16,008 research outputs found
Satellite Alignment: I. Distribution of Substructures and Their Dependence On Assembly History From N-Body Simulations
Observations have shown that the spatial distribution of satellite galaxies
is not random, but aligned with the major axes of central galaxies. This
alignment is dependent on galaxy properties, such that red satellites are more
strongly aligned than blue satellites. Theoretical work done to interpret this
phenomena has found that it is due to the non-spherical nature of dark matter
halos. However, most studies over-predict the alignment signal under the
assumption that the central galaxy shape follows the shape of the host halo. It
is also not clear whether the color dependence of alignment is due to an
assembly bias or an evolution effect. In this paper we study these problems
using a cosmological N-body simulation. Subhalos are used to trace the
positions of satellite galaxies. It is found that the shape of dark matter
halos are mis-aligned at different radii. If the central galaxy shares the same
shape as the inner host halo, then the alignment effect is weaker and agrees
with observational data. However, it predicts almost no dependence of alignment
on the color of satellite galaxies, though the late accreted subhalos show
stronger alignment with the outer layer of the host halo than their early
accreted counterparts. We find that this is due to the limitation of pure
N-body simulations that satellites galaxies without associated subhalos
('orphan galaxies') are not resolved. These orphan (mostly red) satellites
often reside in the inner region of host halos and should follow the shape of
the host halo in the inner region.Comment: 12 pages, 11 figures, Published on Ap
Toward optimal multistep forecasts in non-stationary autoregressions
This paper investigates multistep prediction errors for non-stationary
autoregressive processes with both model order and true parameters unknown. We
give asymptotic expressions for the multistep mean squared prediction errors
and accumulated prediction errors of two important methods, plug-in and direct
prediction. These expressions not only characterize how the prediction errors
are influenced by the model orders, prediction methods, values of parameters
and unit roots, but also inspire us to construct some new predictor selection
criteria that can ultimately choose the best combination of the model order and
prediction method with probability 1. Finally, simulation analysis confirms the
satisfactory finite sample performance of the newly proposed criteria.Comment: Published in at http://dx.doi.org/10.3150/08-BEJ165 the Bernoulli
(http://isi.cbs.nl/bernoulli/) by the International Statistical
Institute/Bernoulli Society (http://isi.cbs.nl/BS/bshome.htm
Detection of small single-cycle signals by stochastic resonance using a bistable superconducting quantum interference device
We propose and experimentally demonstrate detecting small single-cycle and
few-cycle signals by using the symmetric double-well potential of a radio
frequency superconducting quantum interference device (rf-SQUID). We show that
the response of this bistable system to single- and few-cycle signals has a
non-monotonic dependence on the noise strength. The response, measured by the
probability of transition from initial potential well to the opposite one,
becomes maximum when the noise-induced transition rate between the two stable
states of the rf-SQUID is comparable to the signal frequency. Comparison to
numerical simulations shows that the phenomenon is a manifestation of
stochastic resonance.Comment: 5 pages 3 figure
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