Quote:
Originally Posted by Suzy
Robert, I've just had a look at Mike's image and it's mind blowing!!!
I take it that the x/peanut shape one you're talking about is the one I've put an arrow next to it? Because I'm sure I see it there and wow that is really something!
I've never seen anything like that before in images. 
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Hi Suzy,
Yes, that is the galaxy with the boxy/peanut/x shaped bulge. These bulges have a remarkably complex shape.
I may try to answer your question about what causes the boxy and peanut shape of some bulges: I have had the relevant papers on my computer for a long time, but I have not seriously come to grips with how these bulges form and evolve! Box/peanut bulges are a phenomenon that
evolves from the planar disk component of a spiral or S0 galaxy, rather than being spheroidal (elliptical) in shape;
so Box/peanut bulges are
very different from big elliptical bulges like those in M104 and M31.
Another very good example of a giant boxy/peanut bulge is NGC 6771 in the famous galaxy triplet in Pavo:
This bulge is quite bright and it might even be visible to deep sky observers, given a really dark sky and largish aperture (say 10-12 inches plus)
A closer example is NGC 3628; I suspect that the "boxy" nature of its bulge might be a possible for visual detection, though the X-shaped extensions of this bulge are exceedingly faint.
Here is a helpful overlay of the visual Milky Way with the infrared contours that show the shape of the small boxy bulge of the Milky Way Galaxy; our Milky Way bulge is thought to be an elongated bar structure that is seen edge on (Ken Freeman of ANU called it a "bar/bulge") :
[[Incidentally, M31 is not actually "like the Milky Way"; M31 has a biggish spheroidal bulge! ]]
The Milky Way "bulge" that we see in the sky
looks bigger than its actual bulge, so much of the "apparent bulge" that we see must be material relatively near to the sun, which
only looks like it extends a long way above the plane of our galaxy mainly because it is nearby.
It wasn't so long ago that bulges were mostly thought to be spheroidal in shape , with a sort of generic elliptical shape.
But then we found that some bulges are elongated spheroids shaped like a rugby football and other bulges are symmetrical spheroids shaped like a hamburger bun.
Also, various bulges have many and diverse profiles of the falloff of surface brightness with increased radius. All in all, a very complex situation!#$$%%^^!
The small boxy-peanut bulges are probably caused by bending upwards of the material in the planar disk.
Sometimes, a so-called bulge is just an abrupt central brightening in a galaxy, and therefore this "bulge" is barely sticking above the plane of a galaxy;
these flat "disky" bulges are hardly more than an abrupt brightening of the central part of the
planar component of a spiral galaxy (the planar part of a galaxy is usually called its
disk component, to distinguish it from the
spheroidal component(s) )
The
really big boxy/peanut bulges are a puzzle, as they really
do look very very different from a standard "elliptical" bulge, as per this comparison:
Best regards,
Robert Lang
It has been a very long road for astronomers to actually figure out what our own Galaxy looks like; definitely a case of "not seeing the forest because of the trees in the way". The exact bar structure and the spiral arms structure are still controversial!
Here is another helpful graphic, comparing galaxies with various bulge shapes. Note the "disky" bulge, which is just an inner extension of the planar disk, plus the fact that there is a galaxy shown with no bulge:
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