Discovery of a 500 Parsec Shell in the Nucleus of Centaurus A

Spitzer Space Telescope mid-infrared images of the radio galaxy Centaurus A reveal a shell-like, bipolar, structure 500 pc to the north and south of the nucleus. This shell is seen in 5.8, 8.0, and 24 μm broadband images. Such a remarkable shell has not been previously detected in a radio galaxy and, if confirmed, would be the first extragalactic nuclear shell detected at mid-infrared wavelengths. Assuming that it is a coherent expanding structure, we estimate that the shell is a few million years old and has a mass on the order of a million solar masses. A conservative estimate for the mechanical energy in the wind-driven bubble is 10⁵³ ergs. The shell could have been created by a small, few-thousand solar mass, nuclear burst of star formation. Alternatively, the bolometric luminosity of the active nucleus is sufficiently large that it could power the shell. Constraints on the shell's velocity are lacking. However, if the shell is moving at 1000 km s^-1, then the required mechanical energy would be 100 times larger.