Clayton H. Heller

Evolution of Stellar Bars in Live Axisymmetric Halos: Recurrent Buckling and Secular Growth

Evolution of stellar bars in disk galaxies is accompanied by dynamical instabilities and secular changes. Following the vertical buckling instability, the bars are known to weaken dramatically and develop a pronounced boxy/peanut shape when observed edge-on. Using high-resolution N-body simulations of stellar disks embedded in live axisymmetric dark matter halos, we have investigated the long-term changes in the bar morphology, specifically the evolution of the bar size, its vertical structure, and the exchange of angular momentum. We find that following the initial buckling, the bar resumes its growth from deep inside the corotation radius and follows the ultraharmonic resonance thereafter. We also find that this secular bar growth triggers a spectacular secondary vertical buckling instability that leads to the appearance of characteristic boxy/peanut/X-shaped bulges. The secular bar growth is crucial for the recurrent buckling to develop. Furthermore, the secondary buckling is milder, persists over a substantial period of time, ~3 Gyr, and can have observational counterparts. Overall, the stellar bars show recurrent behavior in their properties and evolve by increasing their linear and vertical extents, both dynamically and secularly. We also demonstrate explicitly that the prolonged growth of the bar is mediated by continuous angular momentum transfer from the disk to the surrounding halo and that this angular momentum redistribution is resonant in nature: a large number of lower resonances trap disk and halo particles, and this trapping is robust, in broad agreement with the earlier results in the literature.

Gas Feedback on Stellar Bar Evolution

We have analyzed evolution of live disk-halo systems in the presence of various fractions of gas, fgas � 8% of the disk mass, for 5 Gyr. Specifically, we have addressed the issue of angular momentum (J) transfer from the gas to the stellar bar and its effect on the bar evolution. We find that the weakening of the bar over this time period, reported in the literature, is not related to the J-exchange with the gas, but is caused by the vertical buckling instability in the gas-poor disks and by a steep heating of a stellar velocity dispersion by the central mass concentration (CMC) in the gas-rich disks. On the other hand, the gas has a profound effect on the onset of the buckling — larger fgas brings it forth due to the more massive CMCs. The former process leads to the well-known formation of the boxy/peanut-shaped bulges, while the latter results in the formation of progressively more elliptical bulges, for larger fgas. The subsequent (secular) evolution of the bar differs — the gas-poor models exhibit a growing bar while gas-rich models show a declining bar whose vertical swelling is driven by a secular resonance heating. The border line between the gas-poor and -rich models lies at fgas � 3% in our models, but is model-dependent and will be affected by additional processes, like star formation and feedback from stellar evolution. The overall effect of the gas on the dynamical and secular evolution of the bar is not in a direct J transfer to the stars, but in the loss of J by the gas and its influx to the center that increases the CMC. The more massive CMC damps the vertical buckling instability in the bar and depopulates orbits responsible for the appearance of boxy/peanut-shaped bulges. The combined action of resonant and non-resonant processes in gas-poor and gas-rich disks leads to a converging evolution in the vertical extent of the bar and its stellar dispersion velocities, and to a diverging evolution in the bulge properties. Subject headings: galaxies: bulges – galaxies: evolution – galaxies: formation – galaxies: halos – galaxies: kinematics and dynamics – galaxies: structure

The regeneration of stellar bars by tidal interactions: numerical simulations of fly-by encounters

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. Furthermore, we show that the regenerated bars show the same dynamical properties as their isolated counterparts.

