Aims. Periodic orbits (POs) have been exhaustively studied. On the contrary, to our knowledge, no... more Aims. Periodic orbits (POs) have been exhaustively studied. On the contrary, to our knowledge, no complete and systematic study of higher-multiplicity (ℳ) POs, that is, orbits that close after more than one revolution in phase space, exists. Here, we fill this gap and also extend the standard tools used for studies of the x1 POs to studies of higher multiplicity POs. Methods. We adopted a multi-aspect approach, using surfaces of section, stability diagrams, characteristic diagrams, studies of the shapes of individual orbits, and other properties of the POs. We modified and extended the standard tools used for ℳ = 1, to ℳ > 1 cases, allowing them to use the snapshot information more fully. Our potential is more realistic than those of most previous studies, as it is obtained directly from a snapshot of a fully self-consistent, high-resolution numerical simulation. Results. We find five main pairs of PO families with ℳ = 2. Two of these bifurcate from the x1 family and are direct r...
Monthly Notices of the Royal Astronomical Society, 2018
A method, which we have developed for determining corotation radii, has allowed us to map in deta... more A method, which we have developed for determining corotation radii, has allowed us to map in detail the radial resonant structures of barred spiral galaxies. Here, we have combined this information with new determinations of the bar strength and the pitch angle of the innermost segment of the spiral arms to find relationships between these parameters of relevance to the dynamical evolution of the galaxies. We show how (1) the bar mass fraction, (2) the scaled bar angular momentum, (3) the pitch angle, and (4) the shear parameter vary along the Hubble sequence, and we also plot along the Hubble sequence (5) the scaled bar length, (6) the ratio of bar corotation radius to bar length, (7) the scaled bar pattern speed, and (8) the bar strength. It is of interest to note that the parameters (2), (5), (6), (7), and (8) all show breaks in their behaviour at type Scd. We find that bars with high shear have only small pitch angles, while bars with large pitch angles must have low shear; we also find a generally inverse trend of the pitch angle with bar strength. An inference that at first seems counter-intuitive is that the most massive bars rotate most slowly but have the largest angular momenta. Among a further set of detailed results, we pick out here the 2:1 ratio between the number of spiral arms and the number of corotations outside the bar. These results give a guideline to theories of disc-bar evolution.
1 Research Center for Astronomy, Academy of Athens, Soranou Efessiou 4, GR-11527, Athens, Greece ... more 1 Research Center for Astronomy, Academy of Athens, Soranou Efessiou 4, GR-11527, Athens, Greece [email protected], [email protected] 2 Department of Economic Studies, University of Thessaly, Volos, Greece [email protected] ... Summary. In order to ...
Communicating Astronomy with the Public 2007: Proceedings from the IAU/National Observatory of At... more Communicating Astronomy with the Public 2007: Proceedings from the IAU/National Observatory of Athens/ESA/ESO Conference, Athens, Greece, 8-11 October 2007, Christensen LL, Zoulias M. & Robson, I. (eds.). Published by Eugenides Foundation. ... The Society for Space and ...
The flow in the arms of spiral galaxies is qualitatively different among different morphological ... more The flow in the arms of spiral galaxies is qualitatively different among different morphological types. The stars that reinforce the spiral arms can be either participating in an ordered or in a chaotic flow. Ordered flows are typically described by precessing ellipses corresponding to stable periodic orbits at successive energies (Jacobi constants). Contrarily, there are spiral arms in barred-spiral systems which may be supported by stars in chaotic motion "along" the arms. The trajectories of these stars are associated with the invariant manifolds of unstable periodic orbits at corotations region. We find that the spirals and the outer parts of the bars share the same orbital content. However, there also barredspiral systems with spirals inside corotation, consisting mainly by chaotic orbits, or by ordered flows. The talk will present models where various dynamical mehanisms coexits and will compare them with structures that are possible the result of the concerted actio...
