Expert: Philip Stahl - 7/11/2009

Question
I have been looking around the net as to why bars in spiral galaxies occur and see conflicting information. For instance in one place they say that spiral galaxies oscillate between being barred and not over time. Then in another place they say that the bars appear in the later stages of a galaxy's evolution.
What can you tell me about their origins and if they come and go.
Your response is much appreciated.

Answer
Hello,

Unfortunately, I can't tell you very much at a deep level, which is the level at which you appear to be interested. The reason is that the explicit basis for the emergence of a barred spiral of a given class (Spirals, barred spirals are usually denoted by subclasses a, b, c...e.g. Sc, in terms of how tightly the spiral arms are wound- with 'c' most open, and in terms of the relative size of the nucleus compared to the disk...with 'c' the smallest. Hence the most open configuration and smallest disk dimension go hand in hand)

The apparent conflicts in information you cite - whether spiral galaxies oscillated between "barredness" and non-barredness, or whether the bars emerge at later stages, is really a distinction issuing from the dynamical models used. One such set predicts the former, the other set the latter.

The models themselves are based on the *mode* chosen for particular dynamical equations, especially to do with waves (density waves). In generic dynamical terms, a "mode" is a standing wave that can be supported by a disk of given dimensions, mass.  More broadly, most astronomers who work in this specialist area (I do not, btw) use the term interchangeabley with Fourier m-component. (And it should be understood here that one of the main tools is Fourier analysis of the waves, but alas Fourier analysis is only taught usually to those who take advanced Calculus or analysis courses.

As an example, a particular Fourier coefficient, call it a_n, applicable to a wave - may be defined:

a_n = 1/pi INT (-pi to pi) f(x) cos mx dx

where INT denotes integral, and m is the Fourier m-component)  


What types of modes can one have in these models? One is the "global" or m= 1 mode. Then there are the unstable m= 2 modes.

Whether one mode or another appears (or is used in a spiral galaxy modelling) is critical since it may well determine at what stage a barred spiral develops, if at all. Alas, another complexity enters here since mode analysis is not simply a stand alone but also incorporates a subtle aspect called "marginal stability analysis" wherein one will solve for a quantity Q and if it is very close to 1 one has the case of marginal stability and tightly wound modes or in the case of spirals, around the Sa class.

The trouble is that when one seriously incorporates any heating of the disk for whatever reason (say a massive central black hole sucking up matter and generating much radiation) then the desired values of Q are soon out of range, making it impossible for a given spiral structure to sustain itself.

Why go into all this? Because while on the surface your query appears like a simple answer is possible and the conflicts in information can easily be sorted out, the truth is they cannot.

The conflicts arise precisely because whatever site or sites you perused were presenting the information at the most rudimentary levels that exclude the nasty details of things like modes - whether thay are stable or not, and how this dovetails with marginal stability analysis.

My point is that galaxy evolution is not something that can even be basically understood without also knowing the background and model formats, dynamical bases, which would mean one can then understand how the morphological differences arise.

Sure, it's possible to fob off basic, simplistic responses, but because these don't go to the next level there is no understanding of why a barrd morpphology appears or not, and why at a particular time.

I regret I can't be more helpful here, but the answer to the origins of barred spirals inheres in what particular models one selects - what inputs (e.g. modes) one applies to them, and whether one employs marginal stability analysis or not. (And if one does whether significant heating is permitted or not)

There is no simple answer and if anyone tells you there is, without offering the background, he is fooling you and himself.

The most that can be said is that appears a barred form emerges in spiral galaxy dynamical evolution when certain conditions are met including whether a stable or unstable mode (m) occurs, and how long and to what extent it exists at a marginal stability.

If the marginal stability goes "on" and "off" it is even possible that "barredness" could be a property associated with an oscillation in the galaxy. (Though to be sure, the exact assumptions need to be known)