H i Absorption Toward the Nucleus of the Powerful Radio Galaxy Cygnus A: Evidence for an Atomic Obscuring Torus?

Details H i Absorption Toward the Nucleus of the Powerful Radio Galaxy Cygnus A: Evidence for an Atomic Obscuring Torus? H i Absorption Toward the Nucleus of the Powerful Radio Galaxy Cygnus A: Evidence for an Atomic Obscuring Torus?

Aug 1995

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995apj...449l.131c&link_type=abstract

Astrophysical Journal Letters v.449, p.L131

Astronomy and Astrophysics

Astronomy

66

Galaxies: Active, Galaxies: Individual Name: Cygnus A, Galaxies: Nuclei, Radio Lines: Galaxies, Radio Lines: Ism

Scientific paper

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 x 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.

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