Where do the 'bars' in a barred spiral come from?

[Stark]

I was wandering what the general belief of why barred spiral galaxies, contain 'bars' as straight lines in nature are pretty rare. To my untrained and unqualified eye, it clearly looks - on all the photographs that I've seen - as if there are two 'bars' being emitted from the opposite sides of a small disk at the galactic core. This seems to suggest to me that they are the product of an extremely high speed ejection (hence the straight lines).

My rather modest collection of literature makes no reference to this.

 

[Hurkyl]

I don't know anything about galaxies... but from a naïve point of view we can rule out your hypothesis: ejections would form spirals.

 

[Stratosphere]

The current explanation is that a density wave radiated out from the center of the galaxy and it changed the orbits of the stars orbiting it.

 

[Stark]

I don't know anything about galaxies... but from a naïve point of view we can rule out your hypothesis: ejections would form spirals.

I didn't really mean an ejection in the form of say, a blender with it's lid removed continually throwing its contents out, but more a body of mass restricted by it's own gravity that was getting denser and therefore faster in its rotation until its contents could achieve the escape velocity causing an extremely fast ejection.

 

[Chronos]

See http://www.maia.ub.es/dsg/wsem/documents/romero.pdf

 

[Wallace]

An interesting talk in the link posted by Chronos. The question of the bars in barred spirals has, as the talk details, a long and interesting history and remains unsolved. We do have a reasonable idea, but haven't got all the details worked out yet.

We do know from observering the rotational velocties of the different parts of barred spirals that they are not high speed ejections such as proposed by Stark, in fact they basically rotate at the same rate as the rest of the galaxy. They are almost certainly due to some kind of rotationally instability causing a density wave in the galaxies rotation, but it is the origin of this that is not so certain. It is due to a merger, with another galaxy or some internal process for instance?

 

[taylrl3]

My final year Physics project was looking at the distribution of gas in barred spiral galaxies and as a result I came across the same question. I found this issue quickly became very complex, as simply as I can describe, it is the result of what is known as a Non-axisymmetric potential. This is just means the gravitational potential isn't symmetrical about one axis. This could be due to another galaxy nearby in a cluster or one which is merging. It gets hairy from here on in...and needs some intense maths which I can't really type in here. Simply you can think of it as a galaxy nearby giving a tug each time the star orbits. If you want a proper explanation I'd refer you to "Binney & Tremaine : Galactic dynamics", a very good book if you don't mind some maths. Look at chapter 3.3 it explains it all very thoroughly :)