The way galaxies form and evolve is largely a mystery to astronomers. A particular focus of much head scratching is the bar that appears in many spiral galaxies. How does this form and why only in some spiral galaxies?
One thing that astronomers have studied in great detail is the colour of galaxies. In 1936, Edwin Hubble suggested that galaxies may evolve from elliptical galaxies to spiral ones. Astronomers have since found that galaxies further down this sequence tend to be bluer.
But astronomers also think that the bars at the centre of galaxies must influence colour too, since stars should form in the gas that these bars redistribute in the galaxy which in turn influences the colour of the galaxy.
So a reasonable question to ask is this: what is the difference in colour between spiral galaxies in the same Hubble stage that do and don’t have bars?
Today, Sidney van den Bergh at the Herzberg Institute of Astrophysics in Australia says that nobody has bothered to ask this question, strange though it may seem. So he sets out to do it himself.
He studied 532 barred and spiral galaxies from a database called A Revised Shapley-Ames Catalog of Bright Galaxies published in 1981.
His results indicate that “normal and barred galaxies have indistinguishable colour distributions”. So spiral galaxies at the same Hubble stage are the same colour regardless of whether they have bars or not. In fact their colour is also independent of whether they have other ring-like features, says van den Bergh.
That leads him to a startling conclusion. “Perhaps the apparent independence of the intrinsic colours of spirals from the presence (or absence) of bars hints at the possibility that some bars could be ephemeral structures,” he says, pointing out that it may well be easy for a bar to reform after it has faded away.
So the picture he’s driving at is that in some spiral galaxies, bars form, decay and reform in a cycle. Which means that many of the galaxies that don’t have a bar now may have had one in the recent past and will have one again in the near future. That includes our own galaxy.
That’s an interesting idea but one that will be hard to check. Simulating the shape and evolution of galaxies is notoriously hard because of the complex chemical, magnetic and gravitational interactions that go on.
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