The upper panel in the image above shows the observed average movements of stars within two sample galaxies. The image on the left in the bottom panel shows the typical motion of stars around the short axis of typical low to intermediate mass galaxies, like the Milky Way. The image on the right in the bottom panel shows the motion of stars around the long axis of massive, and super massive galaxies that typically have masses of several times, to hundreds of times that of the Milky Way. In both examples, the blue part of the each sample galaxy moves towards us, while the red parts are rotating away from us.
Although much is known about the processes that drive the formation of galaxies, an equal amount remains unknown or not well understood. However, in a recent study published in the Monthly Notices of the Royal Astronomical Society, the lead investigator, Davor Krajnovic, from the Leibniz Institute for Astrophysics Potsdam (AIP), explains how super massive galaxies, about which the least is known, may have formed.
The author of the study points out that disc shaped galaxies, such as the Milky Way, rotate like a frisbee, and while this is widely accepted, the author of the study also points out the movements of stars within such galaxies can be predicted to within fairly closely constrained limits. While this does not always explain how such galaxies formed, the point the study makes is that with rare exceptions, these types of galaxies largely behave in predictable ways, whereas this cannot be said for super massive galaxies.
To illustrate this point, the study measured the movements of stars within massive galaxies that were all within 800 light years or so from us, and all within dense clusters of galaxies in some of the most densely populated regions of the Local Universe, such as the massive Shapley Supercluster of galaxies. The object of the study was to focus on the most massive galaxies; typically galaxies that are at least one hundred times more massive than the Milky Way, and to measure the mean stellar motions within those galaxies.
Using the MUSE instrument, an advanced integral-field spectrograph mounted on the Very Large Telescope belonging to the ESO at Cerro Paranal in Chile, the researchers found that instead of orbiting the short axis of the galaxies, the stars in the massive galaxies they studied largely orbited the long axes of these galaxies. In practice, these galaxies were rotating much as a rugby ball would spin if the two ends were supported.
In practical terms, this finding suggests that giant, super massive galaxies formed in a very specific way, which is borne out by the fact that the more massive a galaxy is, the more pronounced the rotation pattern of stars around the long axis become. In essence, the finding suggests that super massive galaxies form when two galaxies of similar masses and sizes collide in a certain way.
A collision between two “normal” galaxies is an extremely violent and destructive event that completely destroys the original motions of stars within both galaxies. Over eons, the collisional path imparts new patterns of motion to almost all the stars in both progenitor galaxies, with the result that new, and exceedingly complex streaming patterns arise within the now-merged galaxies that can follow any of the three possible axial paths within a spheroid.
As a practical matter, the most massive galaxies in the Universe represent the final stages of galactic evolution, and as such, it was perhaps to be expected that these galaxies would exhibit the most complex star streaming patterns. Nonetheless, despite the clarity that perfect hindsight brings, it was not until this particular study was performed that one critical piece in the puzzle of how super massive galaxies form fell into place.
All that remains now is to develop models that offer definitive proof of how the progenitor galaxies formed in the first place. While this will not resolve all the remaining unanswered questions about how galaxies form, it will go some way towards improving our collective understanding of the evolution and ultimate fate of the Universe.