Changing Perceptions of the Morphology and Dust Content in Galaxies

Abstract

Optical images of galactic spiral arms invariably show massive, hot and blue stars forming in dense ridges of interstellar gas mixed with dust. Accompanying IR spirals or stellar density waves (if present) in the old stellar disk are invariably bright. Non-interacting galaxies with regular, dark or black IR spiral arms only(i. e those galaxies whose infrared images reveal a clearly defined pattern of pure dust+gas spiral arms only, with little or no star formation in the arms) have hitherto never been encountered. We begin this review by presenting near-infrared imaging of the famous optically flocculent prototype NGC 2841. At K’, a remarkable system of four regular dark spirals, each spanning 160° in azimuth, is reported; NGC 2841 is the first dark IR spiral galaxy to be discovered. The dark IR spirals are so dense that they are obscured at optical wavelengths by scattered light from the bulge. Not only can spiral arms themselves be dark, but the effective dust albedo at K’ in the spiral arms of galaxies can be as high as 0.9 (we derive this result for the M51 arm [dust screen] juxtaposed against the background companion NGC 5195). We conclude the review by discussing the unambiguous detection of dust between the arms of spiral galaxies. Using radiative transfer models (invoking the full eifects of multiple scattering by dust grains) on optical/near-infrared images, it is now possible not only to identify, but also to quantify, widespread interarm dust at unprecedented spatial resolution: a resolution two orders of magnitude better than attainable by IRAS and one order of magnitude better than presently attainable with the largest sub-mm/mm telescopes.