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The galaxies in this Category all appear to deviate significantly from equilibrium forms. For each there is a smaller galaxy close by, which either appears to be causing the pertubation, or is itself distorted. By implication the two disturbances are connected. When the companion is more than about half the size of the main galaxy, the system is placed in Category 2: Interacting Doubles. Other related Categories are Category 8: Galaxies with Apparent Companions, and Category 9: M51-Types. Obviously there are cases which could be classified into two, or three, or more Categories simultaneously. The classification made here is only empirical, from the visual appearance on presently available plates, and could be amended by further photographic, spectroscopic or radio investigations.

Almost all the main galaxies in Category 1 are spirals because it is much easier to detect disturbances in patterned spiral galaxies than in the relatively featureless E galaxies. The galaxies are arranged within each sub-category in the order of closest, strongest interaction first.

1a. Strong Interaction with Spiral Arms

In this sub-category the spirals have well-defined arms so the effect of the interaction on the arms is well seen. The intention here is to furnish examples from which something can be learned about the physical nature of spiral arms and the possible mechanisms responsible for generating them. Note the apparent avoidance of the arm by the companion in AM 2011-705 and AM 2203-281; a similar effect is seen in Atlas of Peculiar Galaxies, No. 111 (Arp 1966). In NGC 1097 (Page 1.3) the material in the arms appears to flow around its companion.

The photograph of AM 0223-245 was taken with the 2.5m Las Campanas reflector. This good resolution plate (CD 1204A) shows large knots in the arm of the barred spiral nearest the companion; the SRC Schmidt Survey plate shows a filament, running due east, connecting the companion to the bar (see Arp 1981, 1982).