Hydrodynamic simulations of the Disc of Gas Around Supermassive black holes (HDGAS) – II. The transition from neutral atomic to molecular gas phases

ABSTRACT We use HDGAS (Hydrodynamic simulations of the Disc of Gas Around Supermassive black holes) hydrodynamic simulations to study the impact of active galactic nucleus (AGN) feedback on the conversion of atomic gas to molecular gas within the circumnuclear disc of a typical AGN-dominated galaxy. The comparison of C i, C ii, and CO line intensities and their ratios in the HDGAS post-processing radiative transfer analysis reveals the complex interplay between AGN activity, cold molecular gas properties, and the physical processes governing the evolution of star formation in galaxies. Our results demonstrate that the C i/CO intensity ratio serves as a reliable indicator of the atomic-to-molecular gas transition. We present the probability distribution function and abundance trends of various metal species related to molecular H2 gas, highlighting differences in clumpiness and intensity maps between AGN-feedback and NoAGN models. The profile of the integrated intensity (moment-0) maps shows that the AGN-feedback model exhibits a lower C i/CO intensity ratio in the vicinity of the supermassive black hole (<50 pc), indicating a smaller atomic gas abundance and the presence of positive AGN feedback. Our simulations have successfully predicted the presence of faint-CO emissions extending to larger radii from the galactic centre. We also explore the relationships between C ii/CO and C i/C ii intensity ratios, as well as the ratios versus CO intensity, which provides insights into the ‘CO-dark’ issues. One notable feature in the later time-scale of the AGN model is the presence of a ‘CO-dark’ region, where the intensity of CO emission ($I_{\mathrm{ CO}}$) is depleted relative to the H$_2$ column density ($N_{\rm H_2}$) compared to the NoAGN model.