A dense gas inflow observed feeding a supermassive black hole at sub-parsec scales

New radio telescope observations of the Circinus galaxy – one of the closest known galaxies to the Milky Way, though it has been little explored – paint a vivid picture of how molecular gas feeds the supermassive blackhole at the galaxy’s heart. The findings present, for the first time, spatially resolved and robust evidence of mass accretion and feedback of an active galactic nucleus (AGN) on sub-parsec scales. Most galaxies contain a supermassive black hole (SMBH) at their center. SMBHs grow by accreting mass from the surrounding galaxy. As this material falls towards the black hole, it heats up and emits energy that can be observed as an AGN. Previous studies have shown that gas can be supplied from the interstellar medium of an entire galaxy to its central region – the area within ~100 parsecs of the core. However, little is known about how gas is delivered at the to the innermost area (< 10 parsecs) surrounding the black hole due to the extreme compactness of the region. Takuma Izumi and colleagues performed submillimeter observations of the active nucleus of the Circinus galaxy with the Atacama Large Millimeter/submillimeter Array (ALMA) radio telescope at a spatial resolution of 0.5 to 2.6 parsecs and supplemented the observations with archival ALMA data. The approach allowed Izumi et al. to study the movement of molecular, atomic, and ionized gas at the parsec-scale, which revealed an inflow of dense molecular gas in the central parsec of the AGN, feeding the SMBH at its center. According to the findings, only a small portion of this inflow – less than 3% – is consumed by the black hole. The rest is expelled by outflows of multiphase gas, providing AGN-driven feedback that may affect star formation in the host galaxy. While the processes responsible for the sub-parsec accretion described remains poorly constrained, the authors suggest that a gravitationally unstable dense gas disk could drive accretion to the central ~1 parsec.