NGC 1232 AND THE GRAND DESIGN
The term “grand design spiral galaxy” is merely a classification, a method of categorizing a galaxy’s structure, but the lofty language accurately captures the majestic appearance of such galaxies.
Grand design spirals, like NGC 1232 shown here, only account for about 10% of all known spiral galaxies, and yet they are iconic. When imagining a galaxy, the perfect spiral shape and clearly-defined arms are common imagery, even though more tightly-wound flocculent spirals or less-defined spirals are far more common.
Perhaps our preference for the perfect spiral is an instinctive aesthetic. The famous Fibonacci spiral, after all, is found throughout nature, occurring in everything from plant structure to the proportions of the human body [Read more about Fibonacci’s sequence here]. There is a certain pleasing symmetry to the pervasiveness of Fibonacci spirals, and our eyes find them easy to follow.
Galaxies such as NGC 1232, located 60 million light-years away in Eridanus, surely have no regard for our aesthetic preferences. Their beautiful arrangements are a natural consequence of physics.
So how do these galaxies get their arms? It’s easy to visualize solid body rotation, or the rotation of points along a flat plane as they are drawn in toward the center. The same phenomenon happens as you watch water circle the shower drain. The problem, however, is that if galaxies followed this pattern, their arms would be tightly wound at a fast rate, and we wouldn’t see such beautiful spirals. This is known as the “winding problem.” Given the mass at the center of a galaxy, it is just not possible that such clearly-defined spiral arms are just stars in a simple orbit around the center.
In the 1960s, scientists C.C. Lin and Frank Shu proposed the density wave theory, which posits that the gravitational interaction between stars and other matter in the galaxy prevents the so-called winding problem. The idea is that the arms are not actually a result of matter, but rather of varying density.
The theory can be compared to a traffic jam. As cars get backed up on the highway, the density of the traffic jam will increase as more cars move toward the center of the jam in an attempt to get to areas of lower density. The location of an individual car might change, but the overall density of the traffic jam does not go through any significant changes Similarly, the stars and other matter that make up NGC 1232’s arms may be pulled along by gravity, but they are moving through the density waves that make up the arms.
Newer research suggests that even this explanation is too simple. Even when accounting for density waves, the longevity of these spiral arms still isn’t quite explained. In 2011, researchers examined 12 nearby spiral galaxies in order to test the density wave theory. If Lin and Shu’s theory was correct, the researchers expected to find a progressive variation in areas of star formation. Instead, the three different phases of star formation they were looking for were scattered throughout the arm. This led to two possible conclusions: either the spiral arms repeatedly dissipate and reform, or the density waves contain variations in pattern speeds that would muddle the areas of star formation.
Ultimately, it is a question that will be settled by more sophisticated modeling processes and increased research. Though the designs of galaxies like NGC 1232 are pleasantly simple, it’s clear that the same can’t be said for the mechanisms by which they form.
Image: FORS, 8.2-meter VLT Antu, ESO
Sources: 1, 2, 3, 4