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Search Results: 1 - 10 of 167565 matches for " E. Athanassoula "
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Bar-halo interactions and their effect on the bar strength and pattern speed
E. Athanassoula
Revista mexicana de astronomía y astrofísica , 2003,
Abstract:
Towards understanding the dynamics of the bar/bulge region in our Galaxy
Athanassoula E.
EPJ Web of Conferences , 2012, DOI: 10.1051/epjconf/20121906004
Abstract: I review some of the work on bars which is closely linked to the bar/bulge system in our Galaxy. Several independent studies, using totally independent methods, come to the same results about the 3D structure of a bar, i.e., that a bar is composed of a vertically thick inner part and a vertically thin outer part. I give examples of this from simulations and substantiate the discussion with input from orbital structure analysis and from observations. The thick part has a considerably shorter radial extent than the thin part. I then see how this applies to our Galaxy, where two bars have been reported, the COBE/DIRBE bar and the Long bar. Comparing their extents and making the reasonable and necessary assumption that our Galaxy has properties similar to those of other galaxies of similar type, leads to the conclusion that these two bars can not form a standard double bar system. I then discuss arguments in favour of the two bars being simply different parts of the same bar, the COBE/DIRBE bar being the thick inner part and the Long bar being the thin outer part of this bar. I also very briefly discuss some related new results. I first consider bar formation and evolution in disc galaxies with a gaseous component – including star formation, feedback and evolution – and a triaxial halo. Then I consider bar formation in a fully cosmological context using hydrodynamical LCDM simulations, where the host galaxies grow, accrete matter and significantly evolve during the formation and evolution of the bar.
Angular momentum redistribution and the evolution and morphology of bars
E. Athanassoula
Physics , 2003, DOI: 10.1007/978-3-540-45040-5_26
Abstract: Angular momentum exchange is a driving process for the evolution of barred galaxies. Material at resonance in the bar region loses angular momentum which is taken by material at resonance in the outer disc and/or the halo. By losing angular momentum, the bar grows stronger and slows down. This evolution scenario is backed by both analytical calculations and by $N$-body simulations. The morphology of the bar also depends on the amount of angular momentum exchanged.
What determines the strength and the slowdown rate of bars ?
E. Athanassoula
Physics , 2003, DOI: 10.1046/j.1365-8711.2003.06473.x
Abstract: Isolated barred galaxies evolve by redistributing their angular momentum, which, emitted by material in the inner disc at resonance with the bar, can be absorbed by resonant material in the outer disc, or in the halo. The amount of angular momentum that can be emitted/absorbed at a given resonance depends on the distribution function of the emitting/absorbing material. It thus depends not only on the amount of material on resonant orbits, but also on the velocity dispersion of that material. As it loses angular momentum, the bar becomes stronger and it also rotates slower. Thus the strength of the bar and the decrease of its pattern speed with time are set by the amount of angular momentum exchanged within the galaxy, which, in turn, is regulated by the mass distribution and the velocity dispersion of the material in the disc and spheroidal components. Correlations between the pattern speed of the bar, its strength and the angular momentum absorbed by the spheroid (halo plus bulge) argue strongly that it is the amount of angular momentum exchanged that determines the strength and the slowdown rate of the bar. The decrease of the bar pattern speed with time should not be used to set constraints on the halo-to-disc mass ratio, since it depends also on the velocity dispersion of the halo and disc material.
Gas flow in barred galaxies
E. Athanassoula
Physics , 2000,
Abstract: I briefly review the properties of the gas flow in and around the region of the bar in a disc galaxy and discuss the corresponding inflow and the loci of star formation. I then review the flow of gas in barred galaxies which have an additional secondary bar. Finally I discuss the signatures of bars in edge-on galaxies.
