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Register a new userThe influence of dynamical friction on the collapse of spherical density pertubation
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We solve numerically the equations of motion for the collapse of a shell of baryonic matter falling into the central regions of a cluster of galaxies, taking into account of the presence of the substructure inducing dynamical friction. The evolution of the expansion parameter a(t) of the perturbation is calculated in spherical systems. The effect of dynamical friction is to reduce the binding radius and the total mass accreted by the central regions. Using a peak density profile given by Bardeen et al. (1986) we show how the binding radius of the perturbation is modified by dinamical friction. We show how dynamical friction modifies the collapse parameter of the perturbation slowing down the collapse.
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We present a rigorous calculation of the dynamical friction force exerted on a spherical massive perturber moving through an infinite homogenous system of field stars. By calculating the shape and mass of the polarization cloud induced by the perturber in the background system, which decelerates the motion of the perturber, we recover Chandrasekhars drag force law with a modified Coulomb logarithm. As concrete examples we calculate the drag force exerted on a Plummer sphere or a sphere with the density distribution of a Hernquist profile. It is shown that the shape of the perturber affects only the exact form of the Coulomb logarithm. The latter converges on small scales, because encounters of the test and field stars with impact parameters less than the size of the massive perturber become inefficient. We confirm this way earlier results based on the impulse approximation of small angle scatterings.
In a recent paper [Phys. Rev. Lett. 80 (1998) 1690], Krim and coworkers have observed that the friction force, acting on a thin physisorbed layer of N_2 sliding on a lead film, abruptly decreases by a factor of ~2 when the lead film is cooled below its superconductivity transition temperature. We discuss the possible mechanisms for the abruptness of the sliding friction drop, and also discuss the relevance of these results to the problem of electronic friction.
Many Local Group dwarf spheroidal galaxies are found in the Galactic halo along great circles in the sky. Some of these stellar systems are thought to be the fragments of larger parent galaxies which have once intruded into and were torn apart by the tide of the Galaxy. Supporting evidences for tidal disruption are found in the form of stellar tidal bridges and tails along the orbits of some dwarf galaxies and globular clusters. In this study, we investigate the influence of mass-loss from star clusters or dwarf galaxies on the rate of their orbit decay due to the effect of dynamical friction. Using a series of numerical N-body simulations, we show that stars, which become unbound from their host-systems, but remain in their vicinity and share their orbits, still contribute to the mass responsible for the dynamical friction. As a rule-by-thumb, the magnitude of dynamical friction at any instance can be approximated by the bound mass plus half of the mass which has already become unbound during the proceeding Galactic orbit. Based on these results, we suggest the tidal disruption of relatively massive satellite stellar systems may be more abrupt than previously estimated.
Following a wave-mechanical treatment we calculate the drag force exerted by an infinite homogeneous background of stars on a perturber as this makes its way through the system. We recover Chandrasekhars classical dynamical friction (DF) law with a modified Coulomb logarithm. We take into account a range of models that encompasses all plausible density distributions for satellite galaxies by considering the DF exerted on a Plummer sphere and a perturber having a Hernquist profile. It is shown that the shape of the perturber affects only the exact form of the Coulomb logarithm. The latter converges on small scales, because encounters of the test and field stars with impact parameters less than the size of the massive perturber become inefficient. We confirm this way earlier results based on the impulse approximation of small angle scatterings.
In this paper, we explore the impact of a galactic bar on the inspiral time-scale of a massive perturber (MP) within a Milky Way-like galaxy. We integrate the orbit of MPs in a multi-component galaxy model via a semi-analytical approach including an accurate treatment for dynamical friction generalized to rotationally supported backgrounds. We compare the MP evolution in a galaxy featuring a Milky Way-like rotating bar to the evolution within an analogous axisymmetric galaxy without the bar. We find that the bar presence may significantly affect the inspiral, sometimes making it shorter by a factor of a few, sometimes hindering it for a Hubble time, implying that dynamical friction alone is greatly insufficient to fully characterize the orbital decay. The effect of the bar is more prominent for initially in-plane, prograde MPs, especially those crossing the bar co-rotation radius or outer Lindblad resonance. In the barred galaxy, we find the sinking of the most massive MPs (>~10^7.5 Msun) approaching the galaxy from large separations (>~8 kpc) to be most efficiently hampered. Neglecting the effect of global torques associated to the non-symmetric mass distribution is thus not advisable even within our idealized, smooth Milky Way model, and it should be avoided when dealing with more complex and realistic galaxy systems. This has important implications for the orbital decay of massive black holes in late-type spirals, the natural candidate sources to be detected with the Laser Interferometer Space Antenna (LISA).
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