Author ORCID Identifier

https://orcid.org/0000-0001-9940-1522

Date Available

12-11-2024

Year of Publication

2024

Document Type

Doctoral Dissertation

Degree Name

Doctor of Philosophy (PhD)

College

Arts and Sciences

Department/School/Program

Physics and Astronomy

Advisor

Dr. Isaac Shlosman

Abstract

The majority of disk galaxies possess stellar bars, which are the main driver in the internal evolution of barred galaxies. Bars represent non-axisymmetric perturbations in the galactic disks, producing strong gravitational torques that facilitate angular momentum transfer from the disk to the spheroidal components, such as central stellar bulges and host dark matter (DM) halos. The redistribution of angular momentum in barred galaxies proceeds mainly via resonant interactions between the bar and orbits in the spheroids. The amount of angular momentum emitted or absorbed at each resonance depends on the mass distribution, density and spin in the spheroidal components. Thus, the properties of the stellar bars, e.g., pattern speed, bar strength, and time of the vertical buckling instability, will differ from galaxy to galaxy, and from model to model. By observing the galactic morphology and kinematics, we can gain an insight into some hidden properties of galactic components, such as DM densities and spin of DM halos, which so far cannot be detected directly.

My thesis aims to study the resonant interaction between stellar bars and their spheroids, bulges and halos. I have constructed a series of models of barred galaxies embedded in DM halos, with and without stellar bulges, and used high-resolution $N$-body numerical simulations to study their evolution as a function of bulge and halo properties. I have developed an orbital analysis tool to determine the resonant frequencies during the vertical buckling instability. I studied various properties of stellar bars that evolved being embedded in different DM densities and spin. Furthermore, to extend this study, I have analyzed models with stellar bulges having different mass concentrations and spin. Unexpectedly, I have found a by-product of this evolution: a new process of `cooling' of the vertical stellar oscillations, from the halo region down to the disk plane.

My main results are as follows: (1) Resonant excitation has an important role in triggering the buckling instability, and the contribution from the non-resonant firehose instability should be re-evaluated; (2) the buckling instability is associated with an abrupt increase in the central mass concentration and leads to circulation cells within the bar, increasing vorticity. This velocity field is absent in classical firehose instability; (3) both the planar and vertical 2:1 resonances appear only with the buckling, and quickly reach the overlapping phase --- thus supporting the energy transfer from horizontal to vertical motions during this instability; (4) with a lower DM density and higher halo spin, stellar bars maintain their pattern speed approximately constant, despite the bar staying strong, up to $\sim 6$\,Gyr. The buckling instability has been delayed for this time period; (5) this absence of a slowdown of the stellar bar follows from the alignment of the stellar bar and the induced DM bar; (6) in models with stellar bulges, the bulge plays a minor role in the exchange of angular momentum between the barred disk and the DM halo, during its spinup and spindown; (7) in bulge models with spinning halos, the buckling process has a prolonged amplitude tail, extending by few Gyrs; (8) the `cooling' process of a small fraction of DM particles has been triggered by the resonant interactions between halo orbits and the stellar bar. Applying this mechanism to the stellar halo, and analyzing the kinematics of the cooled stars, I have concluded that this new phenomenon can provide an explanation for the observed disk population of metal-poor stars.

Digital Object Identifier (DOI)

https://doi.org/10.13023/etd.2024.440

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