The Intrinsic Multiphase Gas—Black Hole Connection across Scales in IllustrisTNG

The relationship between supermassive black holes and the multiphase circumgalactic medium is central to understanding the coevolution of galaxies and their central black holes. We investigate this relationship using the IllustrisTNG100 simulation with a sample of 5089 central galaxies at z = 0, measuring the partial correlation between central black hole mass and the mass of cold ( T < 10 4 K), cool (10 4 ≤ T < 10 5 K), warm (10 5 ≤ T < 10 6 K), and hot ( T ≥ 10 6 K) gas within 0.03 R 200 , 0.15 R 200 , and R 200 , after accounting for stellar and dark matter halo mass. We find that after removing these confounding factors, black hole mass shows a significant negative partial correlation ( ρ ≈ −0.37) with cold gas within R 200 and 0.15 R 200 , whereas warm and hot gas exhibit no substantial intrinsic correlation. The residual plane reveals a threshold pattern: galaxies with overmassive black holes show systematically reduced cold gas, consistent with the cumulative impact of active galactic nuclei (AGN) feedback. The anticorrelation persists across environments with a weak trend in local density, and varies with galaxy type (star-forming, green valley, and quenched). These results provide a quantitative multiphase diagnostic of AGN feedback in IllustrisTNG and support a picture in which feedback progressively removes cold gas, offering testable predictions for future multiwavelength surveys.