It has proven very difficult to estimate the amplitudes of spiralperturbations in disk galaxies f... more It has proven very difficult to estimate the amplitudes of spiralperturbations in disk galaxies from observations due to the variation ofmass-to-light ratio and extinction across spiral arms. Deep, near-infraredimages of grand-design spiral galaxies obtained with HAWK-I/VLT were used toanalyze the azimuthal amplitude and shape of arms, which, even in the K-bandmay,be significantly biased by the presence of young stellar populations. Severaltechniques were applied to evaluate the relative importance of young starsacross the arms, such as surface brightness of the disk with light fromclusters subtracted, number density of clusters detected, and texture of thedisk. The modulation of the texture measurement,which correlates with the numberdensity of faint clusters, yields amplitudes of the spiral perturbation in therange 0.1-0.2. This estimate gives a better estimate of the mass perturbationin the spiral arms, since it is dominated by old clusters.
International Journal of Bifurcation and Chaos, 1994
The problems encountered in the study of three-dimensional Hamiltonian systems by means of the Po... more The problems encountered in the study of three-dimensional Hamiltonian systems by means of the Poincare cross-sections are reviewed. A new method to overcome these problems is proposed. In order to visualize the four-dimensional “space” of section we introduce the use of color and rotation. We apply this method to the case of a family of simple periodic orbits in a three-dimensional potential and we describe the differences in the orbital behavior between regions close to stable and unstable periodic orbits. We outline the differences between the transition from stability to simple instability and the transition from stability to complex instability. We study the changes in the structure of the 4D “spaces” of section, which occur when the family becomes complex unstable after a DU →Δ or a S →Δ transition. We conclude that the orbital behavior after the transition depends on the orbital behavior before it.
Monthly Notices of the Royal Astronomical Society, 2005
We propose particle swarm optimization (PSO) as an alternative method for locating periodic orbit... more We propose particle swarm optimization (PSO) as an alternative method for locating periodic orbits in a three-dimensional (3D) model of barred galaxies. We develop an appropriate scheme that transforms the problem of finding periodic orbits into the problem of detecting global minimizers of a function, which is defined on the Poincaré surface section of the Hamiltonian system. By combining the PSO method with deflection techniques, we succeeded in tracing systematically several periodic orbits of the system. The method succeeded in tracing the initial conditions of periodic orbits in cases where Newton iterative techniques had difficulties. In particular, we found families of two-and three-dimensional periodic orbits associated with the inner 8:1 to 12:1 resonances, between the radial 4:1 and corotation resonances of our 3D Ferrers bar model. The main advantages of the proposed algorithm are its simplicity, its ability to work using function values solely, and its ability to locate many periodic orbits per run at a given Jacobian constant.
Using N-body simulations we study the structures induced on a galactic disc by repeated flybys of... more Using N-body simulations we study the structures induced on a galactic disc by repeated flybys of a companion in decaying eccentric orbit around the disc. Our system is composed by a stellar disc, bulge and live dark matter halo, and we study the system's dynamical response to a sequence of a companion's flybys, when we vary i) the disc's temperature (parameterized by Toomre's Q-parameter) and ii) the companion's mass and initial orbit. We use a new 3D Cartesian grid code: MAIN (Mesh-adaptive Approximate Inverse N-body solver). The main features of MAIN are reviewed, with emphasis on the use of a new Symmetric Factored Approximate Sparse Inverse (SFASI) matrix in conjunction with the multigrid method that allows the efficient solution of Poisson's equation in three space variables. We find that: i) companions need to be assigned initial masses in a rather narrow window of values in order to produce significant and more long-standing non-axisymmetric structure...
