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Recently, many orbital studies in barred galaxy potentials have revealed the existence of orbits which are not trapped around x1 tree orbits, but could be potentially appropriate building blocks for bars. These findings question the uniqueness of the x1 family as the standard paradigm of orbital motion in galactic bars. The main goal of this paper is to investigate the role that such orbits could play in shaping the morphology of bars. We trace the morphological patterns appearing in the face-on and edge-on views of the non-periodic orbits presented in these studies and we show that they are introduced in the system by second type bifurcations of x1. For this purpose, we use a typical 3D Ferrers bar model and follow the radial and vertical bifurcations of the x1 family considered as being multi periodic, with multi = 2, 3, 5. The variation of the stability indices of x1 in the multi = 2, 3 cases give us also the four- and six- periodic orbits, respectively. We tabulate these orbits including all information necessary to assess their role as appropriate building blocks. We discuss their stability and their extent, as well as their size and morphological evolution, as a function of energy. We conclude that even the most important of the multi-periodic orbits presented in Tables 2 to 5 are less appropriate building blocks for bars than the families of the x1 tree at the same energy., 20 pages, 16 figures, 4 tables with small figures, published in Monthly Notices of the Royal Astronomical Society

We study the effect of angular momentum on the surface density profiles of disc galaxies, using high resolution simulations of major mergers whose remnants have downbending radial density profiles (type II). As described in the previous papers of this series, in this scenario, most of the disc mass is acquired after the collision via accretion from a hot gaseous halo. We find that the inner and outer disc scalelengths, as well as the break radius, correlate with the total angular momentum of the initial merging system, and are larger for high angular momentum systems. We follow the angular momentum redistribution in our simulated galaxies, and find that, like the mass, the disc angular momentum is acquired via accretion, i.e. to the detriment of the gaseous halo. Furthermore, high angular momentum systems give more angular momentum to their discs, which affects directly their radial density profile. Adding simulations of isolated galaxies to our sample, we find that the correlations are valid also for disc galaxies evolved in isolation. We show that the outer part of the disc at the end of the simulation is populated mainly by inside-out stellar migration, and that in galaxies with higher angular momentum, stars travel radially further out. This, however, does not mean that outer disc stars (in type II discs) were mostly born in the inner disc. Indeed, generally the break radius increases over time, and not taking this into account leads to overestimating the number of stars born in the inner disc., Comment: 12 pages, 13 figures, accepted for publication in MNRAS

We present evidence of bar-induced secular evolution in galactic discs using 3.6 ${\mu m}$ images of nearby galaxies from the Spitzer Survey of Stellar Structure in Galaxies (S4G). We find that among massive galaxies ($M_{star}/M_{sun}> 10^{10}$), longer bars tend to reside in inner discs having a flatter radial profile. Such galaxies show a light deficit in the disc surrounding the bar, within the bar radius and often show a $\Theta$-shaped morphology. We quantify this deficit and find that among all galaxies explored in this study (with $10^{9}, Comment: Accepted for publication in MNRAS, 12 pages, 8 figures

Barred galaxies have interesting morphological features whose presence and properties set constraints on galactic evolution. Here we examine barlenses, i.e. lens-like components whose extent along the bar major axis is shorter than that of the bar and whose outline is oval or circular. We identify and analyse barlenses in $N$-body plus SPH simulations, compare them extensively with those from the NIRS0S (Near-IR S0 galaxy survey) and the S$^4$G samples (Spitzer Survey of Stellar Structure in Galaxies) and find very good agreement. We observe barlenses in our simulations from different viewing angles. This reveals that barlenses are the vertically thick part of the bar seen face-on, i.e. a barlens seen edge-on is a boxy/peanut/X bulge. In morphological studies, and in the absence of kinematics or photometry, a barlens, or part of it, may be mistaken for a classical bulge. Thus the true importance of classical bulges, both in numbers and mass, is smaller than currently assumed, which has implications for galaxy formation studies. Finally, using the shape of the isodensity curves, we propose a rule of thumb for measuring the barlens extent along the bar major axis of moderately inclined galaxies, thus providing an estimate of which part of the bar is thicker., 21 pages, 11 figures, revised version as published in MNRAS

