Abstract
The majority of spiral galaxies (two-thirds) are barred. Bars are one of the driving forces behind the formation of coherent spiral density waves, along with galaxy interactions. The behavior of barred galaxies can be understood by considering the families of star orbits, which are concentrated around periodic orbits. At the center of galaxies, the orbits are parallel to the bar, the x1 orbits. Then, at each resonance, these orbits rotate 90°, and are therefore perpendicular to the bar outside the corotation. As the bar is not supported by the stellar orbits, it stops at the corotation, as confirmed by numerical simulations. This phenomenon makes it possible to estimate the rotational speed of bar waves, which cannot be measured directly. If there are two internal Lindblad resonances (ILRs), then the stellar orbits will be perpendicular to the bar between the two ILRs. This is a phenomenon of bar self-regulation: over time, the bar traps more and more stars, and expands in radius. Its average speed decreases, slowing down as the precession speed of orbits decreases with radius. A slower bar starts to create one ILR, then two. The bar itself then stops growing, allowing orbits to move in a perpendicular direction. The weakened primary bar will see a smaller, faster secondary bar decouple at the center. Often, the two bars exchange energy, interacting non-linearly at a common resonance. This can be the corotation of the nuclear rod, which corresponds to the ILR of the primary rod.
Numerical simulations have increased our knowledge of bar and spiral phenomena in disk galaxies: in particular, we have learned that Lindblad resonances also occur in the direction perpendicular to the plane. This forms a peanut-like pseudo-bubble of galaxies, highly recognizable in barred galaxies seen from the edge. Star-forming rings also form at resonances, and bars are a way of bringing gas to the center and feeding black holes.
