absorption toward the nucleus of the powerful radio galaxy Cygnus A : evidence for an atomic obscuring torus ?

We report the detection of broad (FWHM 270kms 1) H I 21 cm absorption toward the compact (<15h 1 pc) radio nucleus of the nearby powerful radio galaxy Cygnus A. The absorption corresponds to a column density of hydrogen atoms of at least 2.54 0.44 1019 Tspin cm 2. Observations of OH and H2CO yielded upper limits. While other possibilities exist, we argue that the observed H I absorption plausibly occurs within a circumnuclear obscuring torus which is thought to block our direct view of a quasar nucleus in this object. We have attempted to constrain the properties of the obscuring gas by combining our H I result with upper limits on molecular absorption and estimates of the total obscuring column density from X-ray observations. One possibility is that the majority of the gas is in a hot ( 8000K) mainly atomic phase; we derive limits on the size of such an atomic torus. Alternatively, the H I absorption might be due to atomic gas within a warm ( 1000K) mainly molecular torus. In this case the non-detection of molecular absorption can possibly be explained by radiative excitation due to the central radio source (Maloney, Begelman, & Rees 1994). Follow-up VLBI observations are planned which will further constrain the properties of the absorbing gas and distinguish between the competing models. Subject headings: galaxies: active | galaxies: individual (Cygnus A) | galaxies: nuclei | radio lines: galaxies | radio lines: ISM