Observing AGN fuelling via molecular absorption lines
AGN and their jets are, to a large extent, powered by the accretion of cold molecular gas. Unfortunately, observing this gas on small spatial scales is difficult because it can normally only be seen en masse through emission lines. I will present observations of the molecular gas in massive galaxies on scales of just tens to hundreds of solar masses. We see the gas on this incredibly small mass scale using high angular resolution observations of the galaxies’ bright and extremely compact continuum sources — against which we see the shadows of gas clouds in the form of absorption lines. Our ALMA surveys of several galaxies have detected dozens of ~100 solar mass molecular clouds. The velocities of the clouds are revealed by the absorption’s redshift and in many cases, they’re moving towards their supermassive black hole at several hundred km/s. I will explain how these observations show the early stages of individual clouds accreting onto their host AGN. Characteristics of the absorption lines, such as their strength and width, can then be used to infer the clouds’ mass, temperature, and chemistry. Through this, we show that the gas fuelling AGN and their jets has remarkably similar properties to gas present in the Milky Way.