We examine the effects that the modelling of a Boxy/Peanut (B/P) bulge will have on the estimates of the stellar gravitational potential, forces, orbital structure and bar strength of barred galaxies. We present a method for obtaining the potential of disc galaxies from surface density images, assuming a vertical density distribution (height function), which is let to vary with position, thus enabling it to represent the geometry of a B/P. We construct a B/P height function after the results from a high-resolution, N-body+SPH simulation of an isolated galaxy and compare the resulting dynamical model to those obtained with the commonly used, position-independent "flat" height functions. We show that methods that do not allow for a B/P can induce errors in the forces in the bar region of up to 40% and demonstrate that this has a significant impact on the orbital structure of the model, which in turn determines its kinematics and morphology. Furthermore, we show that the bar strength is reduced in the presence of a B/P. We conclude that neglecting the vertical extent of a B/P can introduce considerable errors in the dynamical modelling. We also examine the errors introduced in the model due to uncertainties in the parameters of the B/P and show that even for generous but realistic values of the uncertainties, the error will be noticeably less than that of not modelling a B/P bulge at all., Comment: Accepted for publication in MNRAS

Boxy/Peanut/X-shaped (B/P/X) bulges are studied using the 3.6 mum images from the Spitzer Survey of Stellar Structure in Galaxies, and the K_s-band images from the Near-IR S0 galaxy Survey. They are compared with the properties of barlenses, defined as lens-like structures embedded in bars, with sizes of about 50% of bars and axial ratios of 0.6-0.9. Based on observations and recent simulation models we show evidence that barlenses are the more face-on counterparts of B/P/X-shaped bulges. Using unsharp masks 18 new X-shaped structures are identified, covering a large range of galaxy inclinations. The similar masses and red B-3.6 mum colors of the host galaxies, and the fact that the combined axial ratio distribution of the host galaxy disks is flat, support the interpretation that barlenses and X-shapes are physically the same phenomenon. In Hubble types -3, Comment: First submission (sent to astro-ph in June 2014) replaced with the version accepted to MNRAS Letters (Vol. 444, L80-84). The biggest difference to the previous submission is Fig. 2

We present a pilot study on the nearby massive galaxy NGC 1291, in which we aim to constrain the dark matter in the inner regions, by obtaining a dynamical determination of the disc mass-to-light ratio (M/L). To this aim, we model the bar-induced dust lanes in the galaxy, using hydrodynamic gas response simulations. The models have three free parameters, the M/L of the disc, the bar pattern speed and the disc height function. We explore the parameter space to find the best fit models, i.e. those in which the morphology of the shocks in the gas simulations matches the observed dust lanes. The best-fit models suggest that the M/L of NGC 1291 agrees with that predicted by stellar population synthesis models in the near-infrared ($\approx$0.6\,$M_{\odot}/L_{\odot}$), which leads to a borderline maximum disc for this galaxy. The bar rotates fast, with corotation radius $\leq$ 1.4 times the bar length. Additionally, we find that the height function has a significant effect on the results, and can bias them towards lower or higher M/L., 3 pages. Proceedings of IAU Symposium 321, "Formation and evolution of galaxy outskirts", Eds. A. Gil de Paz, J. C. Lee & J. H. Knapen, Cambridge University Press, Cambridge

We present the metallicity results from the ARGOS spectroscopic survey of the Galactic bulge. Our aim is to understand the formation of the Galactic bulge: did it form via mergers, as expected from Lambda CDM theory, or from disk instabilities, as suggested by its boxy/peanut shape, or both? We have obtained spectra for 28,000 stars at a spectral resolution of R = 11,000. From these spectra, we have determined stellar parameters and distances to an accuracy of < 1.5 kpc. The stars in the inner Galaxy span a large range in [Fe/H], -2.8 < [Fe/H] < +0.6. From the spatial distribution of the red clump stars as a function of [Fe/H] (Ness et al. 2012a), we propose that the stars with [Fe/H] > -0.5 are part of the boxy/peanut bar/bulge. We associate the lower metallicity stars ([Fe/H] < -0.5) with the thick disk, which may be puffed up in the inner region, and with the inner regions of the metal-weak thick disk and inner halo. For the bulge stars with [Fe/H] > -0.5, we find two discrete populations; (i) stars with [Fe/H] ~ -0.25 which provide a roughly constant fraction of the stars in the latitude interval b = -5 deg to -10 deg, and (ii) a kinematically colder, more metal-rich population with mean [Fe/H] ~ +0.15 which is more prominent closer to the plane. The changing ratio of these components with latitude appears as a vertical abundance gradient of the bulge. We attribute both of these bulge components to instability-driven bar/bulge formation from the thin disk. We do not exclude a weak underlying classical merger-generated bulge component, but see no obvious kinematic association of any of our bulge stars with such a classical bulge component. [abridged], Accepted MNRAS, 21 pages, 20 figures, 6 tables

We discuss the properties of spiral arms in a 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% in about a Gyr.

