Formation and evolution of bars in disc galaxies

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.Comment: 12 pages, 8 figures, includes gh2001-asp.sty, invited review paper for the 4th Guillermo Haro conference on "Disks of Galaxies: Kinematics, Dynamics and Perturbations" at Puebla, Mexico, ASP Conf. Series 275, eds. E. Athanassoula, A. Bosma & R. Mujic

Unravelling the mystery of the M31 bar

The inclination of M31 is too close to edge-on for a bar component to be easily recognised and is not sufficiently edge-on for a boxy/peanut bulge to protrude clearly out of the equatorial plane. Nevertheless, a sufficient number of clues allow us to argue that this galaxy is barred. We use fully self-consistent N-body simulations of barred galaxies and compare them with both photometric and kinematic observational data for M31. In particular, we rely on the near infrared photometry presented in a companion paper. We compare isodensity contours to isophotal contours and the light profile along cuts parallel to the galaxy major axis and offset towards the North, or the South, to mass profiles along similar cuts on the model. All these comparisons, as well as position velocity diagrams for the gaseous component, give us strong arguments that M31 is barred. We compare four fiducial N-body models to the data and thus set constraints on the parameters of the M31 bar, as its strength, length and orientation. Our `best' models, although not meant to be exact models of M31, reproduce in a very satisfactory way the main relevant observations. We present arguments that M31 has both a classical and a boxy/peanut bulge. Its pseudo-ring-like structure at roughly 50' is near the outer Lindblad resonance of the bar and could thus be an outer ring, as often observed in barred galaxies. The shape of the isophotes also argues that the vertically thin part of the M31 bar extends considerably further out than its boxy bulge, i.e. that the boxy bulge is only part of the bar, thus confirming predictions from orbital structure studies and from previous N-body simulations.Comment: 14 pages, 12 figures, minor corrections, accepted by MNRAS. Version with high resolution figures at http://www.oamp.fr/dynamique/pap/M31_th.pd

Towards understanding the dynamics of the bar/bulge region in our Galaxy

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.Comment: 6 pages, 3 figures, invited talk at "Assembling the Puzzle of the Milky Way" conference, April 17-22 2011, Grand Bornan

Topics on bar and bulge formation and evolution

I discuss results from the COSMOS survey, showing that the fraction of disc galaxies that is barred decreases considerably with look-back time from z ~ 0.2 to z ~ 0.8. This decrease is more important for small mass and low luminosity spirals. Classical bar formation theory provides a promising framework for understanding these results. I also discuss the formation of discy bulges using N-body simulations reproducing well the properties of observed discy bulges. Thus, these simulated discy bulges have the shape of a disc, they have Sersic profiles with small values of the shape index and their size is of the order of a kpc. They are formed by radial inflow of material driven by the bar and are thus composed of both gas and stars and have a considerable fraction of young stars. They can harbour spiral structure, or an inner bar.Comment: 4 pages, 2 figures, contributed paper to the Rome meeting on "Formation and Evolution of Galaxy Disks", eds, J. Funes, S.J. and E.M. Corsin

New Results on Bar-Halo Interactions

In this paper I argue that, far from necessarily hindering bar formation in disc galaxies, inner haloes may stimulate it. This constitutes a new instability mechanism by which bars can grow. To show this I use a number of N-body simulations whose initial conditions have identical discs and more or less concentrated haloes. They show that the bar that grows in the more halo-dominated environment is considerably stronger than the bar that grows in the more disc-dominated environment. This result is obtained from simulations with live haloes, i.e. composed of particles which respond to the disc and take part in the evolution. On the other hand, if the halo is rigid, it hinders or quenches bar formation, as expected. Comparison of two simulations which are identical in everything, except that the halo is live in the first one and rigid in the second one, leads me to suggest that the halo response can help the bar grow. Following the orbits of the stars in the halo, I find that a considerable fraction of the halo particles are in resonance with the bar. The halo may thus take angular momentum from the bar and stimulate its growth. I finally discuss whether and how the results of the N-body simulations can be applied to real galaxies.Comment: 11 pages, 4 figures, includes dunk2001_asp.sty, invited paper for the Ken Freeman conference "The Dynamics, Structure and History of Galaxies" at Dunk Island, Australia, ASP Conf. series, eds. G. da Costa & E. Sadle

Gas flow in barred galaxies

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.Comment: 6 pages, 2 figures, style file incl., to appear in the proceedings of an ESO/CTIO/LCO workshop "Stars, Gas and Dust in Galaxies : Exploring the Links", eds. Alloin, Olsen & Galaz (ASP Conf. Series

N-body simulations of interactions and mergings in small galaxy groups

In this paper I focus on three topics related to the dynamical evolution of small galaxy groups, for which the input of N-body simulations has been decisive. These are the merging rates in compact groups, the properties of remnants of multiple mergers, and the evolution of disc galaxies surrounded by one or more satellites. The short dynamical times of compact groups make it difficult to understand why such groups are observed at all. N-body simulations have pointed out two possible classes of solutions to this problem. The first one proposes that there is on-going formation of compact groups, or that the longevity of the group is due to secondary infall. For the second class of solutions the longevity of compact groups is due either to their specific initial conditions, or to a massive common halo, encompassing the whole group. I discuss here these alternatives, together with their respective advantages and disadvantages. I then turn to the structure of remnants of multiple mergers and compare the results of N-body simulations with the properties of observed elliptical galaxies. Finally I discuss the dynamical evolution of a disc galaxy surrounded by one or more spherical satellites.Comment: 10 pages, 2 figures, to appear in the proceedings of IAU Colloquium 174, "Small Galaxy Groups", eds. M. Valtonen & C. Flyn

Bars and the connection between dark and visible matter

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.Comment: 10 pages, 4 figures, includes style file, invited review at IAU Symposium 220,eds S. Ryder, D.J. Pisano, M. Walker & K.C. Freema

Bar slowdown and the distribution of dark matter in barred galaxies

`Conspiracy' between the dark and the baryonic mater 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 characterised by the ratio ${\cal R}$ of the corotation radius $R_{CR}$ to the bar length $L_b$, ${\cal R}=R_{CR}/L_b$. I use a set of $N$-body+SPH simulations, including sub-grid 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 ${\cal R}$ value, well outside the fast bar range (1.2 $\pm$ 0.2). Contrary to these expectations, however, these simulations end up having widely different ${\cal R}$ values, either within the fast bar range, or well outside it. This shows that the ${\cal R}$ value can not 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).Comment: 5 pages, 4 figures, accepted for MNRAS Letter