The smaller galaxy is NGC 5195 . Tidal distortion of
M51's larger arm looks to have been caused by NGC 5195.
Yet, in the early 1990s, NGC 5195 was found to have such
a markedly different red shift from M51, that the dwarf
galaxy was actually a great distance behind M51 and could
not possibly have any effect on it.
I went to a lecture given by a visiting astronomer that had to
do with star formation, galaxies, the Hertzsprung-Russell
diagram, etc. Following the lecture I went up to the
astronomer and asked him about the discrepancy between
what our eyes told us and what the science told us.
With red-faced anger, he told me they were not going to
throw out other scientific knowledge based on one
anomaly.
Nowadays, radio astronomy tells us there are knots of
matter connecting the M51 arm and NGC 5195, which
is not in the distant background, after all. I don't know
when or how the problem of the red shift was solved.