20 research outputs found
A nanomechanical resonator coupled linearly via its momentum to a quantum point contact
We use a Born-Markov approximated master equation approach to study the
symmetrized-in-frequency current noise spectrum and the oscillator steady state
of a nanoelectromechanical system where a nanoscale resonator is coupled
linearly via its momentum to a quantum point contact (QPC). Our current noise
spectra exhibit clear signatures of the quantum correlations between the QPC
current and the back-action force on the oscillator at a value of the relative
tunneling phase (\eta = -\pi/2) where such correlations are expected to be
maximized. We also show that the steady state of the oscillator obeys a
classical Fokker-Planck equation, but can experience thermomechanical noise
squeezing in the presence of a momentum-coupled detector bath and a
position-coupled environmental bath. Besides, the full master equation clearly
shows that half of the detector back-action is correlated with electron
tunneling, indicating a departure from the model of the detector as an
effective bath and suggesting that a future calculation valid at lower bias
voltage, stronger tunneling and/or stronger coupling might reveal interesting
quantum effects in the oscillator dynamics.Comment: 38 pages, 16 figure
Abrupt grain boundary melting in ice
The effect of impurities on the grain boundary melting of ice is investigated
through an extension of Derjaguin-Landau-Verwey-Overbeek theory, in which we
include retarded potential effects in a calculation of the full frequency
dependent van der Waals and Coulombic interactions within a grain boundary. At
high dopant concentrations the classical solutal effect dominates the melting
behavior. However, depending on the amount of impurity and the surface charge
density, as temperature decreases, the attractive tail of the dispersion force
interaction begins to compete effectively with the repulsive screened Coulomb
interaction. This leads to a film-thickness/temperature curve that changes
depending on the relative strengths of these interactions and exhibits a
decrease in the film thickness with increasing impurity level. More striking is
the fact that at very large film thicknesses, the repulsive Coulomb interaction
can be effectively screened leading to an abrupt reduction to zero film
thickness.Comment: 8 pages, 1 figur
The Virial Mass Function of Nearby SDSS Galaxy Clusters
We present a new determination of the cluster mass function and velocity
dispersion function in a volume Mpc using the Fourth Data
Release of the Sloan Digital Sky Survey (SDSS). We use the caustic technique to
remove foreground and background galaxies. The cluster virial mass function
agrees well with recent estimates from both X-ray observations and cluster
richnesses. The mass function lies between those predicted by the First-Year
and Three-Year WMAP data. We constrain the cosmological parameters
and and find good agreement with WMAP and constraints from other
techniques. With the CIRS mass function alone, we estimate
and , or
0.03 when holding fixed. We also use the WMAP
parameters as priors and constrain velocity segregation in clusters. Using the
First and Third-Year results, we infer velocity segregation of
0.05 or 1.280.06 respectively. We
compare the velocity dispersion function of clusters to that of early-type
galaxies and conclude that clusters comprise the high-velocity end of the
velocity dispersion function of dark matter haloes. The evolution of cluster
abundances provides constraints on dark energy models; the mass function
presented here offers an important low redshift calibration benchmark.Comment: 22 pages, 11 figures, ApJ in press, revised figure
Galaxy Orbits for Galaxy Clusters in Sloan Digital Sky Survey and 2dF Galaxy Redshift Survey
We present the results of a study for galaxy orbits in galaxy clusters using
a spectroscopic sample of galaxies in Sloan Digital Sky Survey (SDSS) and 2dF
Galaxy Redshift Survey (2dFGRS). We have determined the member galaxies of
Abell clusters covered by these surveys using the galaxies' redshift and
positional data. We have selected 10 clusters using three criteria: the number
of member galaxies is greater than or equal to 40, the spatial coverage is
complete, and X-ray mass profile is available in the literature. We derive the
radial profile of the galaxy number density and velocity dispersion using all,
early-type, and late-type galaxies for each cluster. We have investigated the
galaxy orbits for our sample clusters with constant and variable velocity
anisotropies over the clustercentric distance using Jeans equation. Using all
member galaxies, the galaxy orbits are found to be isotropic within the
uncertainty for most of sample clusters, although it is difficult to conclude
strongly for some clusters due the large errors and the variation as a function
of the clustercentric distance in the calculated velocity anisotropies. We
investigated the orbital difference between early-type and late-type galaxies
for four sample clusters, and found no significant difference between them.Comment: 59 pages, 21 figures. To appear in ApJ. Paper with high resolution
figures are available at http://astro.kias.re.kr/~hshwang/papers/orbit.pd
Substructure lensing in galaxy clusters as a constraint on low-mass sterile neutrinos in tensor-vector-scalar theory: The straight arc of Abell 2390
Certain covariant theories of the modified Newtonian dynamics paradigm seem
to require an additional hot dark matter (HDM) component - in the form of
either heavy ordinary neutrinos or more recently light sterile neutrinos (SNs)
with a mass around 11eV - to be relieved of problems ranging from cosmological
scales down to intermediate ones relevant for galaxy clusters. Here we suggest
using gravitational lensing by galaxy clusters to test such a marriage of
neutrino HDM and modified gravity, adopting the framework of
tensor-vector-scalar theory (TeVeS). Unlike conventional cold dark matter
(CDM), such HDM is subject to strong phase-space constraints, which allows one
to check cluster lens models inferred within the modified framework for
