The orbital structure of a tidally induced bar

Orbits are the key building blocks of any density distribution and their study helps us understand the kinematical structure and the evolution of galaxies. Here we investigate orbits in a tidally induced bar of a dwarf galaxy, using an $N$-body simulation of an initially disky dwarf galaxy orbiting a Milky Way-like host. After the first pericenter passage, a tidally induced bar forms in the stellar component of the dwarf. The bar evolution is different than in isolated galaxies and our analysis focuses on the period before it buckles. We study the orbits in terms of their dominant frequencies, which we calculate in a Cartesian coordinate frame rotating with the bar. Apart from the well-known x$_1$ orbits we find many other types, mostly with boxy shapes of various degree of elongation. Some of them are also near-periodic, admitting frequency ratios of 4/3, 3/2 and 5/3. The box orbits have various degrees of vertical thickness but only a relatively small fraction of those have banana (i.e. smile/frown) or infinity-symbol shapes in the edge-on view. In the very center we also find orbits known from the potential of triaxial ellipsoids. The elongation of the orbits grows with distance from the center of the bar in agreement with the variation of the shape of the density distribution. Our classification of orbits leads to the conclusion that more than $80 \%$ of them have boxy shapes, while only $8 \%$ have shapes of classical x$_1$ orbits.Comment: 15 pages, 15 figures, accepted for publication in Ap

Tidally induced bars in dwarf galaxies on different orbits around a Milky Way-like host

Bars in galaxies may develop through a global instability or due to an interaction with another system. We study bar formation in disky dwarf galaxies orbiting a Milky Way-like galaxy. We employ $N$-body simulations to study the impact of initial orbital parameters: the size of the dwarf galaxy orbit and the inclination of its disc with respect to the orbital plane. In all cases a bar develops in the center of the dwarf during the first pericenter on its orbit around the host. Between subsequent pericenter passages the bars are stable, but at the pericenters they are usually weakened and shortened. The initial properties and details of the further evolution of the bars depend heavily on the orbital configuration. We find that for the exactly prograde orientation, the strongest bar is formed for the intermediate-size orbit. On the tighter orbit, the disc is too disturbed and stripped to form a strong bar. On the wider orbit, the tidal interaction is too weak. The dependence on the disc inclination is such that weaker bars form in more inclined discs. The bars experience either a very weak buckling or none at all. We do not observe any secular evolution, possibly because the dwarfs are perturbed at each pericenter passage. The rotation speed of the bars can be classified as slow ($R_\mathrm{CR}/l_\mathrm{bar}\sim2-3$). We attribute this to the loss of a significant fraction of the disc's rotation during the encounter with the host galaxy.Comment: 17 pages, 14 figures, accepted to Ap

Induced Nested Galactic Bars Inside Assembling Dark Matter Halos

We investigate the formation and evolution of nested bar systems in disk galaxies in a cosmological setting by following the development of an isolated dark matter (DM) and baryon density perturbation. The disks form within the assembling triaxial DM halos and the feedback from the stellar evolution is accounted for in terms of supernovae and OB stellar winds. Focusing on a representative model, we show the formation of an oval disk and of a first generation of nested bars with characteristic sub-kpc and a few kpc sizes. The system evolves through successive dynamical couplings and decouplings, forcing the gas inwards and settles in a state of resonant coupling. The inflow rate can support a broad range of activity within the central kpc, from quasar- to Seyfert-types, supplemented by a vigorous star formation as a by-product. The initial bar formation is triggered in response to the tidal torques from the triaxial DM halo, which acts as a finite perturbation. This first generation of bars does not survive for more than 4--5 Gyr: by that time the secondary bar has totally dissolved, while the primary one has very substantially weakened, reduced to a fat oval. This evolution is largely due to chaos introduced by the interaction of the multiple non-axisymmetric components.Comment: 4 pages, 4 figures, 1 mpeg animation. To be published by the Astrophysical Journal Letters. The animation can be found at http://www.pa.uky.edu/~shlosman/research/galdyn/movies.html Replaced with an updated version (small text corrections

Halo properties and secular evolution in barred galaxies

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.Comment: 6 pages, 3 figures, to appear in "Island Universes - Structure and Evolution of Disk Galaxies" ed. R. S. de Jon

N-body simulations of galaxies and groups of galaxies with the Marseille GRAPE systems

