2021A&A...645A.120F

We present MUSE at VLT imaging spectroscopy of rest-frame ultraviolet emission lines and ALMA observations of the [CI] ³P₁-³P₀ emission line, probing both the ionized and diffuse molecular medium around the radio galaxy 4C 19.71 at z≃3.6. This radio galaxy has extended Lyα emission over a region ∼100kpc in size preferentially oriented along the axis of the radio jet. Faint Lyα emission extends beyond the radio hot spots. We also find extended CIV and HeII emission over a region of ∼150kpc in size, where the most distant emission lies ∼40kpc beyond the north radio lobe and has narrow full width half maximum (FWHM) line widths of ∼180km/s and a small relative velocity offset Δv∼130km/s from the systemic redshift of the radio galaxy. The [CI] is detected in the same region with FWHM∼100km/s and Δv∼5km/s, while [CI] 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 ∼75kpc 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 [CI]/CIVλλ1548,1551 and CIVλλ1548,1551/HeII 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_☉]<0.1, and diffuse ionization with an ionization parameter (proportional to the ratio of the photon number density and gas density) of logU~-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 ≃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.