A SelfConsistent, TimeDependent Treatment of Dynamical Friction: New Insights regarding Core Stalling and Dynamical Buoyancy
Abstract
Dynamical friction is typically regarded a secular process, in which the subject ('perturber') evolves very slowly (secular approximation), and has been introduced to the host over a long time (adiabatic approximation). These assumptions imply that dynamical friction arises from the LBK torque with nonzero contribution only from pure resonance orbits. However, dynamical friction is only of astrophysical interest if its timescale is shorter than the age of the Universe. In this paper we therefore relax the adiabatic and secular approximations. We first derive a generalized LBK torque, which reduces to the LBK torque in the adiabatic limit, and show that it gives rise to transient oscillations due to nonresonant orbits that slowly damp out, giving way to the LBK torque. This is analogous to how a forced, damped oscillator undergoes transients before settling to a steady state, except that here the damping is due to phase mixing rather than dissipation. Next, we present a selfconsistent treatment, that properly accounts for timedependence of the perturber potential and circular frequency (memory effect), which we use to examine orbital decay in a cored galaxy. We find that the memory effect results in a phase of accelerated, superChandrasekhar friction before the perturber stalls at a critical radius, $R_{\mathrm{crit}}$, in the core (corestalling). Inside of $R_{\mathrm{crit}}$ the torque flips sign, giving rise to dynamical buoyancy, which counteracts friction and causes the perturber to stall. This phenomenology is consistent with $N$body simulations, but has thus far eluded proper explanation.
 Publication:

arXiv eprints
 Pub Date:
 March 2021
 arXiv:
 arXiv:2103.05004
 Bibcode:
 2021arXiv210305004B
 Keywords:

 Astrophysics  Astrophysics of Galaxies
 EPrint:
 Accepted for publication in ApJ