arXiv subjects
Publications and source records attributed to E. Athanassoula.
A very important issue in the area of galactic dynamics is the detection of chaotic and ordered motion inside galaxies. In order to achieve this target, we use the Smaller ALignment Index (SALI) method, which is a very suitable tool for this kind of problems. Here, we apply this index to 3D barred galaxy potentials and we present some results on the chaotic behavior of the model when its main parameters vary.
The halo plays a crucial role in the evolution of barred galaxies. Its near-resonant material absorbs angular momentum emitted from some of the disc particles and helps the bar become stronger. As a result, a bar (oval) forms in the inner parts of the halo of strongly barred disc galaxies. It is thinner in the inner parts (but still considerably fatter than the disc bar) and tends to spherical at larger radii. Its length increases with time, while always staying shorter than the disc bar. It is roughly aligned with the disc bar, which it trails only slightly, and it turns with roughly the same pattern speed. The bi-symmetric component of the halo density continues well outside the halo bar, where it clearly trails behind the disc bar. The length and strength of the disc and halo bars correlate; the former being always much stronger than the latter. If the halo is composed of weakly interacting massive particles, then the formation of the halo bar, by redistributing the matter in the halo and changing its shape, could influence the expected annihilation signal. This is indeed found to be the case if the halo has a core, but not if it has a steep cusp. The formation and evolution of the bar strongly affect the halo orbits. A fraction of them becomes near-resonant, similar to the disc near-resonant orbits at the same resonance, while another fraction becomes chaotic. Finally, a massive and responsive halo makes it harder for a central mass concentration to destroy the disc bar.
From this vast subject, I will pick out and review three specific topics, namely the formation and evolution of bars, the formation of bulges, and the evolution during multiple major mergers. Bars form naturally in galactic discs. Their evolution is driven by the exchange of angular momentum within the galaxy. This is emitted mainly by near-resonant material in the inner disc (bar), and is absorbed by near-resonant material in the outer disc and in the halo. As a result of this, the bar becomes stronger and rotates slower. Bulges are not a homogeneous class of objects. Based on their formation history, one can distinguish three types. Classical bulges are mainly formed before the actual disc component, from collapses or mergers and the corresponding dissipative processes. Boxy/peanut bulges are parts of bars seen edge-on. Finally, disc-like bulges are formed by the inflow of material to the center due to bar torques. Major mergers bring strong and fast evolution and can turn discs into ellipticals. I present results from simulations of multiple mergers in groups of disc or of elliptical galaxies and discuss the orbital anisotropy in the merger remnant.
We have extended previous analytical studies of the interaction of dark halos with galactic disks by introducing for the halo particles anisotropic distribution functions in phase space. For this purpose we have employed the shearing sheet model of a patch of a galactic disk embedded in a homogeneous halo. We find that velocity anisotropy increases considerably the maximum growth factor of perturbations in the disk.
We study the impact of the halo shape and geometry on the expected weakly interacting massive particle (WIMP) dark matter annihilation signal from the galactic center. As the halo profile in the innermost region is still poorly constrained, we consider different density behaviors like flat cores, cusps and spikes, as well as geometrical distortions. We show that asphericity has a strong impact on the annihilation signal when the halo profile near the galactic center is flat, but becomes gradually less significant for cuspy profiles, and negligible in the presence of a central spike. However, the astrophysical factor is strongly dependent on the WIMP mass and annihilation cross-section in the latter case.
Kn-band imaging of a sample of 30 edge-on spiral galaxies with a boxy or peanut-shaped (B/PS) bulge is discussed. Galaxies with a B/PS bulge tend to have a more complex morphology than galaxies with other bulge types, unsharp-masked images revealing structures that trace the major orbit families of three-dimensional bars. Their surface brightness profiles are also more complex, typically containing 3 or more clearly separated regions, including a shallow or flat intermediate region (Freeman Type II profiles), suggestive of bar-driven transfer of angular momentum and radial redistribution of material. The data also suggest abrupt variations of the discs' scaleheights, as expected from the vertical resonances and instabilities present in barred discs but contrary to conventional wisdom. Counter to the standard `bulge + disc' model, we thus propose that galaxies with a B/PS bulge are composed of a thin concentrated disc (a disc-like bulge) contained within a partially thick bar (the B/PS bulge), itself contained within a thin outer disc. The inner disc most likely formed through bar-driven processes while the thick bar arises from buckling instabilities. Both are strongly coupled dynamically and are formed mostly of the same (disc) material.
