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Bulges - classical, "pseudo", and boxy
You are right, Paddy, bulges that look rather like boxes or peanuts are hard to explain.
Professional astronomers often loosely talk of bulges and of elliptical galaxies as being "spheroids" due to fact that, technically, many of them have the geometrical shape of a spheroid.
But Even within the category "spheroid" there are various types of spheroid: e.g. an oblate spheroid (hamburger bun shape) and a prolate spheroid (hot dog bun shape) and a triaxial ellipsoid.
(as an example, some elliptical galaxies are oblate and some elliptical galaxies are prolate)
But, just to make things more complicated, many of the objects that we call bulges are not actually spheroids at all......
In the days of the old photographic galaxy atlases, such as the Hubble Atlas of Galaxies and the Carnegie Atlas of Galaxies, atlases which formed our basic conceptions (and prejudices) about galaxian morphology till about 1980, it was assumed that any discrete large-spatial-scale component, surrounding the centre of a galaxy, that shows a very strong radial brightening towards the centre .......is spheroidal. This is why - for historical reasons - we call all of these features bulges.
However, it has since become apparent that not all bulges are spheroidal!!
The disk (planar) component within a galaxy, in those cases where a galaxy does actually contain a disk component, has an observed surface brightness that tends to rise fairly gently towards the centre of a galaxy. Superposed on this disk component is excess light from a "bulge" component; which generally rises very rapidly towards the galaxian centre.
Anyhow, things have got ever more complicated in bulge studies, and what we observe as a bright central bulge component can have at least three different explanations:
(1) a "classical" bulge, which is genuinely much "fatter" than the typically very thin disk component of a galaxy, and which is spheroidal in shape. These structures tend to be quite massive and in general they contain a lot of very old red stars.
At one time, this was thought to be the only type of bulge that exists.
(2) A pseudo-bulge : when we look at the images of spiral galaxies and we observe a rapid central brightening due to what we usually call a bulge component, a brightening which looks either circular or oval to the eye or camera (it looks this way from our line-of-sight), it is not required that the object we are viewing in 2 dimensions is actually a spheroid in actual (real) three dimensional space.
In fact, a bright central structure which is flat, or fairly flat, can also give the impression of a central bulge.
These relatively flat central objects found in galaxies are therefore called pseudo-bulges (pseudobulges).
While a photograph may show something that looks like it could be spheroidal, the actual shape of the apparent bulge can be relatively flattened and disk-like. These pseudo-bulges are common in galaxies of type Sbc and later (in the Hubble sequence), and they often contain young stars due to ongoing bulge formation at the centre of a galaxy.
(3) Boxy and Peanut bulges:
Some bulges don't even look spheroidal to the eye ; they are noticeably boxy or peanut-shaped. The smaller examples of these boxy/peanut bulges are very likely to be bar structures that are seen edge-on. The bigger examples are not very well understood - they seem to have remarkably complex structures, in some cases.
I would like to include some actual examples of these various bulge types, but this contribution is already too long!
Some of the Really Big boxy bulges do look very odd, relative to the examples of bulges that astronomers used to use as "baseline truth", indeed these bulges have had little detailed study in the literature. I will eventually attach a few examples of giant boxy bulges, as they seem to me show evidence of galaxy interactions.
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