Dynamical friction of massive objects in galactic centres
  Dynamical friction leads to an orbital decay of massive objects like young
compact star clusters or Massive Black Holes in central regions of galaxies.
The dynamical friction force can be well approximated by Chandrasekhar's
standard formula, but recent investigations show, that corrections to the
Coulomb logarithm are necessary. With a large set of N-body simulations we show
that the improved formula for the Coulomb logarithm fits the orbital decay very
well for circular and eccentric orbits. The local scale-length of the
background density distribution serves as the maximum impact parameter for a
wide range of power-law indices of -1 ... -5. For each type of code the
numerical resolution must be compared to the effective minimum impact parameter
in order to determine the Coulomb logarithm. We also quantify the correction
factors by using self-consistent velocity distribution functions instead of the
standard Maxwellian often used. These factors enter directly the decay
timescale and cover a range of 0.5 ... 3 for typical orbits. The new Coulomb
logarithm combined with self-consistent velocity distribution functions in the
Chandrasekhar formula provides a significant improvement of orbital decay times
with correction up to one order of magnitude compared to the standard case. We
suggest the general use of the improved formula in parameter studies as well as
in special applications.