We study the effect of a central mass concentration (CMC) on the secular evolution of a barred disc galaxy. Unlike previous studies, we use fully self-consistent 3D $N$-body simulations with live haloes, which are known to be important for bar evolution. The CMC is introduced gradually, to avoid transients. In all cases where the mass of the CMC is of the order of, or more than, a few per cent of the mass of the disc, the strength of the bar decreases noticeably. The amount of this decrease depends strongly on the bar type. For the same CMC, bars with exponential surface-density profile, which formed in a disk-dominated galaxy (MD-type bars), can be totally destroyed, while strong bars with a flat surface-density profile, whose evolution is largely due to the halo (MH-type bars), witness only a decrease of their strength. This decrease occurs simultaneously from both the innermost and outermost parts of the bar. The CMC has a stronger effect on the Fourier components of higher azimuthal wave number $m$, leading to fatter and/or less rectangular bars. Furthermore, the CMC changes the side-on outline from peanut-shaped to boxy or, for massive CMCs, to elliptical. Similarly, side-on initially boxy outlines can be destroyed. The CMC also influences the velocity dispersion profiles. Most of the decrease of the bar strength occurs while the mass of the CMC increases and it is accompanied by an increase of the pattern speed. In all our simulations, the mass of the CMC necessary in order to destroy the bar is at least several per cent of the mass of the disc. This argues that observed super-massive black holes are not likely to destroy pre-existing bars.
We present a HI synthesis imaging study of NGC 864 (CIG 96), a spiral galaxy well isolated from similarly sized companions, yet presenting an intriguing asymmetry in its integral HI spectrum. The asymmetry in the HI profile is associated with a strong kinematical perturbation in the gaseous envelope of the galaxy, where at one side the decay of the rotation curve is faster than keplerian. We detect a small (M(HI) = 5 x 10^6 Msun) galaxy with a faint optical counterpart at around 80 kpc projected distance from NGC 864. This galaxy is probably not massive enough to have caused the perturbations in NGC 864. We discuss alternatives, such as the accretion of a gaseous companion at a radial velocity lower than the maximum.
A live halo plays an active role in the formation and evolution of bars by participating in the angular momentum redistribution which drives the dynamical evolution. Angular momentum is emitted mainly by near-resonant material in the bar region and is absorbed mainly by near-resonant material in the halo and in the outer disc. This exchange determines the strength of the bar, the decrease of its pattern speed, as well as its morphology. Thus, contrary to previous beliefs, a halo can help the bar grow, so that bars growing in galaxies with responsive massive haloes can become stronger than bars growing in disc dominated galaxies. During the evolution the halo does not stay axisymmetric. It forms a bar which is shorter and fatter than the disc bar and stays so all through the simulation, although its length grows considerably with time. I discuss the orbital structure in the disc and the halo and compare it with periodic orbits in analytical barred galaxy potentials. A central mass concentration (e.g. a central black hole, or a central disc) weakens a bar and increases its pattern speed. The effect of the central mass concentration depends strongly on the model, being less strong in models with a massive concentrated halo and a strong bar.
Present in over 45% of local spirals, boxy and peanut-shaped bulges are generally interpreted as edge-on bars and may represent a key phase in the evolution of bulges. Aiming to test such claims, the kinematic properties of self-consistent 3D N-body simulations of bar-unstable disks are studied. Using Gauss-Hermite polynomials to describe the stellar kinematics, a number of characteristic bar signatures are identified in edge-on disks: 1) a major-axis light profile with a quasi-exponential central peak and a plateau at moderate radii (Freeman Type II profile); 2) a ``double-hump'' rotation curve; 3) a sometime flat central velocity dispersion peak with a plateau at moderate radii and occasional local central minimum and secondary peak; 4) an h3-V correlation over the projected bar length. All those kinematic features are spatially correlated and can easily be understood from the orbital structure of barred disks. They thus provide a reliable and easy-to-use tool to identify edge-on bars. Interestingly, they are all produced without dissipation and are increasingly realized to be common in spirals, lending support to bar-driven evolution scenarios for bulge formation. So called ``figure-of-eight'' position-velocity diagrams are never observed, as expected for realistic orbital configurations. Although not uniquely related to triaxiality, line-of-sight velocity distributions with a high velocity tail (i.e. an h3-V correlation) appear as particularly promising tracers of bars. The stellar kinematic features identified grow in strength as the bar evolves and vary little for small inclination variations. Many can be used to trace the bar length. Comparisons with observations are encouraging and support the view that boxy and peanut-shaped bulges are simply thick bars viewed edge-on.
