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

We present intermediate-resolution near-infrared 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-infrared emission-line kinematics, including line splittings of 200–350 km s−1 and a mixture of narrow (FWHM ∼ 200 km s−1) and broad (FWHM ∼ 700 km s−1) components to the emission lines. It is notable that the Paα 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 active galactic nucleus itself. We suggest that this cloud may be connected to the triggering of the activity in this highly powerful active galactic nucleus. We also detect split H2 components on the nucleus that are likely to originate in the circumnuclear torus.