Flocculent and grand design spiral structure in field, binary and group galaxies

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Abstract
A 12-division morphological system has been developed to classify nearly all spiral galaxies according to the regularity of their spiral arm structure. This system emphasizes arm continuity, length and symmetry, and it may be related to the presence or strength of a density wave. Arm classifications were tabulated for 305 barred and non-barred spiral galaxies; 79 of these are isolated, 52 are in binary systems and 174 are in groups. The fractional occurrences of irregular versus ordered spiral structures were determined for the galaxies in each sample. Among the isolated SA galaxies, 68 ± 10 per cent were found to have irregular and fragmented spiral structures (i.e., flocculent structures); only 32 ± 10 per cent have symmetric spiral arms in the classic grand design pattern. Flocculent spirals are the most common structures found in galaxies without companions or bars. Binary or group membership increases the probability that an SA galaxy has grand design structure to 61 ± 6 per cent, while the presence of a bar (SB or SAB) in a field galaxy increases the fraction that have grand designs to 71 ± 7 per cent. This fraction is only slightly higher, 72 ± 4 per cent, for barred galaxies in groups, but it is much higher, 93 ± 5 per cent, for barred galaxies in binary systems. Since flocculent galaxies may have bars and companions and grand design galaxies may have neither bars nor companions, such perturbations are neither perfectly effective nor always necessary for driving grand design patterns.

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