Morphological study of the extended halo around the Dumbell nebula (NGC 6853).

We present high quality narrow band CCD imaging in H-alpha + (N II), H-beta and (O III) of NGC 6853. We have analyzed these images aiming at a study of the properties of the halo region and their relation to the well known nebula interior. Calibrated fluxes mapping the entire nebula are presented for the first time through simultaneous observations of a standard star. We find that the outer structure of the Dumbell Nebula is conspicuously revealed in our CCD images, as a halo of 1025 sec in its largest dimension. The brightness distribution in H-alpha + N II differs spatially from that of the (O III). In H-beta the halo is fainter, indicating that the halo emission observed through the H-alpha + (N II) filter is mostly due to N II. The H-alpha + N II halo appears limb-brightened and partly disconnected from the main body of the nebula, while the (O III) halo is smoother (especially at the edge) and somewhat less extensive. The bow-shaped appearance of the halo is probably related to the interaction of the nebula with the interstellar medium. The inhomogeneous halo contains various structures, such as radial filaments, knots and arc-like features. The bright jet-like features located at the nebula interior seem to obscure the ionizing radiation resulting in a dimming of the halo along their directions.