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Bars and spiral arms dynamics in N-body and SPH simulations


Seventy percent of galaxies in the local universe are characterized by having a thin disk component, supported by rotation: the galactic disk. Our understanding of the origin and evolution of this galactic component is crucial to understand how the baryons are assembled in the center of dark halos. In galactic disks we very often find more or less prominent spiral patterns, and the origin of these patterns are not yet fully understand, even after decades of theoretical study. These spiral patterns, together with the bar, are the main responsible for the angular momentum transfer in the disk. Furthermore, these patterns concentrate a large fraction of the very recent star formation. Indeed, the HII regions, loci of high star formation rates, are found mainly in the spiral arms. There are currently two different views of the spiral structure in disk galaxies. In one side, we have theories like the "Quasi-Stationary Spiral Structure" (QSSS) from C.C. Lin & F. Shu, or the "Modal Theory" by G. Bertin and collaborators, in which the spiral structure is supposed to be a long lived pattern. In theories like the "Swing Amplification" or the "Stochastic Self-Propagating Star Formation" (SSPSF), the spiral patterns are understood as local patches of waves which are constantly being amplified by different mechanisms, sheared by differential rotation and dumped. The spiral structure is then transient and recurrent. There are observational evidence supporting both theories. We can then suppose that under some conditions the spiral arms could be the density waves proposed by the "long lived" scenario. In other cases, the galaxy formation and evolution processes give advantage to swing amplification mechanisms.In this project, we expect to run a number of N-Body and SPH simulations: N-Body is a collisionless technique and SPH includes a gas component. The models are constructed with disk and halo, and sometimes we include a bulge. By changing some initial parameters, mainly the velocity dispersion and height of the disk, and the ratio disk mass/galaxy mass, we can study the formation and evolution of spiral patterns for different models. In this way, we will try to understand the conditions which favors long or short lived scenarios for the spiral structure in galaxies. The simulations will be analyzed by using uni- and bi-dimensional Fourier Transforms (FT1D/FT2D). These techniques give us the strength, lifetime, and angular speed for different spiral patterns which develop in our simulations. We have already run and preliminary analyzed some simulations with 1.2M and 8M particles. Our main models are three: one disk dominated, other central bulge dominated, and finally one halo dominated. In the first models, the bar formation occurs quite fast, before the first gigayear, and the bar remains for a couple of gigayears. The presence of a bulge delays the formation of the bar, while a very important halo can completely suppresses the bar formation. The spiral patterns appearing in our simulations are amplified by a process similar to the swing amplification. The spiral structures are transients and recurrents, i.e., the spiral patterns are amplified, sheared and dumped in a time scale of some hundreds of million years. (AU)

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