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Abstract

We present MUSE@VLT imaging spectroscopy of rest-frame ultraviolet emission lines and ALMA observations of the [Ci]3P1-3P0emission line, probing both the ionized and diffuse molecular medium around the radio galaxy 4C 19.71 at z'3.6. This radio galaxyhas extended Lyαemission over a region∼100 kpc in size preferentially oriented along the axis of the radio jet. Faint Lyαemissionextends beyond the radio hot spots. We also find extended Civand Heiiemission over a region of∼150 kpc in size, where themost distant emission lies∼40 kpc beyond the north radio lobe and has narrow full width half maximum (FWHM) line widths of∼180 km s−1and a small relative velocity offset∆v∼130 km s−1from the systemic redshift of the radio galaxy. The [Ci] is detectedin the same region with FWHM∼100 km s−1and∆v∼5 km s−1, while [Ci] is not detected in the regions south of the radio galaxy. Weinterpret the coincidence in the northern line emission as evidence of relatively quiescent multi-phase gas residing within the halo ata projected distance of∼75 kpc from the host galaxy. To test this hypothesis, we performed photoionization and photo-dissociatedregion (PDR) modeling, using the code Cloudy, of the three emission line regions: the radio galaxy proper and the northern andsouthern regions. We find that the [Ci]/Civλλ1548,1551 and Civλλ1548,1551/Heiiratios of the two halo regions are consistent witha PDR or ionization front in the circumgalactic medium likely energized by photons from the active galactic nuclei. This modelingis consistent with a relatively low metallicity, 0.03<[Z/Z]<0.1, and diffuse ionization with an ionization parameter (proportional tothe ratio of the photon number density and gas density) of log U∼ −3 for the two circumgalactic line emission regions. Using roughmass estimates for the molecular and ionized gas, we find that the former may be tracing≈2-4 orders of magnitude more mass. Asour data are limited in signal-to-noise due to the faintness of the line, deeper [Ci] observations are required to trace the full extent ofthis important component in the circumgalactic medium.