DARK FILAMENTS IN THE GALAXY NGC-253 - A BOILING GALACTIC DISK

We study the morphology of dark lanes and filaments in the dust-rich g alaxy NGC 253 using an unsharp-masked B-band optical photograph. Dust features are classified as ''arcs,'' which have heights and scale radi us of about 100 to 300 pc, connecting two or more dark clouds, and ''l oops'' and ''bubbles,'' which are developed forms of arcs, expanding i nto the disk-halo interface. These have diameters of a few hundred pc to similar to 1 kpc. Among the bubbles, we notice a peculiar round-sha ped bubble above the nucleus, which could be a large-diameter (similar to 300 pc) supernova remnant exploded in the halo over the nucleus. W e also find ''vertical dust streamers, '' which comprise bunches of na rrow filaments with a thickness of a few tens of pc and are almost per pendicular to the galactic plane, extending coherently for 1 to 2 kpc toward the halo. Finally, we note ''short vertical dust filaments'' (o r spicules) are found in the central region. We interpret these featur es as due to three-dimensional structures of gas extending from the di sk into the halo. We propose a ''boiling disk'' model where the filame ntary features are produced by star-forming activity in the disk as we ll as the influence of magnetic fluxes. We discuss the implication of the model for the chemical evolution of the ISM in a galaxy disk.