OPAQUE SPIRAL GALAXIES

SPlRAL-galaxy disks have long been thought to contain only a modest amount of light absorbing dust, meaning that they are transparent to their own starlight, and that observers see light from their far sides. This view was based on studies of the variation of the average surface brightness as a function of the galactic inclination along the line of sight in small samples of galaxies1,2. We3 have digitized the photographic images of a carefully selected sample of about 16,000 galaxies and determined an extensive set of photometric parameters. These data allow a re-analysis of the effective transparency, using the measured surface brightness profiles for much larger, more strictly defined samples. Several tests provide evidence that the major parts of many spiral disks are opaque, and that in many cases perhaps only the outer layer of stars is observable. This invalidates most determinations of mass-to-light ratios. Various photometric properties point to an obscuring component with a larger exponential scale length than that of the stars, possibly composed of cool compact opaque clouds. This result diminishes the evidence for dark haloes around spirals, as inferred from rotation curves. © 1990 Nature Publishing Group.