Structure Formation Inside Triaxial Dark Matter Halos: Galactic Disks, Bulges and Bars

We investigate formation and evolution of galactic disks immersed in assembling live DM halos. Models have been evolved from cosmological initial conditions and represent the collapse of an isolated density perturbation. The baryons include gas participating in star formation (SF) and stars with the energy feedback onto the ISM. We find that (1) the triaxial halo figure tumbling is insignificant and the angular momentum (J) is channeled into the internal circulation, while the baryonic collapse is stopped by the centrifugal barrier; (2) density response of the (disk) baryons is out of phase with DM, thus washing out the inner halo ellipticity; (3) the total J is neatly conserved, even in models accounting for stellar feedback; (4) the specific J for DM is nearly constant, while that for baryons is decreasing; (5) early stage of disk formation resembles the cats cradle?a small amorphous disk fueled via radial string patterns?followed by growing oval disk whose shape varies with its orientation to the halo major axis; (6) the disk gas layer thins when the SF rate drops below ~5 M? yr-1; (7) about half of the baryons remain outside the disk SF region or in the halo as a hot gas; (8) rotation curves appear to be flat and account for the observed disk/halo contributions; (9) a range of bulge-dominated to bulgeless disks was obtained, depending on the stellar feedback parameter, SF: smaller SF leads to a larger and earlier bulge; lower density threshold for SF leads to a smaller, thicker disk; gas gravitational softening mimics a number of intrinsic processes within the ISM; (10) models are characterized by an extensive bar-forming activity; (11) nested bars form in response to the gas inflow along the primary bars, as shown by Heller, Shlosman, and Athanassoula.

Dynamical Effects of Nuclear Rings in Disk Galaxies

We investigate the dynamical response of stellar orbits in a rotating barred galaxy potential to the perturbation by a nuclear gaseous ring. The change in three-dimensional periodic orbit families is examined as the gas accumulates between the inner Lindblad resonances. It is found that the phase space allowable to the x2 family of orbits is substantially increased, and a vertical instability strip appears with the growing mass of the ring. A significant distortion of the x1 orbits is observed in the vicinity of the ring, which leads to the intersection between orbits with different values of the Jacobi integral. We also examine the dependence of the orbital response to the eccentricity and alignment of the ring with the bar. Misalignment between an oval ring and a bar can leave observational footprints in the form of twisted near-infrared isophotes in the vicinity of the ring. It is inferred that a massive nuclear ring acts to weaken and dissolve the stellar bar exterior to the ring, whereas only weakly affecting the orbits interior to the inner Lindblad resonances. Consequences for gas evolution in the circumnuclear regions of barred galaxies are discussed as well.

EVOLUTION OF THE PHASE-SPACE DENSITY IN DARK MATTER HALOS

The evolution of the phase-space density profile in dark matter (DM) halos is investigated by means of constrained simulations, designed to control the merging history of a given DM halo. Halos evolve through a series of quiescent phases of a slow accretion intermitted by violent events of major mergers. In the quiescent phases the density of the halo closely follows the NFW profile and the phase-space density profile, Q(r) , is given by the Taylor & Navarro power law, r−β, where β ≈ 1.9 and stays remarkably stable over the Hubble time. Expressing the phase-space density by the NFW parameters, Q(r) = Qs(r/Rs)−β, the evolution of Q is determined by Qs. We have found that the effective mass surface density within Rs, Σs ≡ ρsRs, remains constant throughout the evolution of a given DM halo along the main branch of its merging tree. This invariance entails that Qs ∝ Rs−5/2 and Q(r) ∝ Σs−1/2Rs−5/2(r/Rs)−β. It follows that the phase-space density remains constant, in the sense of Qs = const ., in the quiescent phases and it decreases as Rs−5/2 in the violent ones. The physical origin of the NFW density profile and the phase-space density power law is still unknown. Yet, the numerical experiments show that halos recover these relations after the violent phases. The major mergers drive Rs to increase and Qs to decrease discontinuously while keeping Qs × Rs5/2 = const . The virial equilibrium in the quiescent phases implies that a DM halos evolves along a sequence of NFW profiles with constant energy per unit volume (i.e., pressure) within Rs.

