Martin Pugh's portrait of NGC 55 is definitely one of the best pictures I can remember seeing of this galaxy! The picture used by Wikipedia
to show the appearance of NGC 55 is an utter bore compared with Martin Pugh's splendid portrait.
Today's APOD caption calls the core of NGC 55 "bright". That's possible, but the brightness of the core can't be judged visually because of intervening dust lanes. We can
see, however, that the bulge of NGC 55 is faint, pale and small, and it barely bulges. Galaxies with such puny bulges are typically lightweight, because galaxies with bright yellow bulges pack a punch in their bulges when it comes to mass. That is because bright yellow bulges are mostly made up of huge numbers of faint red dwarfs, which, although lightweight, are nevertheless quite massive in relationship to the faint light they produce.
The Large Magellanic Cloud. Photo: Wei-Hao Wang.
I have tried to find an estimate of the mass or even size of NGC 55, but I have come up empty. So I will make a guess myself, based on the appearance of the galaxy. What we can see of it (excluding any dark matter halo) is clearly a lot less massive than the Milky Way, because of the galaxy's puny bulge. But NGC 55 is large and massive enough to be flattened into a pancake shape by rotation, and it has (an admittedly rather thin) central dust lane. It has both a starforming thin disk and a non-starforming thick disk. Its disk is asymmetrical, because it extends farther to the upper right (in today's APOD) than to the lower left as seen from the yellow bulge. But judging by its sideways appearance, it has a lot in common with a spiral galaxy.
I would guess that in terms of mass, NGC 55 might be similar to either M33
or the Large Magellanic Cloud. M33 is definitely a spiral galaxy, while the Large Magellanic Cloud is usually classified as an irregular, but occasionally as a loose barred spiral. Several sources compare NGC 55 to the Large Magellanic Cloud, and Robert Gendler is one of them.
wrote about NGC 55:
Confusion still remains regarding its overall morphology. Conclusions about its morphological type have ranged from barred spiral to Magellenic irregular type. Recent consensus suggests it is a magellenic irregular type with its bar seen end on.