An HI absorbing circumnuclear disk in Cygnus A
Journal article, 2010

We present Very Long Baseline Array (VLBA) HI absorption observations of the core region of the powerful radio galaxy Cygnus A. These data show both broad (FWHM = 231 +/- 21 km s(-1)) and narrow (FWHM < 30 km s(-1)) velocity width absorption components. The broad velocity absorption shows high opacity on the counter-jet, low opacity against the core and no absorption on the jet side. We argue that these results are most naturally explained by a circumnuclear Hi absorbing disk orientated roughly perpendicular to the jet axis. We estimate that the Hi absorbing gas lies at a radius of similar to 80 pc has a scale height of about 20 pc, density n > 10(4) cm(-3) and total column density in the range 10(23)-10(24) cm(-2). Models in which the Hi absorption is primarily from an atomic or a molecular gas phase can both fit our data. Modelling taking into account the effective beam shows that the broad Hi absorbing gas component does not cover the radio core in Cygnus A and therefore does not contribute to the gas column that blocks our view of the hidden quasar nucleus. If however Cygnus A were observed from a different direction, disk gas on similar to 100 pc radius scales would contribute significantly to the nuclear column density, implying that in some radio galaxies gas on these scales may contribute to the obscuration of the central engine. We argue that the circumnuclear torus in Cygnus A contains too little mass to power the AGN over > 10(7) yr but that material in the outer Hi absorbing gas disk can provide a reservoir to fuel the AGN and replenish torus clouds. The second narrow Hi absorption component is significantly redshifted (by 186 km s(-1))with respect to the systemic velocity and probably traces infalling gas which will ultimately fuel the source. This component could arise either within a tidal tail structure associated with a recent (minor) merger or be associated with an observed infalling giant molecular cloud.

galaxies: elliptical and lenticular

SPECTROSCOPY

galaxies:

RADIO GALAXIES

SEYFERT-GALAXIES

ACTIVE GALACTIC NUCLEI

cD

AGN DUSTY TORI

A

galaxies: individual: Cygnus

ABSORPTION

PARSEC

X-RAY

MOLECULAR GAS

galaxies: active

galaxies: kinematics and dynamics

ISM

galaxies: structure

OBSCURING TORUS

Author

C. Struve

Netherlands Institute for Radio Astronomy (ASTRON)

University of Groningen

John Conway

Chalmers, Department of Radio and Space Science, Radio Astronomy and Astrophysics

Astronomy and Astrophysics

0004-6361 (ISSN) 1432-0746 (eISSN)

Vol. 513 (April) (Article Number: A10)- A10

Subject Categories

Astronomy, Astrophysics and Cosmology

DOI

10.1051/0004-6361/200913572

More information

Latest update

5/14/2018