Discovery of a Nuclear Gas Bar Feeding the Active Nucleus in Circinus

Abstract

We report the discovery of gas inflow motions toward the active nucleus of the Circinus galaxy caused by the nonaxisymmetric potential of a nuclear gas bar. Evidence for dust associated with the bar comes from the HST/NICMOS H-K color map, whereas the streaming motions along the gas bar are seen in the velocity field of the H₂ S(1) (1-0) emission line. The gas bar is about 100 pc long with a visual extinction in excess of 10 mag. Indication for the gaseous nature of this bar comes from the lack of a stellar counterpart even in the K band, where the extinction is greatly reduced. We also use the NICMOS emission-line images (Paα, [Si VI], and [Fe II]) to study the innermost region of the ionization cones and the nuclear star-forming activity. We discuss the possible relationship of these components with the gaseous bar. Based on observations with the NASA/ESA Hubble Space Telescope (obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555) and with the Anglo-Australian Telescope (Siding Spring, Australia).