2024A&A...684A..56K

Context. The extent of the effect of active galactic nuclei (AGN) on their host galaxies at high-redshift is not apparent. The processes governing the co-eval evolution of the stellar mass and the mass of the central supermassive black hole, along with the effects of the supermassive black hole on the host galaxy, remain unclear. Studying this effect in the distant universe is a difficult process as the mechanisms of tracing AGN activity can often be inaccurately associated with intense star formation and vice versa.
Aims. Our aim is to better understand the processes governing the interstellar medium (ISM) of the quasar BRI 0952-0952 at z = 4.432, specifically with regard to the individual heating processes at work and to place the quasar in an evolutionary context.
Methods. We analyzed ALMA archival bands 3, 4, and 6 data and combined the results with high-resolution band-7 ALMA observations of the quasar. We detected [C I](2-1), [C II](²P_3/2 - ²P_1/2), CO(5-4), CO(7-6), CO(12-11), OH ²Π_1/2(3/2 - 1/2), and H₂O(2₁₁ - 2₀₂), and we report a tentative detection of OH⁺. We updated the lensing model and we used the radiative transfer code MOLPOP-CEP to produce line emission models which we compared with our observations.
Results. We used the [C I] line emission to estimate the total molecular gas mass in the quasar. We present results from the radiative transfer code MOLPOP-CEP constraining the properties of the CO emission and suggest different possible scenarios for heating mechanisms within the quasar. We extended our results from MOLPOP-CEP to the additional line species detected in the quasar to place stronger constraints on the ISM properties.
Conclusions. Modeling from the CO SLED suggests that there are extreme heating mechanisms operating within the quasar in the form of star formation or AGN activity; however, with the current data, it remains unclear which of the two is the preferred mechanism as both models reasonably reproduce the observed CO line fluxes. The updated lensing model suggests a velocity gradient across the [C II] line, suggestive of ongoing kinematical processes within the quasar. We find that the H₂O emission in BRI 0952 is likely correlated with star-forming regions of the ISM. We used the molecular gas mass from [C I] to calculate a depletion time for the quasar. We conclude that BRI 0952-0952 is a quasar with a significant AGN contribution while also showing signs of extreme starburst activity, indicating that the quasar could be in a transitional phase between a starburst-dominated stage and an AGN-dominated stage.