  NGC 4921 and 7049 are two spiral galaxies presenting narrow, distinct dust
features. A detailed study of the morphology of those features has been carried
out using Hubble Space Telescope archival images. NGC 4921 shows a few but
well-defined dust arms midway to its centre while NGC 7049 displays many more
dusty features, mainly collected within a ring-shaped formation. Numerous dark
and filamentary structures, called outgrowths, are found to protrude from the
dusty arms in both galaxies. The outgrowths point both outwards and inwards in
the galaxies. Mostly they are found to be V-shaped or Y-shaped with the
branches connected to dark arm filaments. Often the stem of the Y appears to
consist of intertwined filaments. Remarkably, the outgrowths show considerable
similarities to elephant trunks in H II regions. A model of the outgrowths,
based on magnetized filaments, is proposed. The model provides explanations of
both the shapes and orientations of the outgrowths. Most important, it can also
give an account for their intertwined structures. It is found that the longest
outgrowths are confusingly similar to dusty spiral arms. This suggests that
some of the outgrowths can develop into such arms. The time-scale of the
development is estimated to be on the order of the rotation period of the arms
or shorter. Similar processes may also take place in other spiral galaxies. If
so, the model of the outgrowths can offer a new approach to the old winding
problem of spiral arms.

Astrophysics of Galaxies