Smooth dark spiral arms in the flocculent 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 are particularly striking in 'grand-design' galaxies, in which the stellar 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 concentrations 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 highly sheared dense clouds, or they might correspond to density waves in the interstellar medium driven by an elongated central bulge, which would not affect the stable stellar disk.