Expert: Philip Stahl - 8/13/2009

Question
Please tell me what is known about the causes of bars in some spiral galaxies. Also, it is hard to understand how they retain the bar shape. One would expect it to distort with the spinning of the galaxy.
I also heard that bars come and go over two billion years in the evolution a galaxy.
Thank you so much for your time.

Answer
Hello,

Again, the "causes" of bars, e.g. in barred spirals, is still up in the air, although some theories have recently come to the fore. Make no mistake, however, as in my previous replies - that these are easy to understand. Thus, even giving you the simplest layout of the (density wave, quasi-stationary) theory, will not necessarily permit a light to flash on for you.

Here is where it sits: the ranking theory now is that the barred spiral shape persists because an essentially standing density wave exists in the galaxy over time. Earlier, in previous replies, I showed how and why these theories came to be, based on the interpretation of modes - to describe the specific properties of the density wave- using a Fourier component, m. (Here, m denotes the mode associated with the wave)

As I said before, a "mode" is essentially a standing wave (Have you taken basic physics - and learned about these ....as in organ pipes, or on strings?)that can be supported by a given disk of mass M, diameter D, thickness t, and angular momentum L.  Thus a specific mode m is associated with a particular density perturbation (and G-field) for a disk assumed to rotate rigidly at a given frequency without propagating in the radial direction.

As I also pointed out, IF one goes into sufficient detail in the "modal-Fourier density wave" theory, one can obtain reasonable explanations (based on the differing m-values, models, marginal stability assumptions) as to why some galaxies are barred and others not.

Here is the problem, the actual detailed understanding means one has to be familiar with: a) the nature of modes and standing waves, b) basic Fourier analysis, and c) concept of marginal stability and how this is used to distinguish spiral models.

There are many sources and texts which fob off all these issues, and details, and simply gimmick the explanation to make it sound easier or more accessible. Unfortunately, most of these over-simplified explanations end up misleading the serious inquirer.

Unfortunately, also, I do not know how much physics you have had, or advanced math, to be able to understand at the level you want to. There is a difference here between wanting to understand the 'causes' at a deep level, and actually having the mathematical-physics background to do so.

For example, in your question you assert (regarding the barred spiral form) that:  "One would *expect it* to distort with the spinning of the galaxy."   Not really so, because this wrongly (or prosaically)  assumes the bars are purely material distributions affected by mechanical forces.  However, if the structure is tied to an underlying potential (V) associated with a *standing* density wave, the result would certainly be at least "quasi-stationary" (given the marginal stability hypothesis) so that we can expect the barred FORM (as a standing density wave) to persist for at least one- two complete galactic rotations.

Since human lifespans don't measure up to even a micro-fraction of a full galactic revolution, the barred structures would always appear to us to be permanent and absolutely stable, though in actual fact they wouldn't be. (This is also why the prefaced "quasi" is used before "stationary", since otherwise the theory would make no sense).

You also added that:  "I also heard that bars come and go over two billion years in the evolution a galaxy", which is possible, since the hypothesis of quasi-stationarity remains a working hypothesis only.

Recall in one of my earlier answers, I noted the effect of altering the marginal stability hypothesis and a particular quantity 'Q' associated with it. Thus, in different model runs (say using different values of mode no. m) it is possible for emerging conditions, such as greater heating, to cause the spiral structure to "vanish" - at least temporarily, especially if the value of Q is caused to diverge significantly from its (marginally-stable) value of Q ~ 1.

Again, I also noted, that numerical simulations uncovered this sort of behavior, and I gave the parameters for the simulations.

Hopefully this answer can suffice, and also - if you can relate to me how much physics, math background you have, it would help in terms of how much detail to provide in future replies on this same topic.  In the meantime, if your physics background is lacking, you might want to google "standing waves" (and even "density waves"). Also, for the math, "Fourier analysis" and "Fourier modes".  You may not grasp all the details in these googles, but it may be enough to understand most of the replies given.