Dark Filaments in the Galaxy NGC 253 - a Boiling Galactic Disk -

Abstract

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 $\sim 1$ kpc. Among the bubbles, we notice a peculiar round-shaped bubble above the nucleus, which could be a large-diameter ($\sim 300$ 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 three-dimensional 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.