Monthly Notices of the Royal Astronomical Society, 2010
We present the orbital analysis of four response models, that succeed in reproducing morphologica... more We present the orbital analysis of four response models, that succeed in reproducing morphological features of NGC 1300. Two of them assume a planar (2D) geometry with Ω p =22 and 16 km s -1 kpc -1 respectively. The two others assume a cylindrical (thick) disc and rotate with the same pattern speeds as the 2D models. These response models reproduce most successfully main morphological features of NGC 1300 among a large number of models, as became evident in a previous study. Our main result is the discovery of three new dynamical mechanisms that can support structures in a barred-spiral grand design system. These mechanisms are presented in characteristic cases, where these dynamical phenomena take place. They refer firstly to the support of a strong bar, of ansae type, almost solely by chaotic orbits, then to the support of spirals by chaotic orbits that for a certain number of pattern revolutions follow an n:1 (n=7,8) morphology, and finally to the support of spiral arms by a combination of orbits trapped around L 4,5 and sticky chaotic orbits with the same Jacobi constant. We have encountered these dynamical phenomena in a large fraction of the cases we studied as we varied the parameters of our general models, without forcing in some way their appearance. This suggests that they could be responsible for the observed morphologies of many barred-spiral galaxies. Comparing our response models among themselves we find that the NGC 1300 morphology is best described by a thick disc model for the bar region and a 2D disc model for the spirals, with both components rotating with the same pattern speed Ω p =16 km s -1 kpc -1 . In such a case, the whole structure is included inside the corotation of the system. The bar is supported mainly by regular orbits, while the spirals are supported by chaotic orbits.
International Journal of Bifurcation and Chaos, 2011
We study the orbital behavior at the neighborhood of complex unstable periodic orbits in a 3D aut... more We study the orbital behavior at the neighborhood of complex unstable periodic orbits in a 3D autonomous Hamiltonian system of galactic type. At a transition of a family of periodic orbits from stability to complex instability (also known as Hamiltonian Hopf Bifurcation) the four eigenvalues of the stable periodic orbits move out of the unit circle. Then the periodic orbits
Monthly Notices of the Royal Astronomical Society, 2016
Using N-body simulations we study the structures induced on a galactic disc by repeated flybys of... more Using N-body simulations we study the structures induced on a galactic disc by repeated flybys of a companion in decaying eccentric orbit around the disc. Our system is composed by a stellar disc, bulge and live dark matter halo, and we study the system's dynamical response to a sequence of a companion's flybys, when we vary i) the disc's temperature (parameterized by Toomre's Q-parameter) and ii) the companion's mass and initial orbit. We use a new 3D Cartesian grid code: MAIN (Mesh-adaptive Approximate Inverse N-body solver). The main features of MAIN are reviewed, with emphasis on the use of a new Symmetric Factored Approximate Sparse Inverse (SFASI) matrix in conjunction with the multigrid method that allows the efficient solution of Poisson's equation in three space variables. We find that: i) companions need to be assigned initial masses in a rather narrow window of values in order to produce significant and more long-standing non-axisymmetric structures (bars and spirals) in the main galaxy's disc by the repeated flyby mechanism. ii) a crucial phenomenon is the antagonism between companion-excited and self-excited modes on the disc. Values of Q > 1.5 are needed in order to allow for the growth of the companion-excited modes to prevail over the the growth of the disc's self-excited modes. iii) We give evidence that the companion-induced spiral structure is best represented by a density wave with pattern speed nearly constant in a region extending from the ILR to a radius close to, but inside, corotation.
I will review the dynamical mechanisms that have been found to shape the observed structures of d... more I will review the dynamical mechanisms that have been found to shape the observed structures of disk galaxies, such as the spirals and the bars. The morphological properties of the spiral arms will be described in association with different dynamical mechanisms that have been proposed to build them. Differences are expected due to the fact that the spirals can be reinforced by stars in regular or in chaotic motion. Also in bars chaos may contribute to the observed morphologies in percentages that vary from case to case. I will also discuss the vertical structure of disk galaxies and the possible contribution of chaotic orbits to their density profiles away from the equatorial plane.
The spiral structure of five ordinary spiral galaxies was studied using deep BVIK' surface ph... more The spiral structure of five ordinary spiral galaxies was studied using deep BVIK' surface photometry maps obtained at the 2.2m ESO/MPI telescope. The detailed shape of the arms was analyzed in terms of the spiral density wave theory. Grand design spirals were found on the K' maps in all five galaxies although at least two would be classified as flocculent
Deep K-band surface photometry of 12 spiral galaxies was used to analyze the spiral structure in ... more Deep K-band surface photometry of 12 spiral galaxies was used to analyze the spiral structure in their disks. Amplitude and shape of the main two-armed spiral arms were determined. Further, synthetic rotation curves were derived using the radial distribution of the K surface photometry and kinematic data available for the galaxies. The radial extent of the main spiral patterns was compared to the locations of major resonances (e.g., ILR, 4:1, CR, OLR) suggested by the rotation curves. This was used to estimate possible values for the angular speed of the spiral patterns.