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 (idealised) 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 $\Lambda$CDM 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 study the bar-driven dynamics in the inner part of the Milky Way by using invariant manifolds. This theory has been successfully applied to describe the morphology and kinematics of rings and spirals in external galaxies, and now, for the first time, we apply it to the Milky Way. We compute the orbits confined by the invariant manifolds of the unstable periodic orbits located at the ends of the bar. We discuss whether the COBE/DIRBE bar and the Long bar compose a single bar or two independent bars and perform a number of comparisons which, taken together, argue strongly in favour of the former. More specifically, we favour the possibility that the so-called COBE/DIRBE bar is the boxy/peanut bulge of a bar whose outer thin parts are the so-called Long bar. This possibility is in good agreement both with observations of external galaxies, with orbital structure theory and with simulations. We then analyse in detail the morphology and kinematics given by five representative Galactic potentials. Two have a Ferrers bar, two have a quadrupole bar and the last one a composite bar. We first consider only the COBE/DIRBE bar and then extend it to include the effect of the Long bar. We find that the large-scale structure given by the manifolds describes an inner ring, whose size is similar to the near and far 3-kpc arm, and an outer ring, whose properties resemble those of the Galactic Molecular Ring. We also analyse the kinematics of these two structures, under the different galactic potentials, and find they reproduce the main over-densities found in the galactic longitude-velocity CO diagram. Finally, we consider for what model parameters, the global morphology of the manifolds may reproduce the two outer spiral arms. We conclude that this would necessitate either more massive and more rapidly rotating bars, or including in the potential an extra component describing the spiral arms.

The distinction between the chaotic and regular behaviour of orbits in galactic models is an important issue and can help our understanding of galactic dynamical evolution. In this paper, we deal with this issue by applying the Smaller ALingment Index (SALI) and Generalized ALingment Index (GALI) techniques to extensive samples of orbits obtained by integrating numerically the equations of motion in a barred galaxy potential. We estimate first the fraction of chaotic and regular orbits for the 2 degrees of freedom (d.o.f.) case [where the galaxy extends only in the (x, y) space] and show that it is a non-monotonic function of the energy. For the 3-d.o.f. extension of this model (in the z-direction), we give similar estimates, both by exploring different sets of initial conditions and by varying the model parameters, like the mass, size and pattern speed of the bar. We find that regular motion is more abundant at small radial distances from the centre of the galaxy, where the relative non-axisymmetric forcing is relatively weak, and at small distances from the equatorial plane, where the trapping around the stable periodic orbits is important. We also find that the variation in the bar pattern speed, within a realistic range of values, does not affect much the phase-space fraction of regular and chaotic motions. Using different sets of initial conditions, we show that chaotic motion is dominant in galaxy models whose bar component is more massive, while models with a fatter or thicker bar present generally more regular behaviour. Finally, we find that the fraction of orbits that are chaotic correlates strongly with the bar strength.

Spitzer Space Telescope Infrared Array Camera (IRAC) imaging provides an opportunity to study all known morphological types of galaxies in the mid-IR at a depth significantly better than ground-based near-infrared and optical images. The goal of this study is to examine the imprint of the de Vaucouleurs classification volume in the 3.6 micron band, which is the best Spitzer waveband for galactic stellar mass morphology owing to its depth and its reddening-free sensitivity mainly to older stars. For this purpose, we have prepared classification images for 207 galaxies from the Spitzer archive, most of which are formally part of the Spitzer Survey of Stellar Structure in Galaxies (S^4G), a Spitzer post-cryogenic ("warm") mission Exploration Science Legacy Program survey of 2,331 galaxies closer than 40 Mpc. For the purposes of morphology, the galaxies are interpreted as if the images are {\it blue light}, the historical waveband for classical galaxy classification studies. We find that 3.6 micron classifications are well-correlated with blue-light classifications, to the point where the essential features of many galaxies look very similar in the two very different wavelength regimes. Drastic differences are found only for the most dusty galaxies. Consistent with a previous study by Eskridge et al. (2002), the main difference between blue light and mid-IR types is an approximately 1 stage interval difference for S0/a to Sbc or Sc galaxies, which tend to appear "earlier" in type at 3.6 microns due to the slightly increased prominence of the bulge, the reduced effects of extinction, and the reduced (but not completely eliminated) effect of the extreme population I stellar component. We present an atlas of all of the 207 galaxies analyzed here, and bring attention to special features or galaxy types that are particularly distinctive in the mid-IR.

