This is another wonderful galaxy portrait by master astrophotographer R. Jay GaBany. Congratulations to David Martinez-Delgado, too, for your collaboration.
The bright yellow swirling star clouds above the bulge of NGC 3521 are interesting. While I have never seen them more clearly than in this picture, other images have also revealed at least traces of them, and their very yellow color has usually been brought out as well. So there can be no doubt that these star clouds are real, nor the fact that they must be composed of old metal-rich stars.
That in itself is interesting. APOD robot wrote:
The shells are likely tidal debris, streams of stars torn from satellite galaxies that have undergone mergers with NGC 3521 in the distant past.
Yes, that seems quite probable. On the other hand, the very yellow color of the stars above the bulge of NGC 3521 is confusing here. If this galaxy has swallowed a member of the menagerie of dwarf lenticular galaxies, which are composed of old metal-poor stars, the star clouds that resulted from swallowing one of these galaxies ought to be fairly neutral-colored. A prime example of the appearance of old metal-poor stars would be the stars of Omega Centauri, the brightest globular cluster of the Milky Way, which may well be the nucleus of a dwarf galaxy that has been incorporated into the Milky Way.
Stars of Omega Centauri, photographed by the Hubble Space Telescope.
Check out this image
of Omega Centauri and its surroundings, where the globular cluster looks quite blue.
The star clouds above the bulge of NGC 3521 might possibly result from the swallowing of a small metal-rich galaxy composed entirely of old stars. A prime example might be M32, the small, compact, very yellow companion galaxy of M31. This is an image of M32 and "friends", including "bully" M31, taken by the Canada-France-Hawaii Telescope.