Optical and Near-Infrared Spectroscopy of Cygnus A

We present optical and near-infrared spectroscopy of the luminous radio galaxy Cygnus A (3C 405). Optical (∼6700–9300 Å) observations were obtained of the central region, including the region where most emission lines are strongest, about 15 northwest of the nucleus. However, high-ionization lines ([Ar XI] and [Fe XI]), as well as an unidentified feature at ∼7608 Å, are stronger in the continuum-dominated southeast component. We detect the near-infrared Ca triplet in absorption, and we use it to obtain a new determination of the redshift and velocity dispersion of the stellar component of Cygnus A. The infrared (∼1.9–2.5 μm) spectrum of the nucleus shows emission from [Si VI], [S XI], and possibly [Si XI]. A number of molecular hydrogen transitions were also detected; their ratios favor X-ray heating from the nuclear source over other possible origins (e.g., shocks) as the most likely excitation mechanism. We see no evidence for broad-line wings on the Paα line profile, confirming that the broad-line region in Cygnus A remains heavily obscured at 2 μm.