In a series of papers, we propose a theory to explain the formation and properties of rings and spirals in barred galaxies. The building blocks of these structures are orbits guided by the manifolds emanating from the unstable Lagrangian points located near the ends of the bar. In this paper, the last of the series, we present more comparisons of our theoretical results to observations and also give new predictions for further comparisons. Our theory provides the right building blocks for the rectangular-like bar outline and for ansae. We consider how our results can be used to give estimates for the pattern speed values, as well as their effect on abundance gradients in barred galaxies. We present the kinematics along the manifold loci, to allow comparisons with the observed kinematics along the ring and spiral loci. We consider gaseous arms and their relations to stellar ones. We discuss several theoretical aspects and stress that the orbits that constitute the building blocks of the spirals and rings are chaotic. They are, nevertheless, spatially well confined by the manifolds and are thus able to outline the relevant structures. Such chaos can be termed `confined chaos' and can play a very important role in understanding the formation and evolution of galaxy structures and in galactic dynamics in general. This work, in agreement with several others, argues convincingly that galactic dynamic studies should not be limited to the study of regular motions and orbits.

Cosmological N-body simulations indicate that the dark matter haloes of galaxies should be generally triaxial. Yet, the presence of a baryonic disc is believed to alter the shape of the haloes. Here we aim to study how bar formation is affected by halo triaxiality and how, in turn, the presence of the bar influences the shape of the halo. We perform a set of collisionless N-body simulations of disc galaxies with triaxial dark matter haloes, using elliptical discs as initial conditions. We study models of different halo triaxialities and, to investigate the behaviour of the halo shape in the absence of bar formation, we run models with different disc masses, halo concentrations, disc velocity dispersions and also models where the disc shape is kept artificially axisymmetric. We find that the introduction of a massive disc causes the halo triaxiality to be partially diluted. Once the disc is fully grown, a strong stellar bar develops within the halo that is still non-axisymmetric, causing it to lose its remaining non-axisymmetry. In triaxial haloes in which the initial conditions are such that a bar does not form, the halo is able to remain triaxial and the circularisation of its shape on the plane of the disc is limited to the period of disc growth. We conclude that part of the circularisation of the halo is due to disc growth, but part must be attributed to the formation of a bar. We find that initially circular discs respond excessively to the triaxial potential and become highly elongated. They also lose more angular momentum than the initially elliptical discs and thus form stronger bars. Because of that, the circularisation that their bars induce on their haloes is also more rapid. We also analyse halo vertical shapes and observe that their vertical flattenings remain considerable, meaning that the haloes become approximately oblate by the end of the simulations. [abridged]

We present a new method for constructing equilibrium phase models for stellar systems, which we call the iterative method. It relies on constrained, or guided evolution, so that the equilibrium solution has a number of desired parameters and/or constraints. This method is very powerful, to a large extent due to its simplicity. It can be used for mass distributions with an arbitrary geometry and a large variety of kinematical constraints. We present several examples illustrating it. Applications of this method include the creation of initial conditions for N-body simulations and the modelling of galaxies from their photometric and kinematic observations., 14 pages, 13 figures, Accepted for publication in MNRAS

The Efstathiou, Lake and Negroponte (1982) criterion can not 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., Comment: 4 pages, 2 figures, accepted for publication in MNRAS Letters

We show that stable double-frequency orbits form the backbone of double bars, because they trap around themselves regular orbits, as stable closed periodic orbits do in single bars, and in both cases the trapped orbits occupy similar volume of phase-space. We perform a global search for such stable double-frequency orbits in a model of double bars by constructing maps of trajectories with initial conditions well sampled over the available phase-space. We use the width of a ring sufficient to enclose a given map as the indicator of how tightly the trajectory is trapped around a double-frequency orbit. We construct histograms of these ring widths in order to determine the fraction of phase-space occupied by ordered motions. We build 22 further models of double bars, and we construct histograms showing the fraction of the phase-space occupied by regular orbits in each model. Our models indicate that resonant coupling between the bars may not be the dominant factor reducing chaos in the system., 15 pages, 9 figures, MNRAS accepted

Bars in galaxies are mainly supported by particles trapped around stable periodic orbits. These orbits represent oscillatory motion with only one frequency, which is the bar driving frequency, and miss free oscillations. We show that a similar situation takes place in double bars: particles get trapped around parent orbits, which in this case represent oscillatory motion with two frequencies of driving by the two bars, and which also lack free oscillations. Thus the parent orbits, which constitute the backbone of an oscillating potential of two independently rotating bars, are the double-frequency orbits. These orbits do not close in any reference frame, but they map onto loops, first introduced by Maciejewski & Sparke (1997). Trajectories trapped around the parent double-frequency orbit map onto a set of points confined within a ring surrounding the loop.

