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Hello there, Leo,
The H-alpha channel adds a lot of interest and additional information to your already very impressive image.
The strong and symmetric two-armed spiral pattern of M51 is, in all likelihood, due to the effects of the encounter with the barred companion of Hubble type S0. The standard physical model of a spiral-shaped wave of increased density occurring in the disk of this galaxy accounts for the symmetric large-scale Two-armed spiral pattern of the prominent HII regions...... which are primarily on the inside of the two main luminous arms.
(in other words, the big HII regions occur where the interstellar gas in M51 is most compressed, and therefore it forms massive O and B stars)
However, the strong two-armed spiral symmetry of the overall pattern of HII regions is strongly disturbed near to the companion!
I am unable to say, from a cursory inspection, whether the gravity of the companion galaxy is pulling the HII regions out from their regular pattern, and/or whether the companion has enough mass to disrupt the underyling mass-dominant stellar population of the disk.
cheers,
Robert
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