Astronomy and Astrophysics – Astrophysics
Scientific paper
Sep 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994apj...432..606m&link_type=abstract
Astrophysical Journal, Part 1 (ISSN 0004-637X), vol. 432, no. 2, p. 606-611
Astronomy and Astrophysics
Astrophysics
28
Active Galactic Nuclei, Emission Spectra, Gas Heating, Interstellar Matter, Molecular Excitation, Radio Emission, Radio Galaxies, Brightness Temperature, Carbon Monoxide, Rotational States, Toruses
Scientific paper
In a recent paper, Barvainis & Antonucci searched for and failed to detect CO J = 1 goes to 0 absorption from the obscuring torus in the nearby powerful radio galaxy Cygnus A. We show that a plausible explanation for the lack of absorption (assuming that the ionization parameter within the torus is low enough for the gas to be molecular) is that radiative excitation of the CO molecules by the nonthermal radio continuum increases the excitation temperature of the lower rotational levels substantially, reducing the optical depths. The excitation temperature may approach the brightness temperature of the radio source at high enough flux-to-density ratios. Heating of the gas by the nonthermal excitation may also be important. We discuss the region of parameter space in which this excitation mechanism will be important and the implications for observations of obscuring tori.
Begelman Mitchell C.
Maloney Philip R.
Rees Martin J.
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