The role of nuclear bars in shaping of the central regions of spiral galaxies
Dr Witold Maciejewski
Double bars, where a nuclear bar (marked by contours in the figure) is nested inside the main bar (yellow elongated structure inside the green disc), are common in disc galaxies. Optical and near-infrared observations reveal that the majority of disc galaxies in the local Universe are barred, including our Milky Way, and nearly 30% of barred galaxies host an inner, secondary bar. There is observational evidence that the two bars rotate independently, and therefore the galaxy is periodically changing in time. The goal of this project is to study why a good fraction of galaxies settles in this mode of dynamics. The starting point will be the orbital analysis of double bars in order to see when such systems are dynamically possible. Some dynamically-possible systems have been already found (Maciejewski & Small 2010, ApJ, 719, 622). The dynamics of double bars will be studied with an elegant method using double-frequency orbits, which form the backbone of double bars. These orbits map onto the so-called loops, which are analogous to closed orbits, and therefore tell us about the extent, morphology, and dynamics of the bars.
The insight from orbital analysis will be then used in order to understand the formation of double bars in numerical simulations. Particular emphasis will be put on the role of the central massive black hole in the formation and destruction of double bars. We know that most, if not all, early-type galaxies contain the central massive black hole, and bars can be destroyed by a central mass concentration in galaxies. Although the mass of the central black hole is too small to destroy the main bar, the inner bar in double-barred galaxies is smaller, and black hole should be able to destroy it. Yet, double bars seem to be most common in early-type galaxies. Thus the second aim of this project is to find how massive the black hole has to be in order to destroy the inner bar and to determine whether this finding can impose an upper limit on the black hole mass in double-barred galaxies.
It has been postulated that double bars may play a role in the feeding of active galactic nuclei. Nuclear activity in a galaxy is caused by gas that falls onto its centre. Gas in a galactic disc can be transported radially inwards if it is dispossessed of some of its angular momentum. Angular momentum can be removed by gravitational torques or non-gravitational interactions, like collisions between clouds on different trajectories. Gravitational torque can be generated by satellites or galaxies passing by, or internally by non-axisymmetric mass distribution in galaxies, that most visibly manifests itself in the presence of bars. This project can be extended to finding double bars that can trigger the feeding of active galactic nuclei.