Double Bars in Disk Galaxies: Dynamical Decoupling of Non-Self-gravitating Gaseous Bars

We find that nuclear rings in barred galaxies can be subject to a new type of non-self-gravitational dynamical instability. The instability leads to the formation of gaseous molecular bars with pattern speeds that are substantially slower than speeds of the primary stellar bars. This spectacular decoupling of nuclear bars from the underlying gravitational potential is triggered but is not driven by the gas viscosity. We find that low-viscosity systems can spend a substantial period of time in a fully decoupled state, with the nuclear bar slowly tumbling in the gravitational field of the primary bar. Higher viscosity systems form nuclear bars that librate about the primary bar. The shape of a nuclear bar, i.e., its eccentricity, correlates strongly with the angle between the bars. We also find that such decoupling, partial or full, most probably will be associated with bursts of star formation and with gas inflow across the inner (Lindblad) resonance zone toward smaller radii.

Kinematics of Ionized and Molecular Hydrogen in the Core of M100

We present high angular and velocity resolution two-dimensional kinematic observations in the spectral lines of Hα and CO J = 1 → 0 of the circumnuclear starburst region in the barred spiral galaxy M100, and compare them with kinematics derived from our previously published numerical modeling. The Hα data, fully sampled and at subarcsecond resolution, show a rotation curve that is rapidly rising in the central ~140 pc, and stays roughly constant, at the main disk value, further out. Noncircular motions are studied from the Hα and CO data by detailed consideration of the velocity fields, residual velocity fields after subtraction of the rotation curve, and sets of position-velocity diagrams. These motions are interpreted as the kinematic signatures of gas streaming along the inner part of the bar, and of density wave streaming motions across a two-armed minispiral. Comparison with a two-dimensional velocity field and rotation curve derived from our 1995 dynamical model shows good qualitative and quantitative agreement for the circular and noncircular kinematic components. Both morphology and kinematics of this region require the presence of a double inner Lindblad resonance in order to explain the observed twisting of the near-infrared isophotes and the gas velocity field. These are compatible with the presence of a global density wave driven by the moderately strong stellar bar in this galaxy. We review recent observational and modeling results on the circumnuclear region in M100, and discuss the implications for bar structure and gas dynamics in the core of M100 and other disk galaxies.

Numerical simulations of interacting gas-rich barred galaxies: vertical impact of small companions

We investigate the dynamical effects of an interaction between an initially barred galaxy and a small spherical companion using an N-body/smoothed-particle-hydrodynamics algorithm. In the models described here the small companion passes through the disc of the larger galaxy nearly 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.

Dark Matter Halos and Evolution of Bars in Disk Galaxies: Varying Gas Fraction and Gas Spatial Resolution

We conduct numerical experiments by evolving gaseous/stellar disks embedded in live dark matter halos aiming at quantifying the effect of gas spatial resolution and gas content on the bar evolution. Three model sequences have been constructed using different resolutions, and the gas fraction has been varied along each sequence within the range of f g = 0%-50%, but keeping the disk and halo properties unchanged. We find that the spatial resolution becomes important with an increase in the gas content. For the higher resolution model sequences, we observe a bimodal behavior in the bar evolution with respect to the gas fraction, especially during the secular phase of this evolution. The switch from the gas-poor to gas-rich behavior is abrupt and depends on the resolution used, being reasonably confined to f g ~ 5%-12%. The diverging evolution has been observed in nearly all basic parameters characterizing bars, such as the bar strength, central mass concentration, bar vertical buckling amplitude, bar size, etc. We find that the presence of the gas component severely limits the bar growth and affects its pattern speed evolution. Gas-poor models display rapidly decelerating bars, while gas-rich models exhibit bars with constant or even slowly accelerating tumbling. We also find that the gas-rich models have bar corotation (CR) radii within the disk at all times, in contrast with gas-poor and purely stellar disks. In addition, the CR-to-bar size ratio is less than 2 for gas-rich models. Next, we have confirmed that the disk angular momentum within the CR remains unchanged in the gas-poor models, as long as the CR stays within the disk, but experiences a sharp drop before leveling off in the gas-rich models. Finally, we discuss a number of observed correlations between various parameters of simulated bars, such as between the bar sizes and the gas fractions, between the bar strength and the buckling amplitude, and between the bar strength and its size, etc.