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.

We present a purely theoretical study of the morphological evolution of self-gravitating systems formed through the dissipationless collapse of N-point sources. We explore the effects of resolution in mass and length on the growth of triaxial structures formed by an instability triggered by an excess of radial orbits. We point out that as resolution increases, the equilibria shift, from mildly prolate, to oblate. A number of particles N ~= 100000 or larger is required for convergence of axial aspect ratios. An upper bound for the softening, e ~ 1/256, is also identified. We then study the properties of a set of equilibria formed from scale-free cold initial mass distributions, ro ~ r^-g with 0

We investigate the dynamical effects of an interaction between an initially barred galaxy and a small spherical companion using an N-body/SPH algorithm. In the models described here the small companion passes through the disc of the larger galaxy near-perpendicular to its plane. The impact positions and times are varied with respect to the phase of the bar and the dynamical evolution of the disc. The interactions produce expanding ring structures, offset bars, spokes,and other asymmetries in the stars and gas. These characteristic signatures of the interaction are present in the disc for about 1 Gyr. We find that in some cases it is possible to destroy the bar while keeping the disc structure. In general the central impacts cause larger damage to the bar and the disc than the peripheral ones. The interaction tends to accelerate the transition from a strongly barred galaxy to a weakly or non-barred galaxy. The final disc morphology is determined more by the impact position relative to the bar rather than the impact time., 19 pages LaTeX, 24 figures (included), 3 tables. Requires mn2e.cls, accepted for publication in MNRAS. High resolution version at http://www.uni-sw.gwdg.de/preprints/2003/pr2003_01.html

Boxy edge-on profiles can be accounted for not only in models of barred galaxies, but also in models of normal (non-barred) galaxies. Thus, the presence of a bar is not a sine qua non condition for the appearance of this feature, as often assumed. We show that a `boxy' or a `peanut' structure in the central parts of a model is due to the presence of vertical resonances at which stable families of periodic orbits bifurcate from the planar x1 family. The orbits of these families reach in their projections on the equatorial plane a maximum distance from the center, beyond which they increase their mean radii by increasing only their deviations from the equatorial plane. The resulting orbital profiles are `stair-type' and constitute the backbone for the observed boxy structures in edge-on views of $N$-body models and, we believe, in edge-on views of disc galaxies. Since the existence of vertical resonances is independent of barred or spiral perturbations in the disc, `boxy' profiles may appear also in almost axisymmetric cases., 5 pages, 4 figures, to appear in MNRAS

We present families, and sets of families, of periodic orbits that provide building blocks for boxy and peanut (hereafter b/p) edge-on profiles. We find cases where the b/p profile is confined to the central parts of the model and cases where a major fraction of the bar participates in this morphology. A b/p feature can be built either by 3D families associated with 3D bifurcations of the x1 family, or, in some models, even by families related with the z-axis orbits and existing over large energy intervals. The ‘X’ feature observed inside the boxy bulges of several edgeon galaxies can be attributed to the peaks of successive x1v1 orbits, provided their stability allows it. However in general, the x1v1 family has to overcome the obstacle of a S → � → S transition in order to support the structure of a b/p feature. Other families that can be the backbones of b/p features are x1v4 and z3.1s. The morphology and the size of the boxy or peanut-shaped structures we find in our models are determined by the presence and stability of the families that support b/p features. The present study favours the idea that the observed edge-on profiles are the imprints of families of periodic orbits that can be found in appropriately chosen Hamiltonian systems, describing the potential of the bar.

