Violent tidal disruptions of atomic hydrogen gas in quasar host galaxies

Violent galactic encounters or mergers are the leading contenders for trigg ering luminous quasar activity at low redshifts: such interactions can lead to the concentration of gas in the host galactic nucleus, thus fueling the suspected central supermassive black hole. Although optical images show a number of violently interacting systems, in many cases, the evidence for su ch interactions is only circumstantial (e.g., asymmetric optical morphologi es, projected nearby companion galaxies) or not at all apparent. Here we im age quasar host galaxies for the first time in the redshifted 21 cm line em ission of neutral atomic hydrogen (H I) gas, which, in nearby galaxies, has proved to be a particularly sensitive as well as enduring tracer of tidal interactions. The three quasars studied have different optical environments that are normally seen around low-redshift quasars, ranging from a perhaps mildly interacting system to a relatively undisturbed host with a projecte d neighboring galaxy to an isolated and apparently serene host galaxy. By c ontrast with their optical appearances, all three quasar host galaxies exhi bit ongoing or remnant tidal H I disruptions tracing galactic encounters or mergers. These observations demonstrate the utility of H I at revealing ti dal interactions in quasar host galaxies and, combined with optical studies , provide a fuller understanding of the likely stage of the interaction.