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

J. E. Conway
National Radio Astronomy Observatory, PO Box 0, Socorro, NM 87801. present address: Onsala Space Observatory, Onsala, S-43992, Sweden

P. R. Blanco

Center for Astrophysics & Space Sciences, University of California, San Diego, 9500 Gilman Drive, La Jolla, CA 92093-0111


We report the detection of broad (FWHM 270 km/s) H I 21 cm absorption toward the compact (<15 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 x 10-19 T_spin 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 (approx. 8000 K) 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 (approx. 1000 K) 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.

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keywords: galaxies: active --- galaxies: individual (Cygnus A) --- galaxies: nuclei --- radio lines: galaxies --- radio lines: ISM