7.245, with the aim of
better understanding the nature of these enigmatic objects. Our g-band image
shows no significant emission coincident with the stellar emission of the host
galaxy, but does reveal faint emission offset by ~3" (26 kpc) therefrom along a
similar position angle to that of the radio jets, reminiscent of the `alignment
effect' often seen in the optically luminous HzRGs. This offset g-band source
is also detected in several UV emission lines, giving it a redshift of 1.609,
with emission line flux ratios inconsistent with photoionization by young stars
or an AGN, but consistent with ionization by fast shocks. Based on its unusual
gas geometry, we argue that in 5C 7.245 we are witnessing a rare (or rarely
observed) phase in the evolution of quasar hosts when stellar mass assembly,
accretion onto the back hole, and powerful feedback activity has eradicated its
cold gas from the central ~20 kpc, but is still in the process of cleansing
cold gas from its extended halo.
