Objects designated as bulges in disc galaxies do not form a homogeneous class. I distinguish three types: the classical bulges, the properties of which 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; and, finally, disc-like bulges, which result from the inflow of (mainly) gas to the centre-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 to distinguish between the three types of bulges and avoid considerable confusion.
ABSTRACT
We discuss the properties of spiral arms in an N‐body simulation of a barred galaxy and present evidence that these are manifold driven. The strongest evidence comes from following the ...trajectories of individual particles. Indeed, these move along the arms while spreading out a little. In the neighbourhood of the Lagrangian points they follow a variety of paths, as expected by manifold‐driven trajectories. Further evidence comes from the properties of the arms themselves, such as their shape and growth pattern. The shape of the manifold arms changes considerably with time, as expected from the changes in the bar strength and pattern speed. In particular, the radial extent of the arms increases with time, thus bringing about a considerable increase of the disc size, by as much as 50 per cent in about 1 Gyr.
'Conspiracy' between the dark and the baryonic matter prohibits an unambiguous decomposition of disc galaxy rotation curves into the corresponding components. Several methods have been proposed to ...counter this difficulty, but their results are widely discrepant. In this paper, I revisit one of these methods, which relies on the relation between the halo density and the decrease of the bar pattern speed. The latter is routinely characterized by the ratio
of the corotation radius R
CR to the bar length L
b,
. I use a set of N-body+SPH simulations, including subgrid physics, whose initial conditions cover a range of gas fractions and halo shapes. The models, by construction, have roughly the same azimuthally averaged circular velocity curve and halo density and they are all submaximal, i.e. according to previous works, they are expected to have all roughly the same
value, well outside the fast bar range (1.2 ± 0.2). Contrary to these expectations, however, these simulations end up having widely different
values, either within the fast bar range or well outside it. This shows that the
value cannot constrain the halo density, nor determine whether galactic discs are maximal or submaximal. I argue that this is true even for early-type discs (S0s and Sas).
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.
2D kinematic signatures of boxy/peanut bulges Iannuzzi, Francesca; Athanassoula, E
Monthly notices of the Royal Astronomical Society,
07/2015, Letnik:
450, Številka:
3
Journal Article
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We study the imprints of boxy/peanut structures on the 2D line-of-sight kinematics of simulated disc galaxies. The models under study belong to a family with varying initial gas fraction and halo ...triaxiality, plus few other control runs with different structural parameters; the kinematic information was extracted using the Voronoi-binning technique and parametrized up to the fourth order of a Gauss–Hermite series. Building on a previous work for the long-slit case, we investigate the 2D kinematic behaviour in the edge-on projection as a function of the boxy/peanut strength and position angle; we find that for the strongest structures the highest moments show characteristic features away from the mid-plane in a range of position angles. We also discuss the masking effect of a classical bulge and the ambiguity in discriminating kinematically this spherically symmetric component from a boxy/peanut bulge seen end-on. Regarding the face-on case, we extend existing results to encompass the effect of a second buckling and find that this phenomenon spurs an additional set of even deeper minima in the fourth moment. Finally, we show how the results evolve when inclining the disc away from perfectly edge-on and face-on. The behaviour of stars born during the course of the simulations is discussed and confronted to that of the pre-existing disc. The general aim of our study is providing a handle to identify boxy/peanut structures and their properties in latest generation Integral Field Unit observations of nearby disc galaxies.
The Efstathiou, Lake and Negroponte (1982) criterion cannot distinguish bar stable from bar unstable discs and thus should not be used in semi-analytic galaxy formation simulations. I discuss the ...reasons for this, illustrate it with examples and point out shortcomings in the recipes used for spheroid formation. I propose an alternative, although much less straightforward, possibility.
We study the role of radial migration of stars on the chemical evolution of the Milky Way disk. We are interested in the impact of that process on the local properties of the disk and on the ...morphological properties of the resulting thick and thin disks. We use a model with several new or up-dated ingredients: atomic and molecular gas phases, star formation that depends on molecular gas, yields from a recent homogeneous grid and observationally inferred SNIa rates. Our model reproduces current values of most of the main global observables of the MW disk and bulge, and also the observed "stacked" evolution of MW-type galaxies. The thick disk extends up to ~11 kpc and has a scale length of 1.8 kpc, which is considerably shorter than the thin disk, because of the inside-out formation scheme. We also show how, in this framework, current and forthcoming spectroscopic observations can constrain the nucleosynthesis yields of massive stars for the metallicity range of 0.1 Z to 2-3 Z.
Bars in hydrodynamical cosmological simulations Scannapieco, Cecilia; Athanassoula, E.
Monthly notices of the Royal Astronomical Society,
September 2012, 2012-09-01, 20120901, 2012-09, Letnik:
425, Številka:
1
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ABSTRACT
We study the properties of two bars formed in fully cosmological hydrodynamical simulations of the formation of Milky Way‐mass galaxies. In one case, the bar formed in a system with disc, ...bulge and halo components, and is relatively strong and long, as could be expected for a system where the spheroid strongly influences the evolution. The second bar is less strong, shorter and formed in a galaxy with no significant bulge component. We study the strength and length of the bars, the stellar density profiles along and across the bars, and the velocity fields in the bar region. We compare them with the results of dynamical (idealized) simulations and with observations, and find, in general, a good agreement, although we detect some important differences as well. Our results show that more or less realistic bars can form naturally in a Λ cold dark matter cosmology, and open up the possibility to study the bar formation process in a more consistent way than previously done, since the host galaxies grow, accrete matter and significantly evolve during the formation and evolution of the bar.
We present an implementation of the Schwarzschild orbit superposition method, which can be used for constructing self-consistent equilibrium models of barred or non-barred disc galaxies, or of ...elliptical galaxies with figure rotation. This is a further development of the publicly available code smile; its main improvements include a new efficient representation of an arbitrary gravitational potential using two-dimensional spline interpolation of Fourier coefficients in the meridional plane, as well as the ability to deal with rotation of the density profile and with multicomponent mass models. We compare several published methods for constructing composite axisymmetric disc–bulge–halo models and demonstrate that our code produces the models that are closest to equilibrium. We also apply it to create models of triaxial elliptical galaxies with cuspy density profiles and figure rotation, and find that such models can be found and are stable over many dynamical times in a wide range of pattern speeds and angular momenta, covering both slow- and fast-rotator classes. We then attempt to create models of strongly barred disc galaxies, using an analytic three-component potential, and find that it is not possible to make a stable dynamically self-consistent model for this density profile. Finally, we take snapshots of two N-body simulations of barred disc galaxies embedded in nearly-spherical haloes, and construct equilibrium models using only information on the density profile of the snapshots. We demonstrate that such reconstructed models are in near-stationary state, in contrast with the original N-body simulations, one of which displayed significant secular evolution.
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.
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