This is a very fine picture of a most interesting galaxy. The picture underscores and to some degree enhances the color differences between the different parts of the galaxy. Star formation in M64 is confined to a very small dust disk close to the center of the galaxy. In the rest of the galaxy, star formation has long ago ceased, and the ripples of star formation,
the galactic spiral arms, are slowly dying out like the ripples in a calm lake, when there are no more disturbances to upset the liquid peace.
I said that the color differences of the different parts of the galaxy have been enhanced. We have every reason to believe that the non-starforming disk of M64 is made up of a mixture of old and intermediate stars, and that its overall color is slightly yellowish.
Personally I think it is very interesting to compare M64 with at least two other galaxies. A near-twin of M64 is
M90. Just like M64, M90 has a rather small inner dust disk with some star formation and a rather large outer disk where the "dead" spiral arms are slowly merging with the underlying smooth thick and thin disks of the galaxy. It is probable that the overall colors of M90 are slightly bluer than the colors of M64, and that the non-starforming disk of M90 contains a somewhat higher percentage of intermediate stars than the disk of M64. The B-V index of M64 is +0.840, whereas the B-V index of M90 is +0.720.
Another galaxy that is somewhat similar to M64 is the large but mostly "dead" spiral galaxy NGC 4921 in the Coma cluster of galaxies, which you can see
here and
here. The filters used for producing this image were clearly not aimed at bringing out subtle color differences across the face of NGC 4921. It is possible, indeed probable, that the inner bulge would look noticeably yellower than the disk of the galaxy if the picture had been taken through RGB filters. What is clear, however, is that NGC 4921 has a weak dust ring (not a dust disk of any kind) which contains clusters of young stars. Whether or not there is any ongoing star formation or any pink emission nebulae in this dust ring is anybody's guess. We can see, however, that the disk has a "dying spiral pattern". The spiral arms are disappearing and merging into the underlying think and thick disks of the galaxy. Colorwise, unsurprisingly, NGC 4921 is redder than both M64 and M90, with a B-V index of +0.870.
M64, M90 and NGC 4921 are three perfect examples of spiral galaxies where star formation has disappeared except from a small inner region close to the center of the galaxy, and where the spiral arms of the outer disk are dying out like the disappearing ripples of a pond, when you stop throwing pebbles into it.
We are actually watching entropy in action.
Ann