The timescale for a galaxy to stop spinning and start collapsing under the influence of gravity depends on several factors, including its mass, angular momentum, and the distribution of matter within it. Generally, galaxies are in a state of dynamic equilibrium, with the gravitational pull inward balanced by the centrifugal force outward due to their rotation.
However, if a galaxy were to lose angular momentum over time, it could eventually cease its rotation and start collapsing. The timescale for this process can vary significantly depending on the specific circumstances. In practice, galaxies do not typically collapse entirely under gravity alone. Other factors, such as dark matter and dark energy, play crucial roles in their evolution and ultimate fate.
In the absence of additional influences, a simple theoretical model known as the "Jeans instability" can provide an estimate of the timescale for gravitational collapse. The Jeans instability describes the conditions under which a self-gravitating system can collapse due to its own gravity. The timescale for collapse, known as the "Jeans time," depends on the mass, size, and density of the system.
For a galaxy, which is a complex and multi-component system, the situation is more complicated. Galaxies consist of stars, gas, dust, and dark matter, each with its own contribution to the overall dynamics. The presence of dark matter, which does not interact electromagnetically but exerts gravitational influence, can significantly affect the collapse timescale.
Moreover, galaxies are subject to various other forces and processes, such as interactions with neighboring galaxies, mergers, and feedback from star formation and active galactic nuclei. These factors can modify the collapse timescale and shape the overall evolution of a galaxy.
In summary, the exact timescale for a galaxy to stop spinning and collapse due to gravity alone is challenging to determine precisely and can vary widely depending on the galaxy's properties and the presence of other influential factors. It is an active area of research in astrophysics to better understand the mechanisms governing galaxy formation and evolution.