ALMA and MUSE observations reveal a quiescent multi-phase circumgalactic medium around the z~3.6 radio galaxy 4C 19.71
Abstract
We present MUSE/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 ~100 kpc 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 ~150 kpc in size, where the most distant emission lies ~40 kpc beyond the north radio lobe and has narrow FWHM line widths of ~180 km/s and a small relative velocity offset ~130 km/s from the systemic redshift of the radio galaxy. The [CI] is detected in the same region with FWHM~100 km/s and ~5 km/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 ~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 [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 log U~3 for the two circumgalactic line emission regions. Using rough mass estimates for the molecular [...]