57,231 research outputs found
Dynamical cooling of galactic discs by molecular cloud collisions -- Origin of giant clumps in gas-rich galaxy discs
Different from Milky-Way-like galaxies, discs of gas-rich galaxies are
clumpy. It is believed that the clumps form because of gravitational
instability. However, a necessary condition for gravitational instability to
develop is that the disc must dissipate its kinetic energy effectively, this
energy dissipation (also called cooling) is not well-understood. We propose
that collisions (coagulation) between molecular clouds dissipate the kinetic
energy of the discs, which leads to a dynamical cooling. The effectiveness of
this dynamical cooling is quantified by the dissipation parameter , which is
the ratio between the free-fall time and the cooling time determined by the cloud collision process . This ratio is related to the ratio between the mean surface density of
the disc and the mean surface density of molecular clouds
in the disc . When (which roughly corresponds to
), cloud collision cooling is
inefficient, and fragmentation is suppressed. When (which roughly
corresponds to ), cloud-cloud
collisions lead to a rapid cooling through which clumps form. On smaller
scales, cloud-cloud collisions can drive molecular cloud turbulence. This
dynamical cooling process can be taken into account in numerical simulations as
a subgrid model to simulate the global evolution of disc galaxies.Comment: MNRAS accepte
Asymptotics for sliced average variance estimation
In this paper, we systematically study the consistency of sliced average
variance estimation (SAVE). The findings reveal that when the response is
continuous, the asymptotic behavior of SAVE is rather different from that of
sliced inverse regression (SIR). SIR can achieve consistency even
when each slice contains only two data points. However, SAVE cannot be
consistent and it even turns out to be not consistent when each
slice contains a fixed number of data points that do not depend on n, where n
is the sample size. These results theoretically confirm the notion that SAVE is
more sensitive to the number of slices than SIR. Taking this into account, a
bias correction is recommended in order to allow SAVE to be
consistent. In contrast, when the response is discrete and takes finite values,
consistency can be achieved. Therefore, an approximation through
discretization, which is commonly used in practice, is studied. A simulation
study is carried out for the purposes of illustration.Comment: Published at http://dx.doi.org/10.1214/009053606000001091 in the
Annals of Statistics (http://www.imstat.org/aos/) by the Institute of
Mathematical Statistics (http://www.imstat.org
Asymptotic Glosten Milgrom equilibrium
This paper studies the Glosten Milgrom model whose risky asset value admits
an arbitrary discrete distribution. Contrast to existing results on insider's
models, the insider's optimal strategy in this model, if exists, is not of
feedback type. Therefore a weak formulation of equilibrium is proposed. In this
weak formulation, the inconspicuous trade theorem still holds, but the
optimality for the insider's strategy is not enforced. However, the insider can
employ some feedback strategy whose associated expected profit is close to the
optimal value, when the order size is small. Moreover this discrepancy
converges to zero when the order size diminishes. The existence of such a weak
equilibrium is established, in which the insider's strategy converges to the
Kyle optimal strategy when the order size goes to zero
Associated production of the top-pions and single top at hadron colliders
In the context of topcolor assisted technicolor(TC2) models, we study the
production of the top-pions with single top quark via the
processes and ,
and discuss the possibility of detecting these new particles at Tevatron and
LHC. We find that it is very difficult to observe the signals of these
particles via these processes at Tevatron, while the neutral and charged
top-pions and can be detecting via considering
the same sign top pair event and the (or )
event at LHC, respectively.Comment: latex files,14 pages, 7 figures. Accepted for publication in Phys.
Rev.
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