ar X iv : a st ro - p h / 94 07 07 4 v 1 2 5 Ju l 1 99 4 DARK FILAMENTS IN THE GALAXY NGC 253 – A BOILING GALACTIC DISK –

We study the morphology of dark lanes and filaments in the dust-rich galaxy NGC 253 using an unsharp-masked B-band optical photograph. Dust features are classified as ‘arcs’, which have heights and scale radius of about 100 to 300 pc, connecting two or more dark clouds, and ‘loops’ and ‘bubbles’, which are developed forms of arcs, expanding into the disk-halo interface. These have diameters of a few hundred pc to ∼ 1 kpc. Among the bubbles, we notice a peculiar round-shaped bubble above the nucleus, which could be a large-diameter (∼ 300 pc) supernova remnant exploded in the halo over the nucleus. We also find ‘vertical dust streamers’, which comprise bunches of narrow filaments with a thickness of a few tens of pc and are almost perpendicular to the galactic plane, extending coherently for 1 to 2 kpc toward the halo. Finally, we note ‘short vertical dust filaments’ (or spicules) are found in the central region. We interpret these features as due to threedimensional structures of gas extending from the disk into the halo. We propose a ‘boiling disk’ model where the filamentary features are produced by star-forming activity in the disk as well as the influence of magnetic fluxes. We discuss the implication of the model for the chemical evolution of the ISM in a galaxy disk. Subject Headings :Galaxies; individual (NGC 253) – Galaxies: Halo – ISM: dust – ISM: Magnetic fields