SMOOTH DARK SPIRAL ARMS IN THE FLOCCULANT GALAXY NGC2841

OPTICAL, images of the arms of spiral galaxies invariably show massive blue stars forming in ridges of interstellar gas and dust(1). These a re particularly striking in 'grand-design' galaxies, in which the stel lar positions are influenced by spiral density waves(1). By contrast, many galaxies have a 'flocculent' appearance, with no obvious evidence of spiral structure at visible wave-lengths, Here we report infrared observations of the prototype flocculent galaxy NGC2841, which reveal a remarkable system of long, dark spiral arms, These arms arise from c oncentrations of dust; they are hidden at optical wavelengths by light scattered from the dust. The mechanism that has organized the gas and dust into these dark arms is at present unclear; the arms might be hi ghly sheared dense clouds, or they might correspond to density waves i n the interstellar medium driven by an elongated central bulge, which would not affect the stable stellar disk.