scholarly journals Ring and Lens Formation

1996 ◽  
Vol 157 ◽  
pp. 286-298
Author(s):  
Françoise Combes
Keyword(s):  
Spiral Structure ◽  
Observational Constraints ◽  
Dynamical Mechanism ◽  
Velocity Anisotropy ◽  
Barred Galaxies ◽  
Feedback Cycle ◽  
Gas Accretion ◽  
Lens Formation ◽  
Bar Formation ◽  
Different Time Scales

AbstractThe dynamical mechanism to form rings at Lindblad resonances in a barred galaxy is now well-known: due to its dissipative character, the gas is forced in a spiral structure, and experiences torques from the bar potential. Angular momentum is transferred until gas accumulates in the resonant rings. Some problems remain however to account for all observations, such as the very different time-scales for nuclear, inner and outer ring formation, while the three are frequently observed in the same galaxy; the shapes, orientations and thickness of the rings, etc... The adequacy of the present gas dynamical modelizations is discussed.Lenses are secondary components of barred galaxies that could originate from bar evolution. No model until now has met the observational constraints, in particular the sharp edge of the lenses, their strong velocity anisotropy, and their small thickness. We propose here that lenses are the result of partial bar destruction, a necessary step in a feedback cycle of bar formation-destruction, a cycle driven by gas accretion.

scholarly journals Secular evolution in galaxies

2006 ◽  
Vol 2 (S235) ◽  
pp. 19-23 ◽  
Author(s):  
F. Combes
Keyword(s):  
Star Formation ◽  
Numerical Simulations ◽  
Significant Role ◽  
High Frequency ◽  
Central Star ◽  
Barred Galaxies ◽  
Secular Evolution ◽  
Gas Accretion ◽  
Bar Formation

AbstractNew observations in favour of a significant role of secular evolution are reviewed: central star formation boosted in pseudo-bulge barred galaxies, relations between bulge and disk, evidence for rejuvenated bulges. Numerical simulations have shown that secular evolution can occur through a cycle of bar formation and destruction, in which the gas plays a major role. Since bars are weakened or destroyed in gaseous disks, the high frequency of bars observed today requires external cold gas accretion, to replenish the disk and allow a new bar formation. The rate of gas accretion from external filaments is compatible with what is observed in cosmological simulations.

scholarly journals Manifold spirals in barred galaxies with multiple pattern speeds

2020 ◽  
Vol 636 ◽  
pp. A44
Author(s):  
C. Efthymiopoulos ◽  
M. Harsoula ◽  
G. Contopoulos
Keyword(s):  
Invariant Manifolds ◽  
Relative Phase ◽  
Spiral Structure ◽  
Galactic Disk ◽  
Time Varying ◽  
Barred Galaxies ◽  
Single Pattern ◽  
Pattern Speed ◽  
Pattern Speeds ◽  
Manifold Theory

In the manifold theory of spiral structure in barred galaxies, the usual assumption is that the spirals rotate with the same pattern speed as the bar. Here, we generalize the manifold theory under the assumption that the spirals rotate with a different pattern speed than the bar. More generally, we consider the case in which one or more modes, represented by the potentials V2, V3, etc., coexist in the galactic disk in addition to the bar’s mode Vbar, but the modes rotate with pattern speeds, Ω2, Ω3, etc., which are incommensurable between themselves and with Ωbar. Through a perturbative treatment (assuming that V2, V3, etc. are small with respect to Vbar), we then show that the unstable Lagrangian points L1 and L2 of the pure bar model (Vbar, Ωbar) are continued in the full model as periodic orbits, in the case of one extra pattern speed, or as epicyclic “Lissajous-like” unstable orbits, in the case of more than one extra pattern speeds. We use GL1 and GL2 to denote the continued orbits around the points L1 and L2. Furthermore, we show that the orbits GL1 and GL2 are simply unstable. As a result, these orbits admit invariant manifolds, which can be regarded as the generalization of the manifolds of the L1 and L2 points in the single pattern speed case. As an example, we computed the generalized orbits GL1, GL2, and their manifolds in a Milky-Way-like model in which bar and spiral pattern speeds were assumed to be different. We find that the manifolds produce a time-varying morphology consisting of segments of spirals or “pseudorings”. These structures are repeated after a period equal to half the relative period of the imposed spirals with respect to the bar. Along one period, the manifold-induced time-varying structures are found to continuously support at least some part of the imposed spirals, except at short intervals around specific times at which the relative phase of the imposed spirals with respect to the bar is equal to ±π/2. The connection of these effects to the phenomenon of recurrent spirals is discussed.

scholarly journals New N-body experiments on the spiral structure of galaxies

1985 ◽  
Vol 106 ◽  
pp. 539-540
Author(s):  
K. O. Thielheim ◽  
H. Wolff
Keyword(s):  
Spiral Structure ◽  
Spiral Structures ◽  
Bar Formation ◽  
Armed Spiral

N-body experiments with stellar disks like those performed by Hohl (1971) and Sellwood (1981) show regular, global, two-armed spiral structures associated with bar formation. The question is whether the spiral is caused by the bar, and if this is true, how the generating mechanism works.

