Smooth red arms in M51 were discovered by Zwicky (1955) through the superposition of two plates exposed in different passbands. Their existence was confirmed by the multicolor surface photometry and image-tube photograph made by Schweizer (1976). Jensen (1977) and Elmegreen (1979) subsequently found spiral structure in the near-infrared passband for other galaxies.
One dominant feature in the grand design spiral galaxies is emphasized by the near-infrared photographs in this atlas and in previous papers. This characteristic is the highly symmetric two-armed spiral pattern. The two main spiral arms show a remarkable degree of smoothness and continuity, particularly in the central regions of grand design spirals. In the barred spiral galaxies, this bimodal pattern manifests itself throughout most of the visible structure.
Many of the galaxies have multiple spiral arms and branches; these are common in later type spirals (Sandage 1961). Multiple arms or branches are part of the regular spiral pattern, and all have the same pitch angle. The arms can be traced back to the central regions. These multiple arms appear in the I as well as in the B photographs. The multiple arms are smoother in the near-infrared, as are the two main spiral arms.
In addition to multiple arms, in many galaxies there are spurs that jut out from the main spiral arms (Weaver 1970, Elmegreen 1980b). The spurs are distinct from branches in that they have a greater pitch angle, and are short segments that do not participate in the basic spiral pattern. The spurs are prominent in both the B and I passbands also.
The primary characteristic of flocculent spiral galaxies is the disjoint nature of the individual arms. The prototype is NGC 2841, but many galaxies in this category differ considerably from this galaxy in their appearance. The number or openness of the arms is not a factor in determining whether a galaxy is flocculent. For example, NGC 7793 is a flocculent spiral galaxy because its spiral arms are fragmented and patchy. The spiral pattern of NGC 2841 clearly is more regular than that of NGC 7793, but this difference is primarily a function of Hubble type. NGC 2841 is an earlier type galaxy than NGC 7793, so it has a more orderly overall design.
Flocculent spiral galaxies do not appear smoother in the near-infrared on the photographs in this atlas; their arms are as patchy in the I as they are in the B. This is in contrast with the grand design spiral galaxies. The patchiness of the flocculents probably reflects the presence of HII regions and complexes of OB and super-giant associations. In this atlas, no underlying bimodal symmetry with an amplitude of some 20% or more can be discerned in the flocculent galaxies, although such bright two-armed patterns clearly are present in the near-infrared for all galaxies classified as grand design spirals. Thus, the morphology of flocculent spiral galaxies is qualitatively distinct from that of grand design galaxies. Forthcoming quantitative results will form the basis for more definitive categorization.
I am grateful to Dr. Bruce Elmegreen for suggesting this survey, and acknowledge helpful discussions with him and Drs. Alan Dressler and Kevin Prendergast. I thank an anonymous referee for his suggestions and comments on the manuscript, John Bedke and Douglas Cunningham for reproducing the galaxy figures, and the staff of the Mt. Wilson and Las Campanas Observatories and Palomar Observatory for the generous allotment of telescope time which made this survey possible. A Carnegie postdoctoral fellowship from the Carnegie Institution of Washington is gratefully acknowledged.