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 at VLT imaging spectroscopy of rest-frame ultraviolet emission lines and ALMA observations of the [C I] 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 C IV and He II 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 full width half maximum (FWHM) line widths of ∼180 km s-1 and a small relative velocity offset Δv ∼ 130 km s-1 from the systemic redshift of the radio galaxy. The [C I] is detected in the same region with FWHM ∼100 km s-1 and Δv ∼ 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 ∼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 IVλλ1548, 1551 and C IVλλ1548, 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⊙] < 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 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 [C I] observations are required to trace the full extent of this important component in the circumgalactic medium. © T. Falkendal et al. 2021.