1,833 research outputs found
The outer profile of dark matter halos: an analytical approach
A steepening feature in the outer density profiles of dark matter halos
indicating the splashback radius has drawn much attention recently. Possible
observational detections have even been made for galaxy clusters.
Theoretically, Adhikari et al. have estimated the location of the splashback
radius by computing the secondary infall trajectory of a dark matter shell
through a growing dark matter halo with an NFW profile. However, since they
imposed a shape of the halo profile rather than computing it consistently from
the trajectories of the dark matter shells, they could not provide the full
shape of the dark matter profile around the splashback radius. We improve on
this by extending the self-similar spherical collapse model of Fillmore \&
Goldreich to a CDM universe. This allows us to compute the dark matter
halo profile and the trajectories simultaneously from the mass accretion
history. Our results on the splashback location agree qualitatively with
Adhikari et al. but with small quantitative differences at large mass accretion
rates. We present new fitting formulae for the splashback radius
in various forms, including the ratios of and
. Numerical simulations have made the puzzling
discovery that the splashback radius scales well with but not
with . We trace the origin of this to be the correlated increase
of and the average halo mass accretion rate with an increasing
redshift.Comment: 10 pages, 10 figures, published in MNRA
Turbulence decay in the density-stratified intracluster medium
Turbulence evolution in a density-stratified medium differs from that of
homogeneous isotropic turbulence described by the Kolmogorov picture. We
evaluate the degree of this effect in the intracluster medium (ICM) with
hydrodynamical simulations. We find that the buoyancy effect induced by ICM
density stratification introduces qualitative changes to the turbulence energy
evolution, morphology, and the density fluctuation - turbulence Mach number
relation, and likely explains the radial dependence of the ICM turbulence
amplitude as found previously in cosmological simulations. A new channel of
energy flow between the kinetic and the potential energy is opened up by
buoyancy. When the gravitational potential is kept constant with time, this
energy flow leaves oscillations to the energy evolution, and leads to a
balanced state of the two energies where both asymptote to power-law time
evolution with slopes shallower than that for the turbulence kinetic energy of
homogeneous isotropic turbulence. We discuss that the energy evolution can
differ more significantly from that of homogeneous isotropic turbulence when
there is a time variation of the gravitational potential. Morphologically, ICM
turbulence can show a layered vertical structure and large horizontal vortical
eddies in the central regions with the greatest density stratification. In
addition, we find that the coefficient in the linear density fluctuation -
turbulence Mach number relation caused by density stratification is in general
a variable with position and time.Comment: 10 pages, 9 figures, published in MNRA
Analytical model for non-thermal pressure in galaxy clusters
Non-thermal pressure in the intracluster gas has been found ubiquitously in
numerical simulations, and observed indirectly. In this paper we develop an
analytical model for intracluster non-thermal pressure in the virial region of
relaxed clusters. We write down and solve a first-order differential equation
describing the evolution of non-thermal velocity dispersion. This equation is
based on insights gained from observations, numerical simulations, and theory
of turbulence. The non-thermal energy is sourced, in a self-similar fashion, by
the mass growth of clusters via mergers and accretion, and dissipates with a
time-scale determined by the turnover time of the largest turbulence eddies.
Our model predicts a radial profile of non-thermal pressure for relaxed
clusters. The non-thermal fraction increases with radius, redshift, and cluster
mass, in agreement with numerical simulations. The radial dependence is due to
a rapid increase of the dissipation time-scale with radii, and the mass and
redshift dependence comes from the mass growth history. Combing our model for
the non-thermal fraction with the Komatsu-Seljak model for the total pressure,
we obtain thermal pressure profiles, and compute the hydrostatic mass bias. We
find typically 10% bias for the hydrostatic mass enclosed within .Comment: 12 pages, 9 figures, published in MNRAS. Discussions and references
added. A factor of 2 corrected in t_dyn (Fig. 2), definition of t_d (Eq. 3)
changed accordingl
How well do third-order aperture mass statistics separate E- and B-modes?
With 3rd-order statistics of gravitational shear it will be possible to
extract valuable cosmological information from ongoing and future weak lensing
surveys which is not contained in standard 2nd-order statistics, due to the
non-Gaussianity of the shear field. Aperture mass statistics are an appropriate
choice for 3rd-order statistics due to their simple form and their ability to
separate E- and B-modes of the shear. However, it has been demonstrated that
2nd-order aperture mass statistics suffer from E-/B-mode mixing because it is
impossible to reliably estimate the shapes of close pairs of galaxies. This
finding has triggered developments of several new 2nd-order statistical
measures for cosmic shear. Whether the same developments are needed for
3rd-order shear statistics is largely determined by how severe this E-/B-mixing
is for 3rd-order statistics. We test 3rd-order aperture mass statistics against
E-/B-mode mixing, and find that the level of contamination is well-described by
a function of , with being the
cut-off scale. At angular scales of , the
decrease in the E-mode signal due to E-/B-mode mixing is smaller than 1
percent, and the leakage into B-modes is even less. For typical small-scale
cut-offs this E-/B-mixing is negligible on scales larger than a few arcminutes.
