466 research outputs found
Substructure in CDM Halos and the Heating of Stellar Disks
Numerical simulations have revealed the presence of long-lived substructure
in Cold Dark Matter (CDM) halos. These surviving cores of past merger and
accretion events vastly outnumber the known satellites of the Milky Way. This
finding has prompted suggestions that substructure in cold dark matter (CDM)
halos may be incompatible with observation and in conflict with the presence of
thin, dynamically fragile stellar disks. N-body simulations of a
disk/bulge/halo model of the Milky Way that includes several hundred dark
matter satellites with masses, densities and orbits derived from
high-resolution cosmological CDM simulations indicate that substructure plays
only a minor dynamical role in the heating of the disk. This is because the
orbits of satellites seldom take them near the disk, where their tidal effects
are greatest. We conclude that substructure might not preclude virialized CDM
halos from being acceptable hosts of thin stellar disks like that of the Milky
Way.Comment: Presented at the Yale Symposium "The Shape of Galaxies and their
Halos", ed. P.Nataraja
Multiple dynamical components in Local Group dwarf spheroidals
The dwarf spheroidal (dSph) satellites of the Local Group have long been
thought to be simple spheroids of stars embedded within extended dark matter
halos. Recently, however, evidence for the presence of spatially and
kinematically distinct stellar populations has been accumulating. Here, we
examine the influence of such components on dynamical models of dwarf galaxies
embedded in cold dark matter halos. We begin by constructing a model of
Andromeda II, a dSph satellite of M31 which shows evidence for spatially
distinct stellar components. We find that the two-component model predicts an
overall velocity dispersion profile that remains approximately constant at
km s out to kpc from the center; this is despite
wide kinematic and spatial differences between the two individual components.
The presence of two components may also help to explain oddities in the
velocity dispersion profiles of other dSphs; we show that velocity dispersion
profiles which appear to rise from the center outwards before leveling
off--such as those of Leo I, Draco, and Fornax--can result from the gradual
transition from a dynamically cold, concentrated component to a second, hotter,
and more spatially extended one, both in equilibrium within the same dark halo.
Dwarf galaxies with two stellar components generally have a leptokurtic
line-of-sight velocity distribution which is well described by a double
Maxwellian. Interestingly, we find that multiple equilibrium components could
also provide a potential alternative origin for ``extra-tidal'' stars (normally
ascribed to tidal effects) in situations where corroborating evidence for tides
may be lacking.Comment: Accepted by MNRAS Letters. Revised version, with addition of new
section and expanded discussio
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