Objects designated as bulges in disc galaxies do not form a homogeneous class. I distinguish three types. The classical bulges, whose properties are similar to those of ellipticals and which form by collapse or merging. Boxy and peanut bulges, which are seen in near edge-on galaxies and which are in fact just a part of the bar seen edge-on. Finally disc-like bulges, which result from the inflow of (mainly) gas to the center-most parts, and subsequent star formation. I make a detailed comparison of the properties of boxy and peanut bulges with those of $N$-body bars seen edge-on and answer previously voiced objections about the links between the two. I also present and analyse simulations where a boxy /peanut feature is present at the same time as a classical spheroidal bulge and compare them with observations. Finally, I propose a nomenclature that can help distinguish between the three types of bulges and avoid considerable confusion.
Angular momentum redistribution within barred galaxies drives their dynamical evolution. Angular momentum is emitted mainly by near-resonant material in the bar region and absorbed by resonant material mainly in the outer disc and in the halo. This exchange determines the strength of the bar, the decrease of its pattern speed, as well as its morphology. If the galaxy has also a gaseous component and/or a companion or satellite, then these also take part in the angular momentum exchange. During the evolution a bar structure forms in the inner parts of the halo as well. This bar is shorter and fatter than the disc bar and stays so all through the simulation, although its length grows considerably with time. Viewed edge-on, the bar in the disc component acquires a boxy or peanut shape. I describe the families of periodic orbits that explain such structures and review the observations showing that boxy/peanut `bulges' are in fact just bars seen edge-on.
A multi-faceted approach is described to constrain the importance of bar-driven evolution in disk galaxies, particularly bulge formation. N-body simulations are used to construct stellar kinematic bar diagnostics for edge-on systems and to quantify the expected vertical structure of bars, and they are compared to observations of 30 edge-on spirals, most with a boxy bulge. Long-slit spectra of the galaxies show characteristic double-hump rotation curves, dispersion profiles with secondary peaks and/or flat maxima, and correlated h3 and V profiles, indicating that most of them harbor edge-on bars. The presence of cold, quasi-axisymmetric central stellar disks is also suggested, presumably formed through bar-driven gaseous inflow and star formation. K-band imaging of the same galaxies spectacularly highlights radial variations of the bars' scaleheights, as expected from vertical disk instabilities. The light profiles also vary radially in shape but never approach a classic de Vaucouleurs law. Filtering of the images further isolates the specific orbit families at the origin of the boxy structure, which can be directly related to periodic orbit calculations in 3D barred potentials. Bars are thus shown to contribute substantially to the formation of both large-scale triaxial bulges and embedded central disks. Relevant results from the SAURON survey of the stellar/ionized-gas kinematics and stellar populations of spheroids are also described. Examples are used to illustrate the potential of coupling stellar kinematics and linestrengths (age and metallicity), here specifically to unravel the dynamical evolution and related chemical enrichment history of bars and bulges. [Abridged]
Isolated barred galaxies evolve by redistributing their internal angular momentum, which is emitted mainly at the inner disc resonances and absorbed mainly at the resonances in the outer disc and the halo. This causes the bar to grow stronger and its pattern speed to decrease with time. A massive, responsive halo enhances this process. I show correlations and trends between the angular momentum absorbed by the halo and the bar strength, pattern speed and morphology. It is thus possible to explain why some disc galaxies are strongly barred, while others have no bar, or only a short bar or an oval. In some cases, a bar is found also in the halo component. This ``halo bar'' is triaxial, but more prolate-like, is shorter than the disc bar and rotates with roughly the same pattern speed. I finally discuss whether bars can modify the density cusps found in cosmological CDM simulations of dark matter haloes.
We study the regeneration of stellar bars triggered by a tidal interaction, using numerical simulations of either purely stellar or stellar+gas disc galaxies. We find that interactions which are sufficiently strong to regenerate the bar in the purely stellar models do not lead to a regeneration in the dissipative models, owing to the induced gas inflow in those models. In models in which the bar can be regenerated, we find a tight correlation between the strength and the pattern speed of the induced bar. This relation can be explained by a significant radial redistribution of angular momentum in the disc due to the interaction, similar to the processes and correlations found for isolated barred spirals. We furthermore show that the regenerated bars show the same dynamical properties as their isolated counterparts.
We argue that inner rings in barred spiral galaxies are associated with specific 2D and 3D families of periodic orbits located just beyond the end of the bar. These are families located between the inner radial ultraharmonic 4:1 resonance and corotation. They are found in the upper part of a type-2 gap of the x1 characteristic, and can account for the observed ring morphologies without any help from families of the x1-tree. Due to the evolution of the stability of all these families, the ring shapes that are favored are mainly ovals, as well as polygons with `corners' on the minor axis, on the sides of the bar. On the other hand pentagonal rings, or rings of the NGC 7020 type hexagon, should be less probable. The orbits that make the rings belong in their vast majority to 3D families of periodic orbits and orbits trapped around them.
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.
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.