A giant molecular cloud falling through the heart of Cygnus A: clues to the triggering of the activity

Abstract

We present intermediate resolution near-IR long-slit spectroscopic data for the nearby radio galaxy Cygnus A (3C 405) (obtained with the NIRSPEC spectrograph on the Keck II telescope). The data reveal considerable complexity in the near-IR emission line kinematics, including line splittings of 200-350 km/s and a mixture of narrow (FWHM ~200 km/s) and broad (FWHM ~700 km/s) components to the emission lines. It is notable that the Pa alpha and H2 emission lines show markedly different kinematics, both on- and off-nucleus. Overall, the data provide evidence for the presence of a giant molecular cloud falling through the heart of the Cygnus A host galaxy, the motion of which is not driven by the AGN itself. We suggest that this cloud may be connected to the triggering of the activity in this highly powerful AGN. We also detect split H2 components on the nucleus that are likely to originate in the circum-nuclear torus.