Driving hot and cold gas flows in galaxies with feedback from massive black holes

Modern astronomy seeks to understand how the galaxies and large clusters of galaxies that we see in the local universe formed in the thirteen billion years since the Big Bang. Computer simulations that include the key physics of gravity and gas cooling can successfully reproduce the distribution of galaxies and clusters on large scales. However, these models fail to recover the structure of individual galaxies. In particular, the models predict too much gas cooling in galaxies producing large cold gas reservoirs and huge bursts of star formation, which are not observed. A heating mechanism is required to prevent this. Whilst low mass galaxies can be heated by winds from stars and supernovae, massive galaxies require the much more powerful jets of high-energy particles launched in the strong gravitational environment around a supermassive black hole. These black holes are a million to several billion times the mass of our Sun and sit at the centre of every galaxy, including our Milky Way. However, we do not yet understand how a black hole that is the size of the solar system is able to heat an entire galaxy and its surroundings on scales roughly a billion times larger. Energetic feedback from a supermassive black hole is one of the most important unsolved problems in galaxy evolution and is the focus of this proposal.

Both the star formation and black hole activity are fuelled by cold gas at the centres of massive galaxies, whose detailed structure can now be resolved by the new ALMA Observatory. ALMA is a large array of 66 high precision radio dishes located in the Atacama Desert of Chile that is transforming our understanding of galaxy evolution by providing the most incredibly detailed view of cold gas in the nearby and distant universe. Over ALMA's four years of science operations, I have led the analyses of five of the most massive gas-rich galaxies that were observed at 'high priority' during the early science phase of the commissioning. I am the PI of three new programs, a co-author of a further six programs and I have led the analysis of observations from each of the four ALMA observing cycles so far. In five of the six galaxies observed, large filaments of cold molecular gas extend a distance equivalent to the diameter of the Milky Way, and form bright, cold rims around giant bubbles of radio-emitting plasma inflated by the jet. The inflation of these huge bubbles pumps a substantial amount of heat into the surroundings that suppresses gas cooling, but my results now show that the bubbles also lift massive gas flows from the galaxy centre. The gas later falls back in a circulating flow feeding central gas disks. These cold gas disks fuel supermassive black holes, thereby sustaining jets and limiting star formation in the host galaxies.

Over the next five years, I will expand my pathfinder observations to a large sample of galaxies with the primary goals of understanding the origin of the cold gas, tracing gas inflows that feed supermassive black holes and determining how the jet-blown bubbles regulate star formation and black hole activity by lifting gas from the galaxy's core. My analysis will focus on mapping the gas velocities in the extended filaments to determine how they can be lifted by the bubbles and identifying correlations between the jet power and the mass, velocity and extent of gas outflows that can be incorporated into galaxy simulations. I will also study the structure of subsequent inflows of gas feeding central disks to show how this feedback from a supermassive black hole can be sustained to restrict the growth of galaxies.