In this series of papers we investigate the orbital structure of 3D models representing barred galaxies. In the present introductory paper we use a fiducial case to describe all families of periodic orbits that may play a role in the morphology of three-dimensional bars. We show that, in a 3D bar, the backbone of the orbital structure is not just the x1 family, as in 2D models, but a tree of 2D and 3D families bifurcating from x1. Besides the main tree we have also found another group of families of lesser importance around the radial 3:1 resonance. The families of this group bifurcate from x1 and influence the dynamics of the system only locally. We also find that 3D orbits elongated along the bar minor axis can be formed by bifurcations of the planar x2 family. They can support 3D bar-like structures along the minor axis of the main bar. Banana-like orbits around the stable Lagrangian points build a forest of 2D and 3D families as well. The importance of the 3D x1-tree families at the outer parts of the bar depends critically on whether they are introduced in the system as bifurcations in $z$ or in $\dot{z}$.

The purpose of this article is to show that when dynamically cold, dissipationless self-gravitating systems collapse, their evolution is a strong function of the symmetry in the initial distribution. We explore with a set of pressure-less homogeneous fluids the time-evolution of ellipsoidal distributions and map the depth of potential achieved during relaxation as function of initial ellipsoid axis ratios. We then perform a series of $N$-body numerical simulations and contrast their evolution with the fluid solutions. We verify an analytic relation between collapse factor ${\cal C}$ and particle number $N$ in spherical symmetry, such that ${\cal C} \propto N^{1/3}$. We sought a similar relation for axisymmetric configurations, and found an empirical scaling relation such that ${\cal C} \propto N^{1/6}$ in these cases. We then show that when mass distributions do not respect spherical- or axial-symmetry, the ensuing gravitational collapse deepens with increasing particle number $N$ but only slowly: 86% of triaxial configurations may collapse by a factor of no more than 40 as $N\to\infty$. For $N\approx 10^5$ and larger, violent relaxation develops fully under the Lin-Mestel-Shu instability such that numerical $N$-body solutions now resolve the different initial morphologies adequately., Uses mn2e latex class; MNRAS, to appear

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.

Using an N-body + SPH code, we have performed numerical simulations to investigate the dynamical effects of an interaction between an initially barred galaxy and a small spherical companion. In the models described here the small companion passes through the disc of the larger galaxy perpendicularly to its plane. The impact positions and times are varied with respect to the evolutionary phase of the bar and disc. The interactions produce expanding ring structures, offset bars, spokes and other asymmetries in the stars and gas. They also affect the strength and pattern speed of the bar.

We use numerical simulations to investigate the cusp at the centre of elliptical galaxies, due to the slow growth of a super-massive black hole. We study this problem for axisymmetric models of galaxies, with or without rotation. The numerical simulations are based on the `Perturbation Particles' method, and use GRAPEs to compute the force due to the cusp. We study how the density cusp is affected by the initial flattening of the model, as well as the role played by initial rotation. The logarithmic slope of the density cusp is found to be very much insensitive to flattening; as a consequence, we deduce that tangential velocity anisotropy -which supports the flattening- is also of little influence on the final cusp. We investigate via two different kinds of rotating models the efficiency with which a rotation velocity component builds within the cusp. A cusp in rotation develops only for models where a net rotation component is initially present at high energy levels. The eventual observation of a central rotational velocity peak in E galaxies has therefore some implications for the galaxy dynamical history., Comment: 16 pages Latex, 19 figures, accepted for publication in MNRAS

In N-body simulations the force calculated between particles representing a given mass distribution is usually softened, to diminish the effect of graininess. In this paper we study the effect of such a smoothing, with the aim of finding an optimal value of the softening parameter. As already shown by Merritt (1996), for too small a softening the estimates of the forces will be too noisy, while for too large a softening the force estimates are systematically misrepresented. In between there is an optimal softening, for which the forces in the configuration approach best the true forces. The value of this optimal softening depends both on the mass distribution and on the number of particles used to represent it. For higher number of particles the optimal softening is smaller. More concentrated mass distributions necessitate smaller softening, but the softened forces are never as good an approximation of the true forces as for not centrally concentrated configurations. We give good estimates of the optimal softening for homogeneous spheres, Plummer spheres, and Dehnen spheres. We also give a rough estimate of this quantity for other mass distributions, based on the harmonic mean distance to the $k$th neighbour ($k$ = 1, .., 12), the mean being taken over all particles in the configuration. Comparing homogeneous Ferrers ellipsoids of different shapes we show that the axial ratios do not influence the value of the optimal softening. Finally we compare two different types of softening, a spline softening (Hernquist & Katz 1989) and a generalisation of the standard Plummer softening to higher values of the exponent. We find that the spline softening fares roughly as well as the higher powers of the power-law softening and both give a better representation of the forces than the standard Plummer softening., 16 pages Latex, 19 figures, accepted for publication in MNRAS, corrected typos, minor changes mainly in sec. 5

It is well known that the interaction between two disk galaxies generates tidal spiral arms and a connection in the form of a bridge. Here we address the question of the formation of tidal arms and bridges from a dynamical point of view. We model the bridges and tails observed in interacting galaxies using the invariant manifolds associated to the Lyapunov orbits of the Lagrangian points of the galactic system, when the two galaxies are considered as two point masses in a circular orbit.

