749 research outputs found
Dynamical Effects of CDM Subhalos on a Galactic Disk
We investigate the dynamical interaction between a galactic disk and
surrounding numerous dark subhalos as expected for a galaxy-sized halo in the
cold dark matter (CDM) models. Our particular interest is to what extent
accretion events of subhalos into a disk are allowed in light of the observed
thinness of a disk. Several models of subhalos are considered in terms of their
internal density distribution, mass function, and spatial and velocity
distributions. Based on a series of N-body simulations, we find that the disk
thickening quantified by the change of its scale height, Delta z_d, depends
strongly on the individual mass of an interacting subhalo M_{sub}. This is
described by the relation, Delta z_d / R_d = 8 Sum_{j=1}^N (M_{sub,j}/M_d)**2,
where R_d is a disk scale length, M_d is a disk mass, and N is the total number
of accretion events of subhalos inside a disk region (< 3R_d). Using this
relation, we find that an observed thin disk has not ever interacted with
subhalos with the total mass of more than 15% disk mass. Also, a less massive
disk with smaller circular velocity V_c is more affected by subhalos than a
disk with larger V_c, in agreement with the observation. Further implications
of our results for the origin of a thick disk component are also discussed.Comment: 12 pages, 9 figures, accepted by PAS
Physical symmetries and gauge choices in the Landau problem
Due to a special nature of the Landau problem, in which the magnetic field is
uniformly spreading over the whole two-dimensional plane, there necessarily
exist three conserved quantities, i.e. two conserved momenta and one conserved
orbital angular momentum for the electron, independently of the choice of the
gauge potential. Accordingly, the quantum eigen-functions of the Landau problem
can be obtained by diagonalizing the Landau Hamiltonian together with one of
the above three conserved operators with the result that the quantum mechanical
eigen-functions of the Landau problem can be written down for arbitrary gauge
potential. The purpose of the present paper is to clarify the meaning of gauge
choice in the Landau problem based on this gauge-potential-independent
formulation, with a particular intention of unraveling the physical
significance of the concept of gauge-invariant-extension of the canonical
orbital angular momentum advocated in recent literature on the nucleon spin
decomposition problem. At the end, our analysis is shown to disclose a
physically vacuous side face of the gauge symmetry.Comment: version to appear in The European Physical Journal
Universal dark halo scaling relation for the dwarf spheroidal satellites
Motivated by a recently found interesting property of the dark halo surface
density within a radius, , giving the maximum circular velocity,
, we investigate it for dark halos of the Milky Way's and
Andromeda's dwarf satellites based on cosmological simulations. We select and
analyze the simulated subhalos associated with Milky Way-sized dark halos and
find that the values of their surface densities, , are in
good agreement with those for the observed dwarf spheroidal satellites even
without employing any fitting procedures. This implies that this surface
density would not be largely affected by any baryonic feedbacks and thus
universal. Moreover, all subhalos on the small scales of dwarf satellites are
expected to obey the relation ,
irrespective of differences in their orbital evolutions, host halo properties,
and observed redshifts. Therefore, we find that the universal scaling relation
for dark halos on dwarf galaxy mass scales surely exists and provides us
important clues to understanding fundamental properties of dark halos. We also
investigate orbital and dynamical evolutions of subhalos to understand the
origin of this universal dark halo relation and find that most of subhalos
evolve generally along the sequence, even
though these subhalos have undergone different histories of mass assembly and
tidal stripping. This sequence, therefore, should be the key feature to
understand the nature of the universality of .Comment: 12 pages, 5 figures and 3 tables, submitted to Ap
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