Uncovering Spiral Structure in Flocculent Galaxies

We present K'(2.1 micron) observations of four nearby flocculent spirals,which clearly show low-level spiral structure and suggest that kiloparsec-scalespiral structure is more prevalent in flocculent spirals than previouslysupposed. In particular, the prototypical flocculent spiral NGC 5055 is shownto have regular, two-arm spiral structure to a radius of 4 kpc in the nearinfrared, with an arm-interarm contrast of 1.3. The spiral structure in allfour galaxies is weaker than that in grand design galaxies. Taken in unbarredgalaxies with no large, nearby companions, these data are consistent with themodal theory of spiral density waves, which maintains that density waves areintrinsic to the disk. As an alternative, mechanisms for driving spiralstructure with non-axisymmetric perturbers are also discussed. Theseobservations highlight the importance of near infrared imaging for exploringthe range of physical environments in which large-scale dynamical processes,such as density waves, are important.Comment: 12 pages AASTeX; 3 compressed PS figures can be retrieved from ftp://ftp.astro.umd.edu/pub/michele as file thornley.tar (1.6Mbytes). Accepted to Ap.J. Letters.(Figures now also available here, and from ftp://ftp.astro.umd.edu/pub/michele , in GIF format.