Molecular hydrogen emission in Cygnus A

We present J, H and K-band spectroscopy of Cygnus A, spanning 1.0–2.4µm in the rest-frame and hence several rovibrational H 2 , H recombination and [FeII] 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 [FeII]) are not wholly produced in the same gas. The broadest line, [FeII]λ1.644, exhibits a non-gaussian profile with a broad base (FWHM ≃ 1040 km s −1), perhaps due to 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 (1996) 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. (1994) 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 ˙ M ≃ 10 M ⊙ yr −1 within this radius.