Hi suzy,
How do bulges form and evolve?
Sometimes they
don't form....as in the case of a "pure disk" galaxy that has only a planar component; such as NGC 5907 (see image later on).
I will try to give some pictorial examples of the various types of bulges, to aid in understanding my previous post about bulge types.
(I should be the expert on this topic, as I have a bulge forming right here.....caused by sitting slumped in front of the computer, day and night.
Ha Ha.)
An example of a galaxy which has a small apparent bulge that is not exactly spheroidal ("elliptical" in shape) is our own Milky Way galaxy.
Here is a COBE/DIRBE infrared map that shows the Milky Way in infrared light....which means that this image is minus most of the
extinction of the background stellar light that we see in visual observations or in our usual CCD images.
Essentially, this image is minus most of the dust (dark nebulae and lanes) that we normally see:
Note how the bulge of our own Galaxy is:
(1) Small (quite unlike the bulge of M31)
(2) Rather box shaped
In this respect the bulge of the Milky Way is similar to the bulge of the galaxy NGC 4565:
But the Milky Way is dissimilar to the galaxy NGC 5907 ...... which has no bulge at all:
It was originally thought that small boxy bulges like N4565 and the Milky Way are just small versions of the Very Large bulges found in galaxies like The Sombrero:
However, in actual fact, Small Boxy Bulges have turned out to be similar to a small bar structure of the sort that can be seen in a face-on spiral galaxy, but which we now see in an edge-on orientation. Small bulges also have a
very different internal structure, as compared to large bulges (for instance: the surface brightness of the Bulge falls off in a very different way)(the falloff of surface brightness with increasing radius)
Small Boxy Bulges also turn out to be
younger objects than the massive spheroidal bulges like those of M104 and M31.......
indeed, small boxy bulges often continue to form stars to this day, and some of them form new stars at a high rate (this is called "ongoing bulge building").
The history of a small boxy bulge is probably very different from the history of a large spheroidal bulge......the big bulge is likely to have have formed most of its constituent stars Early in the history of the universe....... whereas the small boxy bulge may still be building itself up by means of ongoing star formation that is fuelled by gas making its way to the central regions of a galaxy.
Professor Kenneth C. Freeman of ANU, one of the most important extragalactic astronomers of our time, says that large spheroidal bulges are the products of the merger of galaxies; which contrasts with Small Boxy Bulges that form by means of the buckling of the planar disk component of a spiral or S0 galaxy. (Doctor Freeman is one of my gurus. I have abstracted this simplified exposition from his lecture notes!)
Another type of bulge, a so-called pseudobulge, is typified by NGC 4030, and by M94:
In these pseudo-bulges, there is plenty of excess light near the centre, yet these bulges seem to be relatively planar structures rather than actually sticking out of the plane of a galaxy on both sides of its principal plane. Pseudobulges like that of M94 are essentially a continuation of the Flat Disk Component inwards, but at much higher surface brightness. Pseudobulges often have very active current star formation; with OB stars and dust lanes.
In addition to small boxy bulges, large spheroidal bulges, and pseudobulges, it is possible to identify one more type of bulge: a
giant boxy bulge, often containing an X or Peanut shape. A good example of a giant boxy bulge is that of NGC 128:
The origin and evolution of these giant boxy bulges is still a highly contentious matter!!