Astrolunch: Karamveer Kaur (HUJI)

Tue, 26/11/2019 - 12:30 to 13:30

Stalling of Globular Cluster Orbits in Dwarf Galaxies
Some dwarf galaxies are observed to host old globular clusters. This is contrary to the theoretical calculations based on the Chandrasekhar’s dynamical friction, which suggest that angular momentum losses of the orbiting cluster should lead it to the galaxy centre in a few giga years. I will emphasize on the need to go beyond this local approach. This leads to the Tremaine Weinberg theory (1984) of dynamical friction, which correctly takes into account the global orbital structure of galaxy. On this basis, I will discuss the suppression of dynamical friction in the inner core region of the galaxies, leading to stalling of these infalling globular clusters away from the galaxy centre. The drag torque is exerted by stars whose orbits are resonant with the cluster’s orbit. We find that nearly co-rotating stars especially exert dominant torques. Going to smaller radial distances, the retarding torque is highly suppressed because of progressive transition to states in which there are fewer and weaker resonances in operation. The orbital decay slows down drastically, and over astronomically interesting time scales the globular cluster appears to stall at a radius of 220 pc.