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.

Based on observations obtained at the European Organization for Astronomical Research in the Southern Hemisphere under program 097.B-0323(B).