Ingo Berentzen

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.

Stellar Bar Evolution in Cuspy and Flat-cored Triaxial CDM Halos

We analyze the formation and evolution of stellar bars in galactic disks embedded in mildly triaxial cold dark matter (CDM) halos that have density distributions ranging from large flat cores to cuspy profiles. We have applied tailored numerical simulations of analytical and live halos that include the feedback from disk/bar system onto the halo in order to test and extend earlier work by El-Zant and Shlosman. The latter employed the method of Liapunov exponents to analyze the fate of bars in analytical asymmetric halos. We find the following: (1) The bar growth is very similar in all rigid axisymmetric and triaxial halos. (2) Bars in live models experience vertical buckling instability and the formation of a pseudobulge with a boxy/peanut shape, while bars in rigid halos do not buckle. (3) In live axisymmetric halos, the bar strength varies by a factor of 2, in growth or decay, during the secular evolution following the buckling. The bar pattern speed evolution (i.e., deceleration) anticorrelates with the halo core size. In such halos, the bar strength is larger for smaller disk-to-halo mass ratios (D/H) within disk radii, the bar size correlates with the halo core sizes, and the bar pattern speeds correlate with the halo central mass concentration. In contrast, bars embedded in live triaxial halos have a starkly different fate: they dissolve on a timescale of ~1.5-5 Gyr due to the onset of chaos over continuous zones, sometimes leaving behind a weak oval distortion. The onset of chaos is related to the halo triaxiality, the fast-rotating bar, and the halo cuspiness. Before the bar dissolves, the region outside it develops strong spiral structures, especially in the live triaxial halos. (4) More angular momentum is absorbed (fractionally) by the triaxial halos than in the axisymmetric models. The disk-halo angular momentum exchange is mediated by the lower resonances in the latter models. (5) Cuspy halos are more susceptible than flat-core halos to having their prolateness washed out by the action of the bar. The subsequent evolution is then similar to the case of cuspy axisymmetric halos. We analyze the above results on disk and bar evolution in terms of the stability of trajectories and development of chaos in the system. We set important constraints on the triaxiality of dark matter (DM) halos by comparing our predictions to recent observational results on the properties of bars out to intermediate redshifts z ~ 1.

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.

Merger of massive black holes using N-body simulations with post-Newtonian corrections

We present preliminary results from self-consistent, high resolution direct N –body simulations of massive black hole binaries in mergers of galactic nuclei. The dynamics of the black hole binary includes the full Post-Newtonian corrections (up to 2.5PN) to its equations of motion. We show that massive black holes starting at separations of 100 pc can evolve down to gravitational-wave-induced coalescence in less than a Hubble time. The binaries, in our models, often form with very high eccentricity and, as a result, reach separations of 50 Schwarzschild radius with eccentricities which are clearly distinct from zero — even though gravitational wave emission damps the eccentricity during the inspiral. These deviations from exact circular orbits, at such small separations, may have important consequences for LISA data analysis.

Bars in Cuspy Dark Halos

We examine the bar instability in models with an exponential disk and a cuspy NFW-like dark matter (DM) halo inspired by cosmological simulations. Bar evolution is studied as a function of numerical resolution in a sequence of models spanning 10 4 – 10 8 DM particles - including a multi-mass model with an effective resolution of 10 10 . The goal is to find convergence in dynamical behaviour. We characterize the bar growth, the buckling instability, pattern speed decay through resonant transfer of angular momentum, and possible destruction of the DM halo cusp. Overall, most characteristics converge in behaviour for halos containing more than 10 7 particles in detail. Notably, the formation of the bar does not destroy the density cusp in this case. These higher resolution simulations clearly illustrate the importance of discrete resonances in transporting angular momentum from the bar to the halo.

Numerical Simulations of Interacting Gas-Rich Barred Galaxies

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.

Chaos in the centers of galaxies.

ABSTRACT: We compare diverging evolution of a two‐component (gas+stars) galactic disk embedded in a “live” halo with that of an identical pure stellar disk. Our modeling supports the conjecture that the growth of central concentration in galaxies dissolves the main family of regular orbits in the stellar bar and assists in the formation of a galactic bulge.