Molecular hydrogen emission in Cygnus A

We present J , H and K ‐band spectroscopy of Cygnus A, spanning 1.0–2.4 μm in the rest‐frame and hence several rovibrational H 2 , H recombination and [Fe  ii ] emission lines. The lines are spatially extended by up to 6 kpc from the nucleus, but their distinct kinematics indicate that the three groups (H, H 2 and [Fe  ii ]) are not wholly produced in the same gas. The broadest line, [Fe  ii ] λ 1.644, exhibits a non‐Gaussian profile with a broad base (FWHM≃1040 km s −1 ), perhaps because of the interaction with the radio source. Extinctions to the line‐emitting regions substantially exceed earlier measurements based on optical H recombination lines. Hard X‐rays from the quasar nucleus are likely to dominate the excitation of the H 2 emission. The results of Maloney, Hollenbach & Tielens are thus used to infer the total mass of gas in H 2 v=1–0 S(1)‐emitting clouds as a function of radius, for gas densities of 10 3 and 10 5  cm −3 , and stopping column densities N H =10 22 –10 24  cm −2 . Assuming azimuthal symmetry, at least 2.3×10 8  M ⊙ of such material is present within 5 kpc of the nucleus, if the line‐emitting clouds see an unobscured quasar spectrum. Alternatively, if the bulk of the X‐ray absorption to the nucleus inferred by Ueno et al. actually arises in a circumnuclear torus, the implied gas mass rises to ∼10 10  M ⊙ . The latter plausibly accounts for 10 9  yr of mass deposition from the cluster cooling flow, for which within this radius.