Transient Spiral Arms from Out of Equilibrium Gravitational Evolution


Spiral galaxies have the shape of a disk made up of a nucleus with some arms wrapped around it and are some of the most striking and interesting objects of the visible universe revealed by astronomy.  Many theories have tried to explain the origin of these systems, but none has ever established itself as the correct and definitive one. One of the most famous of the last century is that which presupposes the existence of dark matter, that is, a component of matter not directly observable but only perceptible. The rotation curves of spiral galaxies, that is the changes in the orbital velocity of stars as a function of their distance from the center of the galaxy, have so far provided the most solid evidence in favor of dark matter. In this system the stars farthest from the galactic center orbit at almost the same speed as the nearest ones and this phenomenon breaks the third law of Kepler, which defines the velocity of the galaxies inversely proportional to the distance from the center. To explain the unexpected phenomenon of this anomaly, therefore, it was necessary to hypothesize a mass greater than the visible one.

Another explanation of the particular velocities observed in spiral galaxies according to the researchers is provided by the modified Newtonian Dynamics model (Mond):  this theory  theory proposes to modify Newton’s second law, introducing a new fundamental constant.

Left panel of figure: NGC 3310. Right panel: Snapshot of one of our numerical N-body simulations

In both these approaches spiral arms are understood as quasi stationary waves as in this video: the galaxy is in a stationary equilibrum configuration and there are developed, due to some instabilities, density waves that have a spiral configuration. However this kind of features have never been observed in a N-body simulation.


Our work fits into this interesting scenario by providing a third key to understanding the phenomenon. Thanks to computer simulations and theoretical calculations, we have reproduced the gravitational collapse of an ellipsoidal cloud of isolated particles giving them a small initial rotation speed, noting that they derive systems that are qualitatively similar to spiral galaxies, whose arms are not stationary, ie they do not orbit like the planets around the Sun, which are in a state of equilibrium, but are phenomena generated by a dynamics out of equilibrium. Their main characteristic is to have radial as well as circular velocities and the spiral arms are formed precisely because of the combination of these two motions. Instead of having theorized only one theoretical model, we have opened up a broad spectrum of possible models, on which new studies will be based, offering a different reading framework for a fascinating and still mysterious phenomenon.


Read more: