Physics
Scientific paper
Jan 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995phdt........11t&link_type=abstract
Thesis (PH.D.)--UNIVERSITY OF PITTSBURGH, 1995.Source: Dissertation Abstracts International, Volume: 57-01, Section: B, page: 03
Physics
Black Hole
Scientific paper
An essential ingredient of current models of energy production in luminous radio galaxies is a central massive black hole, which accretes matter and efficiently converts it into energy. It has been suggested that these "central engines" may be fueled by flows of cold gas such as neutral hydrogen. A VLBI study in the 21-cm line of neutral hydrogen of three nearby radio galaxies exhibiting HI absorption lines redshifted with respect to their systemic velocity is presented. The unique high spatial resolution of these spectral line observations probes scales of 1 to 100 parsecs and tests the localization of the absorbing HI clouds. In two cases it is convincingly demonstrated that there exist flows of neutral hydrogen towards the core of the radio galaxies. In NGC 5128 (Centaurus A) parsec -size clouds are detected at distances less than 100 pc from the nucleus, showing signs of enhanced HCO ^+ abundances relative to the abundances of the clouds in the dust lane and moving towards the nucleus with a velocity of ~50 km s ^{-1}. In NGC 3894 a circumnuclear torus is detected spiraling towards the core with a rotational velocity of ~100 km s^ {-1}. Its radius is ~ 7 pc and it is approximately 5 pc wide. In both cases, the HI accretion rate is high enough to account for the power of these galaxies at radio wavelengths. In the case of NGC 315 the absorbing clouds are probably more than 1 kpc away from the nucleus and it is not clear if they are related to the fueling process of the radio core. In addition to the spectral line observations, high dynamic range VLBI continuum maps of the three galaxies at 1.4 GHz have been constructed. In all cases they show evidence for diffuse counterjets, which place interesting constraints on the flow along the milliarcsecond jets and their orientation in the sky. In NGC 3894 the jet is edge -brightened and appears to possess an S-symmetry with the counterjet. Possible mechanisms to account for these effects, like helical instabilities, or a nuclear binary black hole, are discussed.
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