A theory related to how galaxies evolve over the course of their lifespan in currently gaining increased support from the international astronomical community. Its premises challenge those of the most commonly-accepted theories of what evolutionary path spiral galaxies take.
One of the most “traditional” galaxy evolution models holds that spiral galaxies retain their trademark shape unless influenced from the outside. These influences can be vary widely.
For example, it is known that spiral galaxies grow in size and mass as they accrete dwarf galaxies. This is how the Milky Way grew to its current size. But what happens when two spiral galaxies collide?
In 3 to 4 billion years, the Milky Way will collide with Andromeda, which is a spiral galaxy of similar mass and size. Chances are high that the merger will produce a massive elliptical galaxy.
With the featureless elliptical form, many things will change. Stars and solar systems will no longer follow a narrow orbital plane as they currently do but will rather have hectic, chaotic orbits.
Therefore, according to this model, this is the only way a spiral galaxy can turn into an elliptical. But the new theory, called secular galaxy evolution, challenges this belief, Universe Today reports.
In this particular context, the term secular means separate, or isolated, and is used to denote the fact that spiral galaxies naturally evolve into ellipticals, without having to undergo mergers or collisions.
This transition is known among specialists as the Hubble sequence. What the new theory says is basically that the Hubble sequence needs no external influences to take place.
One of the basic concepts in the new proposal is the fact that spiral galaxies contain within a mechanism that enables them to transfer angular momentum outwards. What still remains to be established is whether spiral galaxies evolve or degenerate into elliptical ones.
Celestial mechanics seems to indicate that spirals develop their flattened disks and central bulge due to the fact that they spin. The rotation motion was probably imprinted to the entire structure in the earliest stages of its formation.
But the principle of conservation of angular momentum holds that spin will always be preserved. Still, experts now say, it could be that collisions, mergers and other such events might cause spiral galaxies to lose their angular momentum.
The spiral arms, which are actually density waves, may play a critical role in this process. One interesting theory holds that galactic arms are formed by the orbital resonances generated amongst the individual stars of the disk.
As enough momentum is transferred outwards, spirals might gradually come to a halt. The disk would disappear, and be replaced by an amorphous, non-defined mass of hectic star systems. In all respects, the result would be an elliptical galaxy.