Although interarm features in the Population I disk frequently give an impression of a three-fold... more Although interarm features in the Population I disk frequently give an impression of a three-fold symmetry, three-armed spiral structures in the old stellar disk are seldom seen. Such case, like NGC 7137, are of special interest since they can be used to study unique conditions which favor the growth of m=3 modes in stellar disks. The current paper presents the structure of the main family of periodic orbits in a model of the three-armed galaxy NGC 7137 and compares it with near-infrared maps of the galaxy. Finally we examine the relation of the orbital dynamics with 3-armed patterns we find in self-gravitating N-body models.
Monthly Notices of the Royal Astronomical Society, 2010
We study the stellar response in a spectrum of potentials describing the barred spiral galaxy NGC... more We study the stellar response in a spectrum of potentials describing the barred spiral galaxy NGC 1300. These potentials have been presented in a previous paper and correspond to three different assumptions as regards the geometry of the galaxy. For each potential we consider a wide range of Ω p pattern speed values. Our goal is to discover the geometries and the Ω p supporting specific morphological features of NGC 1300. For this purpose we use the method of response models. In order to compare the images of NGC 1300 with the density maps of our models, we define a new index which is a generalization of the Hausdorff distance. This index helps us to find out quantitatively which cases reproduce specific features of NGC 1300 in an objective way. Furthermore, we construct alternative models following a Schwarzschild type technique. By this method we vary the weights of the various energy levels, and thus the orbital contribution of each energy, in order to minimize the differences between the response density and that deduced from the surface density of the galaxy, under certain assumptions. We find that the models corresponding to Ω p ≈ 16km s −1 kpc −1 and Ω p ≈ 22km s −1 kpc −1 are able to reproduce efficiently certain morphological features of NGC 1300, with each one having its advantages and drawbacks.
Aims. Periodic orbits (POs) have been exhaustively studied. On the contrary, to our knowledge, no... more Aims. Periodic orbits (POs) have been exhaustively studied. On the contrary, to our knowledge, no complete and systematic study of higher-multiplicity (ℳ) POs, that is, orbits that close after more than one revolution in phase space, exists. Here, we fill this gap and also extend the standard tools used for studies of the x1 POs to studies of higher multiplicity POs. Methods. We adopted a multi-aspect approach, using surfaces of section, stability diagrams, characteristic diagrams, studies of the shapes of individual orbits, and other properties of the POs. We modified and extended the standard tools used for ℳ = 1, to ℳ > 1 cases, allowing them to use the snapshot information more fully. Our potential is more realistic than those of most previous studies, as it is obtained directly from a snapshot of a fully self-consistent, high-resolution numerical simulation. Results. We find five main pairs of PO families with ℳ = 2. Two of these bifurcate from the x1 family and are direct r...
Monthly Notices of the Royal Astronomical Society, 2018
A method, which we have developed for determining corotation radii, has allowed us to map in deta... more A method, which we have developed for determining corotation radii, has allowed us to map in detail the radial resonant structures of barred spiral galaxies. Here, we have combined this information with new determinations of the bar strength and the pitch angle of the innermost segment of the spiral arms to find relationships between these parameters of relevance to the dynamical evolution of the galaxies. We show how (1) the bar mass fraction, (2) the scaled bar angular momentum, (3) the pitch angle, and (4) the shear parameter vary along the Hubble sequence, and we also plot along the Hubble sequence (5) the scaled bar length, (6) the ratio of bar corotation radius to bar length, (7) the scaled bar pattern speed, and (8) the bar strength. It is of interest to note that the parameters (2), (5), (6), (7), and (8) all show breaks in their behaviour at type Scd. We find that bars with high shear have only small pitch angles, while bars with large pitch angles must have low shear; we also find a generally inverse trend of the pitch angle with bar strength. An inference that at first seems counter-intuitive is that the most massive bars rotate most slowly but have the largest angular momenta. Among a further set of detailed results, we pick out here the 2:1 ratio between the number of spiral arms and the number of corotations outside the bar. These results give a guideline to theories of disc-bar evolution.
