Morphological studies of galaxies began with thc discovery of the spiral shape of M51 by Lord Rosse in 1845. Most photographic surveys, such as the Hubble Atlas (Sandage 1961), have been made in the B or V passbands. These wavelengths are suitable for examining the young stars and dust obscuration, which provide information about the locations of star formation and of large-scale gas compressions in a galaxy. The distribution of older stars is more evident at longer wavelengths. Schweizer (1976) showed that the contribution of OB stars to the total intensity is only 15% in the near-infrared, compared with 65% in the blue; most of the light in the smooth underlying red arms is emitted by older disk main sequence and giant stars. The stellar morphology is also prominent in the I band because the opacity of dust is 2.6 times less than in the B (Johnson 1966).
This paper introduces a near-infrared photographic atlas of spiral galaxies with Hubble types Sa through Sd. The primary classification of spiral galaxies is based upon the size of the bulge or nuclear region and the tightness of the winding of the spiral arms (See review by Sandage 1975). In addition to this formal classification, subdivisions may be made according to the structure of the spiral arms. Although a continuum of arm subdivisions may exist, it is useful to mention the two extreme cases. The first comprises the M51-type galaxies, which are said to have a "global" or "grand design" spiral pattern. In these galaxies, the arms are long and continuous, and there is a two-armed axial symmetry. The second group consists of galaxies in which the spiral arms are highly fragmented in visual photographs. These galaxies have arms whose continuity extends only over small angles; the spiral pattern is composed of many patchy arm segments. Sandage (1961) first alluded to these galaxies by mentioning several early-type galaxies that are similar to the prototype, NGC 2841. Woltjer (1965) distinguished such galaxies from the grand design spirals by referring to them as "spiral-like". Kormendy (1977) also emphasized their distinctiveness as a group. These NGC 2841-type or spiral-like galaxies will be referred to as "flocculent" spirals to underscore their patchy appearance, a characteristic that is independent of Hubble type.
The contrasting appearance of grand design and flocculent spiral galaxies suggests that their predominant morphologies may be a result of different spiral structure formation mechanisms (e.g., compare Lin and Shu 1964 or Roberts, Roberts, and Shu 1975 with Mueller and Arnett 1976 or Gerola and Seiden 1978). There is probably some interplay between density waves and stochastic or other processes in creating spiral morphology. The possibility of the presence of more than one process was first suggested and examined theoretically by B. Elmegreen (1979), and has been inferred observationally in detail for one galaxy, M83 (Jensen, Talbot, and Dufour 1981).
The galaxies in this atlas have been subdivided into grand design and flocculent groups. At the present, these are subjective categories that are based on the appearances of the most prominent features that are present on sky-limited blue plates. Forthcoming surface photometry on the galaxies in this atlas will provide a quantitative analysis that will allow intermediate cases to be discerned more easily; for example, there may be low amplitude or narrow patterns of symmetric, continuous arms present in the galaxies classified as flocculent. The present atlas serves to illustrate the main characteristics of grand design and flocculent galaxies. One motivation for this study was to determine whether galaxies with flocculent spiral structure in visual wavelengths appear to be smoother and more regular in the near-infrared, as are the grand design spirals. It is shown here that blue-flocculent galaxies are also patchy in the near-infrared.