The Centaurus A Northern Middle Lobe as a Buoyant Bubble

We model the northern middle radio lobe of Centaurus A (NGC 5128) as abuoyant bubble of plasma deposited by an intermittently active jet. The extentof the rise of the bubble and its morphology imply that the ratio of itsdensity to that of the surrounding ISM is less than 10^{-2}, consistent withour knowledge of extragalactic jets and minimal entrainment into the precursorradio lobe. Using the morphology of the lobe to date the beginning of its risethrough the atmosphere of Centaurus A, we conclude that the bubble has beenrising for approximately 140Myr. This time scale is consistent with thatproposed by Quillen et al. (1993) for the settling of post-merger gas into thepresently observed large scale disk in NGC 5128, suggesting a strong connectionbetween the delayed re-establishment of radio emission and the merger of NGC5128 with a small gas-rich galaxy. This suggests a connection, for radiogalaxies in general, between mergers and the delayed onset of radio emission.In our model, the elongated X-ray emission region discovered by Feigelson etal. (1981), part of which coincides with the northern middle lobe, is thermalgas that originates from the ISM below the bubble and that has been upliftedand compressed. The "large-scale jet" appearing in the radio images of Morgantiet al. (1999) may be the result of the same pressure gradients that cause theuplift of the thermal gas, acting on much lighter plasma, or may represent ajet that did not turn off completely when the northern middle lobe started tobuoyantly rise. We propose that the adjacent emission line knots (the "outerfilaments") and star-forming regions result from the disturbance, in particularthe thermal trunk, caused by the bubble moving through the extended atmosphereof NGC 5128.