An H I absorbing circumnuclear disk in Cygnus A

¹ Netherlands Foundation for Research in Astronomy, Postbus 2, 7990 AA Dwingeloo, The Netherlands e-mail: struve@astron.nl
² Kapteyn Institute, University of Groningen, Landleven 12, 9747 AD Groningen, The Netherlands
³ Onsala Space Observatory, 439 92 Onsala, Sweden e-mail: john.conway@chalmers.se

Received: 29 October 2009
Accepted: 19 December 2009

Abstract

We present Very Long Baseline Array (VLBA) H i absorption observations of the core region of the powerful radio galaxy Cygnus A. These data show both broad (FWHM = 231 ± 21 km s^-1) and narrow (FWHM < 30 km s^-1) velocity width absorption components. The broad velocity absorption shows high opacity on the counter-jet, low opacity against the core and no absorption on the jet side. We argue that these results are most naturally explained by a circumnuclear H i absorbing disk orientated roughly perpendicular to the jet axis. We estimate that the H i absorbing gas lies at a radius of  pc has a scale height of about 20 pc, density n > 10⁴ cm^-3 and total column density in the range 10²³ – 10²⁴ cm^-2. Models in which the H i absorption is primarily from an atomic or a molecular gas phase can both fit our data. Modelling taking into account the effective beam shows that the broad H i absorbing gas component does not cover the radio core in Cygnus A and therefore does not contribute to the gas column that blocks our view of the hidden quasar nucleus. If however Cygnus A were observed from a different direction, disk gas on ~ 100 pc radius scales would contribute significantly to the nuclear column density, implying that in some radio galaxies gas on these scales may contribute to the obscuration of the central engine. We argue that the circumnuclear torus in Cygnus A contains too little mass to power the AGN over > 10⁷ yr but that material in the outer H i absorbing gas disk can provide a reservoir to fuel the AGN and replenish torus clouds. The second narrow H i absorption component is significantly redshifted (by 186 km s^-1) with respect to the systemic velocity and probably traces infalling gas which will ultimately fuel the source. This component could arise either within a tidal tail structure associated with a recent (minor) merger or be associated with an observed infalling giant molecular cloud.