H [CSC]i[/CSC] Absorption toward the Nucleus of the Powerful Radio Galaxy Cygnus A: Evidence for an Atomic Obscuring Torus?

Abstract

We report the detection of broad (FWHM 270 km s-1) H I 21 cm absorption toward the compact (<15 h-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 (≈ 8000 K) mainly atomic phase; we derive limits on the size of such an atomic torus. Alternatively, the H I absorption might be caused by atomic gas within a warm (≈ 1000 K) mainly molecular torus. In this case, the nondetection of molecular absorption can possibly be explained by radiative excitation due to the central radio source. Follow-up VLBI observations are planned which will further constrain the properties of the absorbing gas and distinguish between the competing models.