The thickening of the bar in barred disk galaxies has a strong influence in shaping the morphology in the inner regions of a disk galaxy above the galactic plane. The result of such a secular evolutionary process can be observed in galaxies with box/peanut shaped (b/ps) bulges. We have applied a one–dimensional fitting method to our sample of 30 edge–on disk galaxies using different fitting function approaches. A clear increase in scale height can be observed in the area of the most prominent b/ps isophotes compared to the neighbouring disk and bulge areas, in agreement with the predictions of the bar thickening model.

Disc galaxies have a number of structures, such as bars, spirals, rings, discy bulges, m = 1 asymmetries, thick discs, warps etc. I will summarise what is known about their origin and in particular whether it is due to an external or an internal process. The former include interactions, major or minor mergers etc, while the latter include instabilities, or driving by another component of the same galaxy, as e.g. the bar or the halo. In cases where more than one process is eligible, I will analyse whether it is possible to distinguish between different origins, and what it would take to do so. This discussion will show that, at least in some cases, it is difficult to distinguish between an internal and an external origin.

International audience; Context. Many haloes of nearby disc galaxies contain faint and extended features, including loops, which are often interpreted as relics of satellite infall in the main galaxy's potential well. In most cases, however, the residual nucleus of the satellite is not seen, although it is predicted by numerical simulations. Aims. We test whether such faint and extended features can be associated to gas-rich, major mergers, which may also lead to disc rebuilding and thus be a corner stone for the formation of spiral galaxies. Our goal is to test whether the major merger scenario can provide a good model for a particularly difficult case, that of NGC 5907, and to compare to the scenario of a satellite infall. Methods. Using the TreeSPH code GADGET-2, we model the formation of an almost bulge-less galaxy similar to NGC 5907 (B/T less than or similar to 0.2) after a gas-rich major merger. First, we trace tidal tail particles captured by the galaxy gravitational potential to verify whether they can form loops similar to those discovered in the galactic haloes. Results. We indeed find that 3:1 major mergers can form features similar to the loops found in many galactic haloes, including in NGC 5907, and can reproduce an extended thin disc, a bulge, as well as the pronounced warp of the gaseous disc. Relatively small bulge fractions can be reproduced by a large gas fraction in the progenitors, as well as appropriate orbital parameters. Conclusions. Even though it remains difficult to fully cover the large volume of free parameters, the present modelling of the loops in NGC 5907 proves that they could well be the result of a major merger. It has many advantages over the satellite infall scenario; e.g., it solves the problem of the visibility of the satellite remnant, and it may explain some additional features in the NGC 5907 halo, as well as some gas properties of this system. For orbital parameters derived from cosmological simulations, the loops in NGC 5907 can be reproduced by major mergers (3:1 to 5:1) and possibly by intermediate mergers (5:1 to 12:1). The major merger scenario thus challenges the minor merger one and could explain many properties that haloes of spiral galaxies have in common, including their red colours and the presence of faint extended features.

