Molecular hydrogen emission in Cygnus A

We present J, H and K-band spectroscopy of Cygnus A, spanning 1.0-2.4 mum i n 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]lambda1.644, exhibits a non-Gaussian profile with a broad base (FWHM similar or equal to 1040 km s(-1)), perhaps because of the interacti on with the radio source. Extinctions to the line-emitting regions substant ially exceed earlier measurements based on optical H recombination lines. Hard X-rays from the quasar nucleus are likely to dominate the excitation o f the H-2 emission. The results of Maloney, Hollenbach & Tielens are thus u sed to infer the total mass of gas in H-2 v=1-0 S(1)-emitting clouds as a f unction of radius, for gas densities of 10(3) and 10(5) cm(-3), and stoppin g column densities N-H=10(22)-10(24) cm(-2). Assuming azimuthal symmetry, a t least 2.3x10(8) M-. of such material is present within 5 kpc of the nucle us, if the line-emitting clouds see an unobscured quasar spectrum. Alternat ively, 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 similar to 10(10) M-.. The latter plausibly accounts for 10(9) yr of ma ss deposition from the cluster cooling flow, for which (M) over dot similar or equal to 10M(.)yr(-1) within this radius.