Halo Properties and Secular Evolution in Barred Galaxies

The halo plays a crucial role in the evolution of barred galaxies. Its near-resonant material absorbs angular momentum emitted from some of the disc particles and helps the bar become stronger. As a result, a bar (oval) forms in the inner parts of the halo of strongly barred disc galaxies. It is thinner in the inner parts (but still considerably fatter than the disc bar) and tends to spherical at larger radii. Its length increases with time, while always staying shorter than the disc bar. It is roughly aligned with the disc bar, which it trails only slightly, and it turns with roughly the same pattern speed. The bi-symmetric component of the halo density continues well outside the halo bar, where it clearly trails behind the disc bar. The length and strength of the disc and halo bars correlate; the former being always much stronger than the latter. If the halo is composed of weakly interacting massive particles, then the formation of the halo bar, by redistributing the matter in the halo and changing its shape, could influence the expected annihilation signal. This is indeed found to be the case if the halo has a core, but not if it has a steep cusp. The formation and evolution of the bar strongly affect the halo orbits. A fraction of them becomes near-resonant, similar to the disc near-resonant orbits at the same resonance, while another fraction becomes chaotic. Finally, a massive and responsive halo makes it harder for a central mass concentration to destroy the disc bar. 1. The role of the halo in the evolution of barred galaxies As shown by N -body simulations, barred galaxies undergo considerable secular evolution (e.g. Hernquist & Weinberg 1992; Debattista & Sellwood 2000; Athanassoula & Misiriotis 2002 [AM02]; Athanassoula 2002 [A02]; Athanassoula 2003 [A03]; O’Neil & Dubinski 2003; Valenzuela & Klypin 2003; Martinez-Valpuesta & Shlosman 2004; Athanassoula 2005a; MartinezValpuesta, Shlosman & Heller 2005). The halo plays a major role in this, since it participates in the angular momentum exchange within the galaxy. In an isolated disc galaxy with little or no gas, angular momentum is emitted mainly by near-resonant material in the inner disc (bar region) and absorbed by near-