Therefore, 3rd-order aperture mass statistics can safely be used to separate E-
and B-modes and infer cosmological information, for ground-based surveys as
well as forthcoming space-based surveys such as Euclid.Comment: references added, A&A publishe
Multi-scale analysis of turbulence evolution in the density stratified intracluster medium
The diffuse hot medium inside clusters of galaxies typically exhibits
turbulent motions whose amplitude increases with radius, as revealed by
cosmological hydrodynamical simulations. However, its physical origin remains
unclear. It could either be due to an excess injection of turbulence at large
radii, or faster turbulence dissipation at small radii. We investigate this by
studying the time evolution of turbulence in the intracluster medium (ICM)
after major mergers, using the Omega500 non-radiative hydrodynamical
cosmological simulations. By applying a novel wavelet analysis to study the
radial dependence of the ICM turbulence spectrum, we discover that faster
turbulence dissipation in the inner high density regions leads to the
increasing turbulence amplitude with radius. We also find that the ICM
turbulence at all radii decays in two phases after a major merger: an early
fast decay phase followed by a slow secular decay phase. The buoyancy effects
resulting from the ICM density stratification becomes increasingly important
during turbulence decay, as revealed by a decreasing turbulence Froude number
. Our results indicate that the stronger density
stratification and smaller eddy turn-over time are the likely causes of the
faster turbulence dissipation rate in the inner regions of the cluster.Comment: 8 pages, 7 figures, accepted to MNRA
The Role of Early Recurrence in Improving Visual Representations
This dissertation proposes a computational model of early vision with recurrence, termed as early recurrence. The idea is motivated from the research of the primate vision. Specifically, the proposed model relies on the following four observations. 1) The primate visual system includes two main visual pathways: the dorsal pathway and the ventral pathway; 2) The two pathways respond to different visual features; 3) The neurons of the dorsal pathway conduct visual information faster than that of the neurons of the ventral pathway; 4) There are lower-level feedback connections from the dorsal pathway to the ventral pathway. As such, the primate visual system may implement a recurrent mechanism to improve visual representations of the ventral pathway.
Our work starts from a comprehensive review of the literature, based on which a conceptualization of early recurrence is proposed. Early recurrence manifests itself as a form of surround suppression. We propose that early recurrence is capable of refining the ventral processing using results of the dorsal processing.
Our work further defines a set of computational components to formalize early recurrence. Although we do not intend to model the true nature of biology, to verify that the proposed computation is biologically consistent, we have applied the model to simulate a neurophysiological experiment of a bar-and-checkerboard and a psychological experiment involving a moving contour illusion. Simulation results indicated that the proposed computation behaviourally reproduces the original observations.
The ultimate goal of this work is to investigate whether the proposal is capable of improving computer vision applications. To do this, we have applied the model to a variety of applications, including visual saliency and contour detection. Based on comparisons against the state-of-the-art, we conclude that the proposed model of early recurrence sheds light on a generally applicable yet lightweight approach to boost real-life application performance
Dynamical heating of the X-ray emitting intracluster medium: the roles of merger shocks and turbulence dissipation
The diffuse plasma inside clusters of galaxies has X-ray emitting
temperatures of a few keV. The physical mechanisms that heat this intracluster
medium (ICM) to such temperatures include the accretion shock at the periphery
of a galaxy cluster, the shocks driven by merger events, as well as a somewhat
overlooked mechanism -- the dissipation of intracluster turbulent motions. We
study the relative role of these heating mechanisms using galaxy clusters in
Lagrangian tracer particle re-simulations of the Omega500 cosmological
simulation. We adopt a novel analysis method of decomposing the temperature
increase at each time step into the contribution from dissipative heating and
that from adiabatic heating. In the high-resolution spatial-temporal map of
these heating rates, merger tracks are clearly visible, demonstrating the
dominant role of merger events in heating the ICM. The dissipative heating
contributed by each merger event is extended in time and also occurs in the
rarefaction regions, suggesting the importance of heating by the dissipation of
merger-induced turbulence. Quantitative analysis shows that turbulence heating,
rather than direct heating at merger shocks, dominates the temperature increase
of the ICM, especially at inner radii . In addition, we find
that many merger shocks can propagate with almost constant velocity to very
large radii , some even reach and join with the accretion
shock and becoming the outer boundary of the ICM. Altogether, these results
suggest that the ICM is heated more in an `inside-out' fashion rather than
`outside-in' as depicted in the classical smooth accretion picture.Comment: 12 pages, 13 figures, published in MNRA
Analytical model for non-thermal pressure in galaxy clusters - III. Removing the hydrostatic mass bias
Non-thermal pressure in galaxy clusters leads to underestimation of the mass
of galaxy clusters based on hydrostatic equilibrium with thermal gas pressure.
This occurs even for dynamically relaxed clusters that are used for calibrating
the mass-observable scaling relations. We show that the analytical model for
non-thermal pressure developed in Shi & Komatsu 2014 can correct for this
so-called 'hydrostatic mass bias', if most of the non-thermal pressure comes
from bulk and turbulent motions of gas in the intracluster medium. Our
correction works for the sample average irrespective of the mass estimation
method, or the dynamical state of the clusters. This makes it possible to
correct for the bias in the hydrostatic mass estimates from X-ray surface
brightness and the Sunyaev-Zel'dovich observations that will be available for
clusters in a wide range of redshifts and dynamical states.Comment: 9 pages, 8 figures, published in MNRA
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