1 Research Center for Astronomy, Academy of Athens, Soranou Efessiou 4, GR-11527, Athens, Greece ... more 1 Research Center for Astronomy, Academy of Athens, Soranou Efessiou 4, GR-11527, Athens, Greece [email protected], [email protected] 2 Department of Economic Studies, University of Thessaly, Volos, Greece [email protected] ... Summary. In order to ...
Communicating Astronomy with the Public 2007: Proceedings from the IAU/National Observatory of At... more Communicating Astronomy with the Public 2007: Proceedings from the IAU/National Observatory of Athens/ESA/ESO Conference, Athens, Greece, 8-11 October 2007, Christensen LL, Zoulias M. & Robson, I. (eds.). Published by Eugenides Foundation. ... The Society for Space and ...
The flow in the arms of spiral galaxies is qualitatively different among different morphological ... more The flow in the arms of spiral galaxies is qualitatively different among different morphological types. The stars that reinforce the spiral arms can be either participating in an ordered or in a chaotic flow. Ordered flows are typically described by precessing ellipses corresponding to stable periodic orbits at successive energies (Jacobi constants). Contrarily, there are spiral arms in barred-spiral systems which may be supported by stars in chaotic motion "along" the arms. The trajectories of these stars are associated with the invariant manifolds of unstable periodic orbits at corotations region. We find that the spirals and the outer parts of the bars share the same orbital content. However, there also barredspiral systems with spirals inside corotation, consisting mainly by chaotic orbits, or by ordered flows. The talk will present models where various dynamical mehanisms coexits and will compare them with structures that are possible the result of the concerted actio...
It has proven very difficult to estimate the amplitudes of spiralperturbations in disk galaxies f... more It has proven very difficult to estimate the amplitudes of spiralperturbations in disk galaxies from observations due to the variation ofmass-to-light ratio and extinction across spiral arms. Deep, near-infraredimages of grand-design spiral galaxies obtained with HAWK-I/VLT were used toanalyze the azimuthal amplitude and shape of arms, which, even in the K-bandmay,be significantly biased by the presence of young stellar populations. Severaltechniques were applied to evaluate the relative importance of young starsacross the arms, such as surface brightness of the disk with light fromclusters subtracted, number density of clusters detected, and texture of thedisk. The modulation of the texture measurement,which correlates with the numberdensity of faint clusters, yields amplitudes of the spiral perturbation in therange 0.1-0.2. This estimate gives a better estimate of the mass perturbationin the spiral arms, since it is dominated by old clusters.
International Journal of Bifurcation and Chaos, 1994
The problems encountered in the study of three-dimensional Hamiltonian systems by means of the Po... more The problems encountered in the study of three-dimensional Hamiltonian systems by means of the Poincare cross-sections are reviewed. A new method to overcome these problems is proposed. In order to visualize the four-dimensional “space” of section we introduce the use of color and rotation. We apply this method to the case of a family of simple periodic orbits in a three-dimensional potential and we describe the differences in the orbital behavior between regions close to stable and unstable periodic orbits. We outline the differences between the transition from stability to simple instability and the transition from stability to complex instability. We study the changes in the structure of the 4D “spaces” of section, which occur when the family becomes complex unstable after a DU →Δ or a S →Δ transition. We conclude that the orbital behavior after the transition depends on the orbital behavior before it.
Monthly Notices of the Royal Astronomical Society, 2005
We propose particle swarm optimization (PSO) as an alternative method for locating periodic orbit... more We propose particle swarm optimization (PSO) as an alternative method for locating periodic orbits in a three-dimensional (3D) model of barred galaxies. We develop an appropriate scheme that transforms the problem of finding periodic orbits into the problem of detecting global minimizers of a function, which is defined on the Poincaré surface section of the Hamiltonian system. By combining the PSO method with deflection techniques, we succeeded in tracing systematically several periodic orbits of the system. The method succeeded in tracing the initial conditions of periodic orbits in cases where Newton iterative techniques had difficulties. In particular, we found families of two-and three-dimensional periodic orbits associated with the inner 8:1 to 12:1 resonances, between the radial 4:1 and corotation resonances of our 3D Ferrers bar model. The main advantages of the proposed algorithm are its simplicity, its ability to work using function values solely, and its ability to locate many periodic orbits per run at a given Jacobian constant.
