Observations of spiral arm types

From inspection of photographs of spiral galaxies Reynolds (1925) noted that some galaxies had ‘massive’ arms, whereas others exhibited ‘filamentous’ spiral structure. Another early attempt to classify galaxies on the basis of arm morphology was made by Danver (1942). More recently Elmegreen & Elmegreen (1982, 1987) have devised a twelve-stage classification system for spiral arms. These classifications range from Type 1 ‘flocculent’ arms, which are ragged, patchy, or chaotic to Type 12 ‘grand design’ arms, which are long, symmetrical, sharply defined, and dominate the appearance of the spiral galaxy in which they occur. After excluding barred spirals Elmegreen & Elmegreen (1982) find that 32 ± 10% of isolated objects exhibit well-developed spiral structure, compared to 67 ± 6% of members of binary pairs or groups. These results show that the formation of ‘grand design’ spiral structure is strongly favored by tidal interactions. Not unexpectedly Elmegreen & Elmegreen find a significant correlation between their spiral arm classification types and the luminosity classes of van den Bergh (1960a,b,c). Galaxies with patchy, fragmentary arms of Type 1 are all of low luminosity, whereas spirals with grand design spiral arms of Type 12 are, without exception, objects of high luminosity. The fact that spirals with very late Hubble types (Sd, Sdm, Sm) all have chaotic fragmentary arms of Types 1 and 2 is, no doubt, due to the low intrinsic luminosities of many very late-type spirals.