New models of Jupiter in the context of Juno and Galileo: signature of a decoupling between the atmosphere and the interior

2020 ◽  
Author(s):  
Florian Debras ◽  
Gilles Chabrier
Keyword(s):  
Extrasolar Planets ◽  
Convective Zone ◽  
Giant Planets ◽  
Atmospheric Composition ◽  
Observational Constraints ◽  
Physical Processes ◽  
Hot Jupiters ◽  
Formation And Evolution ◽  
Gas Accretion

<p><span lang="en-US">Juno's observations of Jupiter's gravity field have revealed extremely low values for the gravitational moments that are difficult to reconcile with the high abundance of metals observed in the atmosphere by Galileo. Recent studies chose to arbitrarily get rid of one of these two constraints in order to build models of Jupiter.</span></p> <p><span lang="en-US">In this presentation, I will detail our new Jupiter structure models reconciling Juno and Galileo observational constraints. These models confirm the need to separate Jupiter into at least 4 layers: an outer convective shell, a non-convective zone of compositional change, an inner convective shell and a diluted core representing about 60 percent of the planet in radius. Compared to other studies, these models propose a new idea with important consequences: a decrease in the quantity of metals between the outer and inner convective shells. This would imply that the atmospheric composition is not representative of the internal composition of the planet, contrary to what is regularly admitted, and would strongly impact the Jupiter formation scenarios (localization, migration, accretion).</span></p> <p><span lang="en-US">In particular, the presence of an internal non-convective zone prevents mixing between the two convective envelopes. I will detail the physical processes of this semi-convective zone (layered convection or H-He immiscibility) and explain how they may persist during the evolution of the planet.</span></p> <p><span lang="en-US">These models also impose a limit mass on the compact core, which cannot be heavier than 5 Earth masses. Such a mass, lower than the runaway gas accretion minimum mass, needs to be explained in the light of our understanding of the formation and evolution of giant planets.</span></p> <p><span lang="en-US">Using these models of Jupiter, I will finally detail the application of our new understanding of the interior of this planet to giant exoplanets. At a time of direct imaging of extrasolar planets and atmospheric characterization of hot Jupiters, a good understanding of the internal processes of planets in the solar system is paramount to make the best use of all the observations.</span></p>

scholarly journals Gas accretion on spiral galaxies: Bar formation and renewal

2002 ◽  
Vol 392 (1) ◽  
pp. 83-102 ◽  
Author(s):  
F. Bournaud ◽  
F. Combes
Keyword(s):  
Spiral Galaxies ◽  
Gas Accretion ◽  
Bar Formation

scholarly journals Bar Formation in Milky Way type Galaxies

2016 ◽  
Vol 25 (4) ◽  
Author(s):  
E. V. Polyachenko ◽  
P. Berczik ◽  
A. Just
Keyword(s):  
Milky Way ◽  
Barred Galaxies ◽  
Numerical Evidence ◽  
Pattern Speed ◽  
Widespread Belief ◽  
Bar Formation ◽  
Lindblad Resonance ◽  
Inner Lindblad Resonance

AbstractMany barred galaxies, possibly including the Milky Way, have cusps in their centers. There is a widespread belief, however, that the usual bar instability, which occurs in bulgeless galaxy models, is impossible for cuspy models because of the presence of the inner Lindblad resonance for any pattern speed. At the same time, there is numerical evidence that the bar instability can form a bar. We analyze this discrepancy by performing accurate and diverse

scholarly journals On the Lengths, Colours and Ages of Bars

2006 ◽  
Vol 2 (S235) ◽  
pp. 98-98
Author(s):  
Dimitri A. Gadotti ◽  
Ronaldo E. de Souza
Keyword(s):  
Theoretical Models ◽  
Previous Article ◽  
Age Difference ◽  
Observational Constraints ◽  
Average Difference ◽  
Barred Galaxies ◽  
Secular Evolution ◽  
Analytical Results

AbstractIn an effort to obtain further observational evidences for secular evolution processes in galaxies, as well as observational constraints to current theoretical models of secular evolution, we have used BVRI and Ks images of a sample of 18 barred galaxies to measure the lengths and colours of bars, create colour maps and estimate global colour gradients. In addition, applying a method we developed in a previous article, we could distinguish for 7 galaxies in our sample those whose bars have been recently formed from the ones with already evolved bars. We estimated an average difference in the optical colours between young and evolved bars that may be translated to an age difference of the order of 10 Gyr, meaning that bars may be long standing structures. Moreover, our results show that, on average, evolved bars are longer than young bars. This seems to indicate that, during its evolution, a bar grows longer by capturing stars from the disk, in agreement with recent numerical and analytical results.

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