Buckling instability in tidally induced galactic bars

Strong galactic bars produced in simulations tend to undergo a period of buckling instability that weakens and thickens them and forms a boxy/peanut structure in their central parts. This theoretical prediction has been confirmed by identifying such morphologies in real galaxies. The nature and origin of this instability remains however poorly understood with some studies claiming it to be due to fire-hose instability while others relating it to vertical instability of stellar orbits supporting the bar. One of the channels for the formation of galactic bars is via the interaction of disky galaxies with perturbers of significant mass. Tidally induced bars offer a unique possibility of studying buckling instability because their formation can be controlled by changing the strength of the interaction while keeping the initial structure of the galaxy the same. We use a set of four simulations of flyby interactions where a galaxy on a prograde orbit forms a bar, which is stronger for stronger tidal forces. We study their buckling by calculating different kinematic signatures, including profiles of the mean velocity in vertical direction, as well as distortions of the bars out of the disk plane. Although our two strongest bars buckle most strongly, there is no direct relation between the ratio of vertical to horizontal velocity dispersion and the bar's susceptibility to buckling, as required by the fire-hose instability interpretation. While our weakest bar buckles, a stronger one does not, its dispersion ratio remains low and it grows to become the strongest of all at the end of evolution. Instead, we find that during buckling the resonance between the vertical and radial orbital frequencies becomes wide and therefore able to modify stellar orbits over a significant range of radii. We conclude that the vertical orbital instability is the more plausible explanation for the origin of buckling.Comment: 9 pages, 11 figures, accepted for publication in A&

Damping of the Milky Way bar by manifold-driven spirals

We describe a new phenomenon of `bar damping' that may have played an important role in shaping the Milky Way bar and bulge as well as its spiral structure. We use a collisionless N-body simulation of a Milky Way-like galaxy initially composed of a dark matter halo and an exponential disk with Toomre parameter slightly above unity. In this configuration, dominated by the disk in the center, a bar forms relatively quickly, after 1 Gyr of evolution. This is immediately followed by the formation of two manifold-driven spiral arms and the outflow of stars that modifies the potential in the vicinity of the bar, apparently shifting the position of the L_1/L_2 Lagrange points. This modification leads to the shortening of the bar and the creation of a next generation of manifold-driven spiral arms at a smaller radius. The process repeats itself a few times over the next 0.5 Gyr resulting in further substantial weakening and shortening of the bar. The time when the damping comes to an end coincides with the first buckling episode in the bar which rebuilds the orbital structure so that no more new spiral arms are formed. The morphology of the bar and the spiral structure at this time show remarkable similarity to the present properties of the Milky Way. Later on, the bar starts to grow rather steadily again, weakened only by subsequent buckling episodes occurring at more distant parts of the disk.Comment: 6 pages, 5 figures, revised version accepted for publication in ApJ Letter

Evolution of peaks in weakly nonlinear density field and dark halo profiles

Using the two-point Edgeworth series up to second order we construct the weakly nonlinear conditional probability distribution function for the density field around an overdense region. This requires calculating the two-point analogues of the skewness parameter $S_{3}$. We test the dependence of the two-point skewness on distance from the peak for scale-free power spectra and Gaussian smoothing. The statistical features of such conditional distribution are given as the values obtained within linear theory corrected by the terms that arise due to weakly nonlinear evolution. The expected density around the peak is found to be always below the linear prediction while its rms fluctuation is always larger than in the linear case. We apply these results to the spherical model of collapse as developed by Hoffman & Shaham (1985) and find that in general the effect of weakly nonlinear interactions is to decrease the scale from which a peak gathers mass and therefore also the mass itself. In the case of open universe this results in steepening of the final profile of the virialized protoobject.Comment: Latex, 23 pages with 5 postscript figures included, submitted to MNRA

Universal profile of dark matter halos and the spherical infall model

I propose a modification of the spherical infall model for the evolution of density fluctuations with initially Gaussian probability distribution and scale-free power spectra in Einstein-de Sitter universe as developed by Hoffman & Shaham. I introduce a generalized form of the initial density distribution around an overdense region and cut it off at half the inter-peak separation accounting in this way for the presence of the neighbouring fluctuations. Contrary to the original predictions of Hoffman & Shaham the resulting density profiles within virial radii no longer have power-law shape but their steepness increases with distance. The profiles of halos of galactic mass are well fitted by the universal profile formula of changing slope obtained as a result of N-body simulations by Navarro, Frenk & White. The trend of steeper profiles for smaller masses and higher spectral indices is also reproduced. The agreement between the model and simulations is better for smaller masses and lower spectral indices which suggests that galaxies form mainly by accretion while formation of clusters involves merging.Comment: 12 pages + 5 figures, significantly revised (collapse factor properly calculated, agreement with N-body simulations dramatically improved), accepted for publication in MNRA

The spherical collapse model in a universe with cosmological constant

We generalize the spherical collapse model for the formation of dark matter halos to apply in a universe with arbitrary positive cosmological constant. We calculate the critical condition for collapse of an overdense region and give exact values of the characteristic densities and redshifts of its evolution.Comment: 6 pages, 3 figures, contribution to the proceedings of the 3rd International Workshop on the Identification of Dark Matter (IDM2000) in York, in pres