167 research outputs found
The formation and dynamical evolution of young star clusters
Recent observations have revealed a variety of young star clusters, including
embedded systems, young massive clusters, and associations. We study the
formation and dynamical evolution of these clusters using a combination of
simulations and theoretical models. Our simulations start with a turbulent
molecular cloud that collapses under its own gravity. The stars are assumed to
form in the densest regions in the collapsing cloud after an initial free-fall
times of the molecular cloud. The dynamical evolution of these stellar
distributions are continued by means of direct -body simulations. The
molecular clouds typical for the Milky Way Galaxy tend to form embedded
clusters which evolve to resemble open clusters. The associations were
initially considerably more clumpy, but lost their irregularity in about a
dynamical time scale due to the relaxation process. The densest molecular
clouds, which are absent in the Milky Way but are typical in starburst
galaxies, form massive young star clusters. They indeed are rare in the Milky
Way. Our models indicate a distinct evolutionary path from molecular clouds to
open clusters and associations or to massive star clusters. The mass-radius
relation for both types of evolutionary tracks excellently matches the
observations. According to our calculations the time evolution of the half-mass
radius for open clusters and associations follows , whereas for massive star clusters . Both trends are consistent with
the observed age-mass-radius relation for clusters in the Milky Way.Comment: 16 pages, 9 figures, accepted for publication in Ap
Dynamical friction on satellite galaxies
For a rigid model satellite, Chandrasekhar's dynamical friction formula
describes the orbital evolution quite accurately, when the Coulomb logarithm is
chosen appropriately. However, it is not known if the orbital evolution of a
real satellite with the internal degree of freedom can be described by the
dynamical friction formula. We performed N-body simulation of the orbital
evolution of a self-consistent satellite galaxy within a self-consistent parent
galaxy. We found that the orbital decay of the simulated satellite is
significantly faster than the estimate from the dynamical friction formula. The
main cause of this discrepancy is that the stars stripped out of the satellite
are still close to the satellite, and increase the drag force on the satellite
through two mechanisms. One is the direct drag force from particles in the
trailing tidal arm, a non-axisymmetric force that slows the satellite down. The
other is the indirect effect that is caused by the particles remaining close to
the satellite after escape. The force from them enhances the wake caused in the
parent galaxy by dynamical friction, and this larger wake in turn slows the
satellite down more than expected from the contribution of its bound mass. We
found these two have comparable effects, and the combined effect can be as
large as 20% of the total drag force on the satellite.Comment: 15 pages, 10 figures, submitted to PASJ; v2: 14 pages, 13 figures,
accepted by PAS
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