Self-Perpetuating Spiral Arms

 

The precise nature of spiral structure in galaxies remains uncertain. Recent studies suggest that spirals result from interactions between disks and satellite galaxies. Instead, leaving aside the grand bi-symmetric spirals, here we consider the possibility that the multi-armed spiral features originate from density inhomogeneities orbiting within disks. Using high-resolution N-body simulations, we follow the motions of stars under the influence of gravity, and show that mass concentrations with properties similar to those of giant molecular clouds can encourage the formation of spiral arms through a process termed swing amplification.  However, unlike in earlier work, we demonstrate that the eventual response of the disk can be highly non-linear, significantly modifying the development and longevity of the resulting patterns. Contrary to expectation, ragged spiral structures can survive at least in a statistical sense long after the original perturbing influence has been removed, motivating a new interpretation of various phenomena, including disk heating, radial migration, and galaxy pattern speeds.


This is an artistic visualization of our simulation on the formation of spiral features  done in collaboration with SCI in Utah.



Disk with 1,000 giant molecular clouds corotating shows formation of spiral arms

(the disk is displayed face-on on cartesian coordinates on the left panel while surface density residuals are shown in polar polar coordinates on the right panel).












Spiral arms are density waves and are not material: a patch of stars is identified in the overdenity region of the arm and colored in black at the time when the arms are fully deveoped. The stars in the patch are followed forward in time and spread out confirming that spiral arms in our simulations are density waves.





Disk once the 1,000 perturbers are removed: arms are surprisingly long-lived.













A stellar disk is perturbed by a single object with mass of 10^7 Msun forced to circle on a circular orbit: evidence of non-linear effects after less than 1 galactic year.




After 1 galactic year the object of 10^7 Msun is removed: the arms become self-perpetuating.






The disk is bombarded by substructures as predicted by cosmological simulations that leads to the formation of rings and arcs but not ragged spiral arms.