Was the Whirlpool (M51) a Spiral Galaxy before the Encounter with the   Companion NGC 5195?
  Using the B-I color index in the radial photometry of M51, applied to defined azimuthal sectors, we reveals asymmetries in the shape of the density pattern of the galactic arms. The anomalous shape is present only in the side close to the companion and not in the far side. Two more non-interacting check-spirals do not show similar behaviour. The anomalous sawtooth-like shape is similar to that produced by a progressive wave propagating in a preexisting density pattern. In this hypothesis we have to conclude that the galaxy M51 could be a spiral galaxy formerly, i.e. before the encounter with the companion. 
F. Natali, L. Di Fino, G. Natali (CNR-IAS, Area Ricerca Tor Vergata,   Roma, Italy)