The first studies of pairs of interacting galaxies was undertaken in
the early 1920's. Knut Lundmark, then at Upsaala Observatory in
Sweden, carried out investigations of ``double nebulae". (1920, 1926,
1927) Assuming that both of the galaxies are at the same
distance from the observer, Lundmark deduced the relative properties
such as size and luminosity of the different morphological types of
nebulae. Based on this research, he was able to conclude that given
the large sizes of the "nebulae" relative to their separations in
space, "collisions or encounters must be rather common among these
objects.
Lindblad (1926) further theorized that "sharp encounters
between nebulae...must be considered as highly inelastic and must tend
to convert translational (i.e. orbtial) into rotational
(i.e. internal) kinetic energy. An encounter of this kind may even
lead to a fusion of the respective bodies."