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,
present address: Onsala Space Observatory, Onsala,
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
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
preprint (gzip Postscript)
reprint (Adobe Acrobat PDF)
keywords: galaxies: active --- galaxies: individual (Cygnus A) ---
galaxies: nuclei --- radio lines: galaxies --- radio lines: ISM