We present MUSE @ VLT imaging spectroscopy of rest-frame ultraviolet emission lines and ALMA observations of the [ C i ] ^ { 3 } P _ { 1 } - ^ { 3 } P _ { 0 } emission line , probing both the ionized and diffuse molecular medium around the radio galaxy 4C 19.71 at z \simeq 3.6 .
This radio galaxy has extended Ly \alpha emission over a region \sim 100 kpc in size preferentially oriented along the axis of the radio jet .
Faint Ly \alpha emission extends beyond the radio hot spots .
We also find extended C iv and He ii emission over a region of \sim 150 kpc in size , where the most distant emission lies \sim 40 kpc beyond the north radio lobe and has narrow full width half maximum ( FWHM ) line widths of \sim 180 km s ^ { -1 } and a small relative velocity offset \Delta v \sim 130 km s ^ { -1 } from the systemic redshift of the radio galaxy .
The [ C i ] is detected in the same region with FWHM \sim 100 km s ^ { -1 } and \Delta v \sim 5 km s ^ { -1 } , while [ C i ] is not detected in the regions south of the radio galaxy .
We interpret the coincidence in the northern line emission as evidence of relatively quiescent multi-phase gas residing within the halo at a projected distance of \sim 75 kpc from the host galaxy .
To test this hypothesis , we performed photoionization and photo-dissociated region ( PDR ) modeling , using the code Cloudy , of the three emission line regions : the radio galaxy proper and the northern and southern regions .
We find that the [ C i ] /C iv \lambda \lambda 1548,1551 and C iv \lambda \lambda 1548,1551/He ii ratios of the two halo regions are consistent with a PDR or ionization front in the circumgalactic medium likely energized by photons from the active galactic nuclei .
This modeling is consistent with a relatively low metallicity , 0.03 < [ Z/Z _ { \odot } ] < 0.1 , and diffuse ionization with an ionization parameter ( proportional to the ratio of the photon number density and gas density ) of log U \sim - 3 for the two circumgalactic line emission regions .
Using rough mass estimates for the molecular and ionized gas , we find that the former may be tracing \approx 2-4 orders of magnitude more mass .
As our data are limited in signal-to-noise due to the faintness of the line , deeper [ CI ] observations are required to trace the full extent of this important component in the circumgalactic medium .