Full list of authors: de Blok, W. J. G.; Athanassoula, E.; Bosma, A.; Combes, F.; English, J.; Heald, G. H.; Kamphuis, P.; Koribalski, B. S.; Meurer, G. R.; Román, J.; Sardone, A.; Verdes-Montenegro, L.; Bigiel, F.; Brinks, E.; Chemin, L.; Fraternali, F.; Jarrett, T.; Kleiner, D.; Maccagni, F. M.; Pisano, D. J.; Serra, P.; Spekkens, K.; Amram, P.; Carignan, C.; Dettmar, R. -J.; Gibson, B. K.; Holwerda, B. W.; Józsa, G. I. G.; Lucero, D. M.; Oosterloo, T. A.; Ramaila, A. J. T.; Ramatsoku, M.; Sheth, K.; Walter, F.; Wong, O. I.; Zijlstra, A. A.; Bloemen, S.; Groot, P. J.; Le Poole, R.; Klein-Wolt, M.; Körding, E. G.; McBride, V. A.; Paterson, K.; Pieterse, D. L. A.; Vreeswijk, P.; Woudt, P. A., Aims. We present the results of three commissioning H I observations obtained with the MeerKAT radio telescope. These observations make up part of the preparation for the forthcoming MHONGOOSE nearby galaxy survey, which is a MeerKAT large survey project that will study the accretion of gas in galaxies and the link between gas and star formation. Methods. We used the available H I data sets, along with ancillary data at other wavelengths, to study the morphology of the MHONGOOSE sample galaxy, ESO 302-G014, which is a nearby gas-rich dwarf galaxy. Results. We find that ESO 302-G014 has a lopsided, asymmetric outer disc with a low column density. In addition, we find a tail or filament of H I clouds extending away from the galaxy, as well as an isolated H I cloud some 20 kpc to the south of the galaxy. We suggest that these features indicate a minor interaction with a low-mass galaxy. Optical imaging shows a possible dwarf galaxy near the tail, but based on the current data, we cannot confirm any association with ESO 302-G014. Nonetheless, an interaction scenario with some kind of low-mass companion is still supported by the presence of a significant amount of molecular gas, which is almost equal to the stellar mass, and a number of prominent stellar clusters, which suggest recently triggered star formation. Conclusions. These data show that MeerKAT produces exquisite imaging data. The forthcoming full-depth survey observations of ESO 302-G014 and other sample galaxies will, therefore, offer insights into the fate of neutral gas as it moves from the intergalactic medium onto galaxies. © ESO 2020., It is a pleasure to thank the MeerKAT commissioning team without whom this paper would not have been possible. The MeerKAT telescope is operated by the South African Radio Astronomy Observatory, which is a facility of the National Research Foundation, an agency of the Department of Science and Innovation. (Part of) the data published here have been reduced using the CARACal pipeline, partially supported by ERC Starting grant number 679627 "FORNAX", MAECI Grant Number ZA18GR02, DST-NRF Grant Number 113121 as part of the ISARP Joint Research Scheme, and BMBF project 05A17PC2 for D-MeerKAT. Information about CARACal can be obtained online under the URL: https://caracal.readthedocs.io.This work is partly based on data obtained with the MeerLICHT telescope, located at the SAAO Sutherland station, South Africa. The MeerLICHT telescope is run by the MeerLICHT consortium, on behalf of Radboud University, the University of Cape Town, the Netherlands Foundation for Scientific Research (NWO), the National Research Facility of South Africa through the South African Astronomical Observatory, the University of Oxford, the University of Manchester and the University of Amsterdam. The work of PK is partially supported by the BMBF project 05A17PC2 for D-MeerKAT. This project has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (grant agreement no. 679627; project name FORNAX). BKG acknowledges the UK's Science & Technology Facilities Council (STFC), through the University of Hull Consolidated Grant (ST/R000840/1). KS acknowledges funding from the Natural Sciences and Engineering Council of Canada (NSERC). AS is supported by an NSF Astronomy and Astrophysics Postdoctoral Fellowship under award AST-1903834. EA and AB thank the CNES for financial support. The work of DJP was partially supported by NSF CAREER grant AST-1149491. FB acknowledges funding from the European Union's Horizon 2020 research and innovation programme (grant agreement no. 726384/EMPIRE). LVM and JR acknowledge financial support from the grants AYA2015-65973-C3-1-R and RTI2018-096228-B-C31 (MINECO/FEDER, UE), as well as from the State Agency for Research of the Spanish MCIU through the "Center of Excellence Severo Ochoa" award to the Instituto de Astrofisica de Andalucia (SEV-2017-0709).

We computed the chemical evolution of Seyfert galaxies, residing in spiral bulges, based on an updated model for the Milky Way bulge with updated calculations of the Galactic potential and of the feedback from the central supermassive black hole (BH) in a spherical approximation. We followed the evolution of bulges of masses $2\times 10^{9}-10^{11}M_{\odot}$ by scaling the star-formation efficiency and the bulge scalelenght as in the inverse-wind scenario for ellipticals. We successfully reproduced the observed relation between the BH mass and that of the host bulge, and the observed peak nuclear bolometric luminosity. The observed metal overabundances are easily achieved, as well as the constancy of chemical abundances with the redshift., Comment: 2 pages, 1 figure, to appear the proceedings of IAUS245 "Formation and Evolution of Galaxy Bulges"