Dynamical evolution driven by bars and interactions : Input from numerical simulations
E. Athanassoula
Physics , 2002, DOI: 10.1023/A:1019595111382
Abstract: We discuss the evolution of a disc galaxy due to the formation of a bar and, subsequently, a peanut. After the formation stage there is still considerable evolution, albeit slower. In purely stellar cases the pattern speed of the bar decreases with time, while its amplitude grows. However, if a considerable gaseous component is present in the disc, the pattern speed may increase with time, while the bar strength may decrease. In some cases the gas can be brought sufficiently close to the center to create a strong central concentration, which, in turn, may modify the properties of the bar. More violent evolution can take place during interactions, so that some disc substructures can be either formed or destroyed in a time scale which is small compared to a Hubble time. These include spirals, bars, bridges, tails, rings, thick discs and bulges. In some cases interactions may lead to mergings. We briefly review comparisons of the properties of merger remnants with those of elliptical galaxies, both for the case of pairwise mergings and the case of multiple mergings.
Bar-Halo Interaction and Bar Growth
E. Athanassoula
Physics , 2002, DOI: 10.1086/340784
Abstract: I show that strong bars can grow in galactic discs, even when the latter are immersed in haloes whose mass within the disc radius is comparable to, or larger than, the mass of the disc. I argue that this is due to the response of the halo and in particular to the destabilising influence of the halo resonant stars. Via this instability mechanism the halo can stimulate, rather than restrain, the growth of the bar.
Formation and evolution of bars in disc galaxies
E. Athanassoula
Physics , 2002,
Abstract: I follow a bar from its formation, via its evolution, to its destruction and, perhaps, regeneration. I discuss the main features at each stage and particularly the role of the halo. Bars can form even in sub-maximum discs. In fact, such bars can be stronger than bars which have grown in maximum discs. This is due to the response of the halo and, in particular, to the exchange of energy and angular momentum between the disc particles constituting the bar and the halo particles at resonance with it. The bar slowdown depends on the initial central concentration of the halo and the initial value of the disc Q. Contrary to the halo mass distribution, the disc changes its radial density profile considerably during the evolution. Applying the Sackett criterion, I thus find that discs become maximum in many simulations in which they have started off as sub-maximum. I briefly discuss the evolution if a gaseous component is present, as well as the destruction and regeneration of bars.
Boxy/peanut/X bulges, barlenses and the thick part of galactic bars: What are they and how did they form?
E. Athanassoula
Physics , 2015, DOI: 10.1007/978-3-319-19378-6_14
Abstract: Bars have a complex three-dimensional shape. In particular their inner part is vertically much thicker than the parts further out. Viewed edge-on, the thick part of the bar is what is commonly known as a boxy-, peanut- or X- bulge and viewed face-on it is referred to as a barlens. These components are due to disc and bar instabilities and are composed of disc material. I review here their formation, evolution and dynamics, using simulations, orbital structure theory and comparisons to observations.
N-body simulations of interacting disc galaxies
E. Athanassoula
Physics , 1998,
Abstract: Disc galaxies can be substantially modified by close encounters and mergers, since their discs are very responsive components. Close interactions can be held responsible for the formation of bridges and tails, as well as for the formation of some bars, asymmetries and grand design spirals. Bound clumps can form in the tails, due to self-gravity, and could evolve to dwarf galaxies. Off-centerings and asymmetries in the central parts of barred galaxies can be made by off-centered and/or oblique impacts of sufficiently massive and compact companions. Similar impacts, but preferably centered, on non-barred galaxies can form ring galaxies. Companions on initially near-circular orbits can also cause changes to the target disc as they spiral gradually inwards. Low density companions are disrupted before reaching the center of the target and their debris form a thick disc. On the other hand most of the mass of the high density companions reaches the center, where it may form a bulge, thus entailing evolution along the Hubble sequence. Such companions thicken and expand the target disc and may also destroy bars in it. If their initial orbital plane is at an angle to the plane of the disc of the target, they can cause the latter to tilt substantially, depending on their mass and initial inclination.
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