MikeA wrote:I'm noticing a faint donut shape around the smaller elliptical bottom right .... ??
That is barred galaxy that has mostly lost all of its features except its bar.
The Large Magellanic Cloud.
Photo: Eckhard Slawik.
Consider the Large Magellanic Cloud. Practically everyone is most interest in the Tarantula Nebula when it comes to the LMC, but it is really the bar that is the LMC:s most stable and long-lasting feature. If the LMC were to "run out of gas" by some means - by having it stolen by the Milky Way? - so that its star formation stopped, most of the galaxy's blue features would simply fade away and become insignificant. What would be left is the bar.
Most barred galaxies have what the LMC lacks, namely a bright nucleus and a set of spiral arms. Many also have a ring. Consider NGC 1073. It has a bright nucleus and a ring surrounding the bar. (The ring may be two overlapping spiral arms, but never mind.) If you compare the picture of the LMC with the picture of NGC 1073, you can see that the LMC also seems to have the beginnings of ring fragments at the ends of its bar.
When a galaxy runs out of gas, which often happens to galaxies in dense clusters, star formation will come to an end, the brightest stars will disappear and die, and the overall populations will become ever more yellow. Also the the interesting features of the galaxy - the spiral arms, the dust lanes - will become ever more blurred and muted. But the brightest, most well-established features will remain the longest - such as the galactic bar, and even some hints of a ring. The ring will typically be brightest near the ends of the bar, as is the case in NGC 936.
So I'd say that the elongated, "donut-shaped" galaxy at bottom left in the APOD is a barred galaxy that has lost almost all of its features. Like the LMC, it may never have had a bright nucleus. It did have a ring, and we can actually still see a faint ring. And we can see typical bar-end enhancements, which seem to make the bar flare out at both ends.