An HST Archival Survey of Feathers in Spiral Galaxies

We present a survey of spiral arm extinction substructure referred to as feathers in 223 spiral galaxies using HST WFPC2 images. The sample includes all galaxies in the RC3 catalog with cz < 5000 km/s, B_T < 15, i < 60 degrees, and types Sa--Sd with well-exposed broadband WFPC2 images. The detection frequency of delineated, periodic feathers in this sample is 20% (45 of 223). This work is consistent with Lynds (1970), who concluded that feathers are common in prototypical Sc galaxies; we find that feathers are equally common in Sb galaxies. Sb--Sc galaxies without clear evidence for feathers either had poorer quality images, or flocculent or complex structure. We did not find clearly defined feathers in any Scd--Sd galaxy. The probability of detecting feathers was highest (83%) for spirals with well-defined primary dust lanes (PDLs; the lanes which line the inner edge of an arm); well-defined PDLs were only noted in Sab--Sc galaxies. Consistent with earlier work, we find that neighboring feathers tend to have similar shapes and pitch angles. OB associations are often found lining feathers, and many feathers transition to the stellar substructures known as spurs (Elmegreen 1980). We find that feathers are coincident with interarm filaments strikingly revealed in Spitzer 8 micron images. Comparison with CO 1-0 maps of NGC 0628 and NGC 5194 from BIMA SONG shows that feathers originate at the PDL coincident with gas surface density peaks. Contrary to the appearance at 8 microns, the CO maps show that gas surface density in feathers decreases rapidly with distance from the PDL. Also, we find that the spacing between feathers decreases with increasing gas surface density, consistent with formation via a gravitational instability