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NGC 4921 and 7049 are two spiral galaxies presenting narrow, distinct dustfeatures. A detailed study of the morphology of those features has been carriedout using Hubble Space Telescope archival images. NGC 4921 shows a few butwell-defined dust arms midway to its centre while NGC 7049 displays many moredusty features, mainly collected within a ring-shaped formation. Numerous darkand filamentary structures, called outgrowths, are found to protrude from thedusty arms in both galaxies. The outgrowths point both outwards and inwards inthe galaxies. Mostly they are found to be V-shaped or Y-shaped with thebranches connected to dark arm filaments. Often the stem of the Y appears toconsist of intertwined filaments. Remarkably, the outgrowths show considerablesimilarities to elephant trunks in H II regions. A model of the outgrowths,based on magnetized filaments, is proposed. The model provides explanations ofboth the shapes and orientations of the outgrowths. Most important, it can alsogive an account for their intertwined structures. It is found that the longestoutgrowths are confusingly similar to dusty spiral arms. This suggests thatsome of the outgrowths can develop into such arms. The time-scale of thedevelopment is estimated to be on the order of the rotation period of the armsor shorter. Similar processes may also take place in other spiral galaxies. Ifso, the model of the outgrowths can offer a new approach to the old windingproblem of spiral arms.

Model of Outgrowths in the Spiral Galaxies NGC 4921 and NGC 7049 and the Origin of Spiral Arms