Using N-body simulations we study the structures induced on a galactic disc by repeated flybys of... more Using N-body simulations we study the structures induced on a galactic disc by repeated flybys of a companion in decaying eccentric orbit around the disc. Our system is composed by a stellar disc, bulge and live dark matter halo, and we study the system's dynamical response to a sequence of a companion's flybys, when we vary i) the disc's temperature (parameterized by Toomre's Q-parameter) and ii) the companion's mass and initial orbit. We use a new 3D Cartesian grid code: MAIN (Mesh-adaptive Approximate Inverse N-body solver). The main features of MAIN are reviewed, with emphasis on the use of a new Symmetric Factored Approximate Sparse Inverse (SFASI) matrix in conjunction with the multigrid method that allows the efficient solution of Poisson's equation in three space variables. We find that: i) companions need to be assigned initial masses in a rather narrow window of values in order to produce significant and more long-standing non-axisymmetric structure...
Monthly Notices of the Royal Astronomical Society, 2010
We present the orbital analysis of four response models, that succeed in reproducing morphologica... more We present the orbital analysis of four response models, that succeed in reproducing morphological features of NGC 1300. Two of them assume a planar (2D) geometry with Ω p =22 and 16 km s -1 kpc -1 respectively. The two others assume a cylindrical (thick) disc and rotate with the same pattern speeds as the 2D models. These response models reproduce most successfully main morphological features of NGC 1300 among a large number of models, as became evident in a previous study. Our main result is the discovery of three new dynamical mechanisms that can support structures in a barred-spiral grand design system. These mechanisms are presented in characteristic cases, where these dynamical phenomena take place. They refer firstly to the support of a strong bar, of ansae type, almost solely by chaotic orbits, then to the support of spirals by chaotic orbits that for a certain number of pattern revolutions follow an n:1 (n=7,8) morphology, and finally to the support of spiral arms by a combination of orbits trapped around L 4,5 and sticky chaotic orbits with the same Jacobi constant. We have encountered these dynamical phenomena in a large fraction of the cases we studied as we varied the parameters of our general models, without forcing in some way their appearance. This suggests that they could be responsible for the observed morphologies of many barred-spiral galaxies. Comparing our response models among themselves we find that the NGC 1300 morphology is best described by a thick disc model for the bar region and a 2D disc model for the spirals, with both components rotating with the same pattern speed Ω p =16 km s -1 kpc -1 . In such a case, the whole structure is included inside the corotation of the system. The bar is supported mainly by regular orbits, while the spirals are supported by chaotic orbits.
International Journal of Bifurcation and Chaos, 2011
We study the orbital behavior at the neighborhood of complex unstable periodic orbits in a 3D aut... more We study the orbital behavior at the neighborhood of complex unstable periodic orbits in a 3D autonomous Hamiltonian system of galactic type. At a transition of a family of periodic orbits from stability to complex instability (also known as Hamiltonian Hopf Bifurcation) the four eigenvalues of the stable periodic orbits move out of the unit circle. Then the periodic orbits
Monthly Notices of the Royal Astronomical Society, 2016
Using N-body simulations we study the structures induced on a galactic disc by repeated flybys of... more Using N-body simulations we study the structures induced on a galactic disc by repeated flybys of a companion in decaying eccentric orbit around the disc. Our system is composed by a stellar disc, bulge and live dark matter halo, and we study the system's dynamical response to a sequence of a companion's flybys, when we vary i) the disc's temperature (parameterized by Toomre's Q-parameter) and ii) the companion's mass and initial orbit. We use a new 3D Cartesian grid code: MAIN (Mesh-adaptive Approximate Inverse N-body solver). The main features of MAIN are reviewed, with emphasis on the use of a new Symmetric Factored Approximate Sparse Inverse (SFASI) matrix in conjunction with the multigrid method that allows the efficient solution of Poisson's equation in three space variables. We find that: i) companions need to be assigned initial masses in a rather narrow window of values in order to produce significant and more long-standing non-axisymmetric structures (bars and spirals) in the main galaxy's disc by the repeated flyby mechanism. ii) a crucial phenomenon is the antagonism between companion-excited and self-excited modes on the disc. Values of Q > 1.5 are needed in order to allow for the growth of the companion-excited modes to prevail over the the growth of the disc's self-excited modes. iii) We give evidence that the companion-induced spiral structure is best represented by a density wave with pattern speed nearly constant in a region extending from the ILR to a radius close to, but inside, corotation.
