We observed the nuclear region of NGC 4261 ( 3C270 ) with VLBI to determine the morphology of the central radio source on parsec scales , and in particular to see if the inner radio axis remained in the same direction as the kpc-scale jets or whether it was aligned with the apparent rotation axis of the nuclear disk imaged by HST .
The position angle of the radio axis in our VLBA images agrees , within the errors , with the position angle of the VLA-scale jet .
Thus , there is no evidence for precession of the jets on time scales shorter than the material propagation time from the nucleus to the diffuse radio lobes .
Our dual frequency observations also reveal basically symmetric radio structures at both 1.6 and 8.4 GHz .
Analysis of these images shows that most of the central 10 pc of this source is not significantly affected by free-free absorption , even though HST images of the the nucleus of the galaxy show it to contain a nearly edge-on disk of gas and dust on larger scales .
The lack of detectable absorption over most of the central 10 pc implies that the density of ionized gas in this region is less than \sim 10 ^ { 3 } { cm } ^ { -3 } , assuming a temperature of \sim 10 ^ { 4 } K. Our highest angular resolution images show a very narrow absorption feature just east of the radio core , suggesting that there may be a small , dense inner accretion disk whose width is less than 0.1 pc .
If the inclination of this inner disk is close to that of the larger-scale HST disk it becomes optically thin to 8.4 GHz radiation at a deprojected radius of about 0.8 pc .
The brightness of the pc-scale jets falls off very rapidly on both sides of the core , suggesting that the jets are rapidly expanding during the first several pc of their travel .
The rate of jet expansion must slow when the internal pressure falls below that of the external medium .
We suggest that this occurs between about 10 and 200 pc from the core because the rate of decrease in radio brightness is far slower > 200 pc from the core than it is within 10 pc of the core .
It appears that there is a small dense inner disk centered on the radio core ( the base of the jets ; < 1 pc ) , a low density , presumably hot “ bubble ” filling most of the the inner several pc of the nucleus ( within which the radio jets expand rapidly ; \sim 10 pc ) , and a surrounding cool , higher density region ( of which the HST absorption absorption disk is part ; > 10 pc ) within which the transverse expansion of the radio jets , as implied by the rate of decrease in jet brightness , is nearly halted .