Doctoral Dissertation uoadl:3414408 7 Read counter

Department of Physics

Library of the School of Science

Library of the School of Science

2024-08-31

2024

Ζουλουμη Κωνσταντινα

Χρήστος Ευθυμιόπουλος (κύριος επιβλέπων), Αναπληρωτής Καθηγητής, Department of Mathematics Tullio Levi-Civita, University of Padua

Μαρία Χαρσούλα, Ερευνήτρια A΄, Κ.Ε.Α.Ε.Μ. Ακαδημίας Αθηνών

Πέτρος Ιωάννου, αφυπ. Καθηγητής

Θεοχάρης Αποστολάτος, Καθηγητής, Τμήμα Φυσικής, ΕΚΠΑ,

Νεκτάριος Βλαχάκης, Καθηγητής, Τμήμα Φυσικής, ΕΚΠΑ,

Φώτιος Διάκονος, Καθηγητής, Τμήμα Φυσικής, ΕΚΠΑ,

Στυλιανός Καζαντζίδης, Επίκουρος Καθηγητής, Τμήμα Φυσικής, ΕΚΠΑ

Μαρία Χαρσούλα, Ερευνήτρια A΄, Κ.Ε.Α.Ε.Μ. Ακαδημίας Αθηνών

Πέτρος Ιωάννου, αφυπ. Καθηγητής

Θεοχάρης Αποστολάτος, Καθηγητής, Τμήμα Φυσικής, ΕΚΠΑ,

Νεκτάριος Βλαχάκης, Καθηγητής, Τμήμα Φυσικής, ΕΚΠΑ,

Φώτιος Διάκονος, Καθηγητής, Τμήμα Φυσικής, ΕΚΠΑ,

Στυλιανός Καζαντζίδης, Επίκουρος Καθηγητής, Τμήμα Φυσικής, ΕΚΠΑ

SPIRAL STRUCTURE AND CHAOS IN N‐ BODY MODELS OF GALACTIC DISCS

English

Greek

Greek

Spiral structure and chaos in N-body models of galactic discs

Understanding the nature of spiral arms in galaxies is a difficult open astronomical problem. There are many open problems in the study of the spiral structure, concerning the origin, evolution, and, in particular, the longevity of the spiral arms. The present thesis discusses several dynamical approaches allowing to interpret the observed galactic morphologies. Such approaches elucidate, in particular, the role of chaos and non-linear phenomena in the stability and coherence of the spiral structures. In the last decades the results of several researches in galactic dynamics have shown that the galaxies undergo substantial secular evolution. Merging events can also affect directly the dynamics of the system. Moreover, the observations, as well as N-body simulations of barred spiral galaxies give strong indications for the presence of multiple pattern speeds in galactic discs. This is in contradiction to the single pattern speed assumption inherent in many basic theoretical models of spiral structure. Density wave theory, as well as the manifold theory of the spiral structure are examples of such models. In the light of these remarks, it becomes evident that dynamical models of spiral structure need to be developed and adjust to the recent results coming from observations and N-body simulations.

In the present thesis we construct dynamical models of the spiral structure in two different cases: (a) in the case of an N-body simulation of a barred spiral galaxy, and (b) in the case of a Milky Way-like theoretical potential model. The nature of the stellar orbits that support the spiral arms differs in these two cases, thus giving rise to two different mechanisms of support of the spiral structure. In the first case, of barred spiral galaxies, the spirals are generated by chaotic orbits (‘manifold spirals’), while in the second case, of normal galaxies, the spiral arms are constructed by ordered orbits (’precessing ellipses’).

In the case of the N-body simulation we have a self-consistent N-body model of a barred spiral galaxy, where stemming from the positions and the velocities of the bodies we can obtain important information used in the modelling, namely the bodies, the surface density, mean velocitiy and gravitational potential at each point of the galactic disc plane. We then proceed with the computation of manifold spirals with a single pattern speed or with multiple pattern speeds. We also observe the evolution of the manifold spirals along with the secularly evolving structures in the disc. In the case of the Milky Way-like theoretical model, we compute, instead, models of spirals based on the superposition of periodic orbits of the family of ”precessing ellipses”. In this case, we investigate how the three main parameters of the model, mamely, the pattern speed of the spiral arms, the amplitude of the spiral pertubation and the pitch angle, collaborate so as to reproduce realistic spiral density waves.

In the present thesis we construct dynamical models of the spiral structure in two different cases: (a) in the case of an N-body simulation of a barred spiral galaxy, and (b) in the case of a Milky Way-like theoretical potential model. The nature of the stellar orbits that support the spiral arms differs in these two cases, thus giving rise to two different mechanisms of support of the spiral structure. In the first case, of barred spiral galaxies, the spirals are generated by chaotic orbits (‘manifold spirals’), while in the second case, of normal galaxies, the spiral arms are constructed by ordered orbits (’precessing ellipses’).

In the case of the N-body simulation we have a self-consistent N-body model of a barred spiral galaxy, where stemming from the positions and the velocities of the bodies we can obtain important information used in the modelling, namely the bodies, the surface density, mean velocitiy and gravitational potential at each point of the galactic disc plane. We then proceed with the computation of manifold spirals with a single pattern speed or with multiple pattern speeds. We also observe the evolution of the manifold spirals along with the secularly evolving structures in the disc. In the case of the Milky Way-like theoretical model, we compute, instead, models of spirals based on the superposition of periodic orbits of the family of ”precessing ellipses”. In this case, we investigate how the three main parameters of the model, mamely, the pattern speed of the spiral arms, the amplitude of the spiral pertubation and the pitch angle, collaborate so as to reproduce realistic spiral density waves.

Science

galaxies, spiral structure, barred galaxies, normal galaxies, chaos, chaotic orbits, invariant manifolds, precessing ellipses, density waves

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0

Yes

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