I will review the dynamical mechanisms that have been found to shape the observed structures of d... more I will review the dynamical mechanisms that have been found to shape the observed structures of disk galaxies, such as the spirals and the bars. The morphological properties of the spiral arms will be described in association with different dynamical mechanisms that have been proposed to build them. Differences are expected due to the fact that the spirals can be reinforced by stars in regular or in chaotic motion. Also in bars chaos may contribute to the observed morphologies in percentages that vary from case to case. I will also discuss the vertical structure of disk galaxies and the possible contribution of chaotic orbits to their density profiles away from the equatorial plane.
The spiral structure of five ordinary spiral galaxies was studied using deep BVIK' surface ph... more The spiral structure of five ordinary spiral galaxies was studied using deep BVIK' surface photometry maps obtained at the 2.2m ESO/MPI telescope. The detailed shape of the arms was analyzed in terms of the spiral density wave theory. Grand design spirals were found on the K' maps in all five galaxies although at least two would be classified as flocculent
Deep K-band surface photometry of 12 spiral galaxies was used to analyze the spiral structure in ... more Deep K-band surface photometry of 12 spiral galaxies was used to analyze the spiral structure in their disks. Amplitude and shape of the main two-armed spiral arms were determined. Further, synthetic rotation curves were derived using the radial distribution of the K surface photometry and kinematic data available for the galaxies. The radial extent of the main spiral patterns was compared to the locations of major resonances (e.g., ILR, 4:1, CR, OLR) suggested by the rotation curves. This was used to estimate possible values for the angular speed of the spiral patterns.
Although interarm features in the Population I disk frequently give an impression of a three-fold... more Although interarm features in the Population I disk frequently give an impression of a three-fold symmetry, three-armed spiral structures in the old stellar disk are seldom seen. Such case, like NGC 7137, are of special interest since they can be used to study unique conditions which favor the growth of m=3 modes in stellar disks. The current paper presents the structure of the main family of periodic orbits in a model of the three-armed galaxy NGC 7137 and compares it with near-infrared maps of the galaxy. Finally we examine the relation of the orbital dynamics with 3-armed patterns we find in self-gravitating N-body models.
Monthly Notices of the Royal Astronomical Society, 2010
We study the stellar response in a spectrum of potentials describing the barred spiral galaxy NGC... more We study the stellar response in a spectrum of potentials describing the barred spiral galaxy NGC 1300. These potentials have been presented in a previous paper and correspond to three different assumptions as regards the geometry of the galaxy. For each potential we consider a wide range of Ω p pattern speed values. Our goal is to discover the geometries and the Ω p supporting specific morphological features of NGC 1300. For this purpose we use the method of response models. In order to compare the images of NGC 1300 with the density maps of our models, we define a new index which is a generalization of the Hausdorff distance. This index helps us to find out quantitatively which cases reproduce specific features of NGC 1300 in an objective way. Furthermore, we construct alternative models following a Schwarzschild type technique. By this method we vary the weights of the various energy levels, and thus the orbital contribution of each energy, in order to minimize the differences between the response density and that deduced from the surface density of the galaxy, under certain assumptions. We find that the models corresponding to Ω p ≈ 16km s −1 kpc −1 and Ω p ≈ 22km s −1 kpc −1 are able to reproduce efficiently certain morphological features of NGC 1300, with each one having its advantages and drawbacks.
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Papers by P. Patsis