I review the Marseille GRAPE systems and the N-body simulations done with them. I first describe briefly the available hardware and software, their possibilities and their limitations. I then describe work done on interacting galaxies and groups of galaxies. This includes simulations of the formation of ring galaxies, simulations of bar destruction by massive compact satellites, of merging in compact groups and of the formation of brightest members in clusters of galaxies.Comment: 13 pages, 5 figures, to be published in "Non-linear Dynamics and Chaos in Astrophysics", eds. J.R. Buchler, S. Gottesman, J. Hunter and H. Kandrup, Annals of the New York Academy of Science

Star formation in the warped outer pseudoring of the spiral galaxy NGC 3642

NGC 3642 was classified as a spiral galaxy with three rings and no bar. We have performed an HI and optical study of this nearly face-on galaxy. We find that the nuclear ring might in fact be part of an inner one-armed spiral, that could be driving nuclear accretion and feeding the central activity in the inner kpc. The inner ring is faint, and the outer ring is a rather ill-defined pseudoring. Furthermore, the size ratio of the rings is such that they cannot be due to a single pattern speed linking them together. The outer pseudoring is peculiar, since it lies in the faint outer parts of the disk, where star formation is still going on at 1.4 times the optical radius. Higher HI column densities are associated with these regions and the atomic gas layer is warped. These perturbations affect only the outer disk, since the kinematics within the main body conforms well to an ordinary differentially rotating disk. We propose here that both nuclear activity and star formation in the warped outer parts might be linked to the fact that NGC 3642 is located in a rich environment, where its close neighbors show clear signs of merging. Our suggestion is that NGC 3642 has captured recently a low-mass, gas-rich dwarf, and star formation was triggered in this infalling external gas that produced also a pronounced warp in the gaseous disk.Comment: Accepted for publication in A&A. Full resolution version available at http://www.iaa.es/~lourdes/3642/H3551.tar.g

The length of stellar bars in SB galaxies and N-body simulations

We have investigated the accuracy and reliability of six methods used to determine the length of stellar bars in galaxies or N-body simulations. All these methods use ellipse fitting and Fourier decomposition of the surface brightness. We have applied them to N-body simulations that include stars, gas, star formation, and feedback. Stellar particles were photometrically calibrated to make B and K-band mock images. Dust absorption is also included. We discuss the advantages and drawbacks of each method, the effects of projection and resolution, as well as the uncertainties introduced by the presence of dust. The use of N-body simulations allows us to compare the location of Ultra Harmonic Resonance (UHR or 4/1) and corotation (CR) with measured bar lengths. We show that the minimum of ellipticity located just outside the bulk of the bar is correlated with the corotation, whereas the location of the UHR can be approximated using the phase of the fitted ellipses or the phase of the m=2 Fourier development of the surface brightness. We give evidence that the classification of slow/fast bars, based on the ratio R = Rcr/Rbar could increase from 1 (fast bar) to 1.4 (slow bar) just by a change of method. We thus conclude that one has to select the right bar-length estimator depending on the application, since these various estimators do not define the same physical area.Comment: Major revision, A&A in pres

Bar Diagnostics in Edge-On Spiral Galaxies. I. The Periodic Orbits Approach

We develop diagnostics to detect the presence and orientation of a bar in an edge-on disk, using its kinematical signature in the position-velocity diagram (PVD) of a spiral galaxy observed edge-on. Using a well-studied barred spiral galaxy mass model, we briefly review the orbital properties of two-dimensional non-axisymmetric disks and identify the main families of periodic orbits. We use those families as building blocks to model real galaxies and calculate the PVDs obtained for various realistic combinations of periodic orbit families and for a number of viewing angles with respect to the bar. We show that the global structure of the PVD is a reliable bar diagnostic in edge-on disks. Specifically, the presence of a gap between the signatures of the families of periodic orbits in the PVD follows directly from the non-homogeneous distribution of the orbits in a barred galaxy. Similarly, material in the two so-called forbidden quadrants of the PVD results from the elongated shape of the orbits. We show how the shape of the signatures of the dominant x1 and x2 families of periodic orbits in the PVD can be used efficiently to determine the viewing angle with respect to the bar and, to a lesser extent, to constrain the mass distribution of an observed galaxy. We also address the limitations of the models when interpreting observational data.Comment: 22 pages, 9 figures (AASTeX, aaspp4.sty). Accepted for publication in The Astrophysical Journa