NGC 3521 is a fascinating galaxy, and today's APOD is very fine. Please compare the Hanson/Selby image with the one by R. Jay GaBany at right. Note how "thick" the yellow streams are, and how they seem to rise from the central part of the galaxy like "smoke". This sort of thing isn't "normal", or at least, it isn't at all common. It's not the streams themselves that are unusual, but their brightness is.
For myself, I find the streams' deep yellow-orange color remarkable, too. Streams usually originate from dwarf galaxies that have been captured by a larger galaxy and have been cannibalized by the larger bully. But dwarf galaxies, even those that consists of old stars only, aren't usually so red. That's because dwarf galaxies tend to be relatively metal-poor, and metal-poor stars are a little bluer (or more non-red) than metal-rich ones at the same spectral class and temperature.
Note in the Hanson/Selby image that one of the yellow streams seems to contain a truly red component too, as if it contained some ionized hydrogen. It would be interesting if that was the case.
To me the yellow streams rising from the center of NGC 3521 appear somewhat similar to water splashing from a pond when you have thrown a stone into it. Is it possible that something similar is going on in NGC 3521? Can the yellow stars of the streams actually be yellow stars from the bulge of NGC 3521 itself that are "splashing" from center because something has been "thrown" into it? (I can hear Chris say no.
Because when a galaxy hits another galaxy the result looks like, say, the Tadpole galaxy, not like water splashing from a pond.)
I keep insisting that the shells of NGC 3521 are unusually yellow. Let's compare it with two shell galaxies in Pisces, NGC 474 and NGC 467:
In my opinion, Martin Pugh's image has a bluish cast to it, which is to say that I believe that the shells of NGC 474 are probably not as bluish as they appear to be here. On the other hand, we can say without a doubt that the shells of NGC 474 are bluer than the yellow core of NGC 474, and I'm not sure that the same thing can be said about NGC 3521. In other words, the shells of NGC 3521 appear to be the same deep yellow color as the core of this galaxy.
The shells of NGC 467 appears to be a little yellower than the shells of NGC 474, but still not as yellow as the shells of NGC 3521.
Read about NGC 474 here