consistency. Since the considered HDM particles cannot collapse into
arbitrarily dense clumps and only form structures well above the galactic
scale, systems which indicate the need for dark substructure are of particular
interest. As a first example, we study the cluster lens Abell 2390 and its
impressive straight arc with the help of numerical simulations. Based on our
results, we outline a general and systematic approach to model cluster lenses
in TeVeS which significantly reduces the calculation complexity. We further
consider a simple bimodal lens configuration, capable of producing the straight
arc, to demonstrate our approach. We find that such a model is marginally
consistent with the hypothesis of 11eV SNs. Future work including more detailed
and realistic lens models may further constrain the necessary SN distribution
and help to conclusively assess this point. Cluster lenses could therefore
provide an interesting discriminator between CDM and such modified gravity
scenarios supplemented by SNs or other choices of HDM.Comment: 22 pages, 14 figures, 2 tables; minor changes to match accepted
versio
The mass and anisotropy profiles of galaxy clusters from the projected phase space density: testing the method on simulated data
We present a new method of constraining the mass and velocity anisotropy
profiles of galaxy clusters from kinematic data. The method is based on a model
of the phase space density which allows the anisotropy to vary with radius
between two asymptotic values. The characteristic scale of transition between
these asymptotes is fixed and tuned to a typical anisotropy profile resulting
from cosmological simulations. The model is parametrized by two values of
anisotropy, at the centre of the cluster and at infinity, and two parameters of
the NFW density profile, the scale radius and the scale mass. In order to test
the performance of the method in reconstructing the true cluster parameters we
analyze mock kinematic data for 20 relaxed galaxy clusters generated from a
cosmological simulation of the standard LCDM model. We use Bayesian methods of
inference and the analysis is carried out following the Markov Chain Monte
Carlo approach. The parameters of the mass profile are reproduced quite well,
but we note that the mass is typically underestimated by 15 percent, probably
due to the presence of small velocity substructures. The constraints on the
anisotropy profile for a single cluster are in general barely conclusive.
Although the central asymptotic value is determined accurately, the outer one
is subject to significant systematic errors caused by substructures at large
clustercentric distance. The anisotropy profile is much better constrained if
one performs joint analysis of at least a few clusters. In this case it is
possible to reproduce the radial variation of the anisotropy over two decades
in radius inside the virial sphere.Comment: 11 pages, 10 figures, accepted for publication in MNRA
Mass profiles and galaxy orbits in nearby galaxy clusters from the analysis of the projected phase-space
We analyze kinematic data of 41 nearby (z<0.1) relaxed galaxy clusters in
terms of the projected phase-space density using a phenomenological, fully
anisotropic model of the distribution function. We apply the Markov Chain Monte
Carlo approach to place constraints on total mass distribution approximated by
the universal NFW profile and the profile of the anisotropy of galaxy orbits.
We find the normalization of the mean mass-concentration relation is
c=6.9_{-0.7}^{+0.6} at the virial mass M_v=5x10^{14}M_sun. Assuming a
one-to-one correspondence between sigma_8 and the normalization of the
mass-concentration relation in the framework of the concordance model we
estimate the normalization of the linear power spectrum to be
sigma_8=0.91_{-0.08}^{+0.07}. Our constraints on the parameters of the mass
profile are compared with estimates from other methods. We show that galaxy
orbits are isotropic at the cluster centres (with the mean ratio of the
radial-to-tangential velocity dispersions sigma_r/sigma_theta=0.97+/-0.04) and
radially anisotropic at the virial sphere (with the mean ratio
sigma_r/sigma_theta=1.75^{+0.23}_{-0.19}). Although the value of the central
anisotropy appears to be universal, the anisotropy at the virial radius differs
between clusters within the range 1<(sigma_r/sigma_theta)<2. Utilizing the
Bautz-Morgan morphological classification and information on the prominence of
a cool core we select two subsamples of galaxy clusters corresponding to less
and more advanced evolutionary states. It is demonstrated that less evolved
clusters have shallower mass profiles and their galaxy orbits are more radially
biased at the virial sphere. This property is consistent with the expected
evolution of the mass profiles as well as with the observed orbital segregation
of late and early type galaxies.Comment: 16 pages, 12 figures, accepted for publication in MNRA
The importance of interloper removal in galaxy clusters: saving more objects for the Jeans analysis
We study the effect of contamination by interlopers in kinematic samples of
galaxy clusters. We demonstrate that without the proper removal of interlopers
the inferred parameters of the mass distribution in the cluster are strongly
biased towards higher mass and lower concentration. The interlopers are removed
using two procedures previously shown to work most efficiently on simulated
data. One is based on using the virial mass estimator and calculating the
maximum velocity available to cluster members and the other relies on the ratio
of the virial and projected mass estimators. We illustrate the performance of
the methods in detail using the example of A576, a cluster with a strong
uniform background contamination, and compare the case of A576 to 15 other
clusters with different degree of contamination. We model the velocity
dispersion and kurtosis profiles obtained for the cleaned data samples of these
clusters solving the Jeans equations to estimate the mass, concentration and
anisotropy parameter. We present the mass-concentration relation for the total
sample of 22 clusters.Comment: 13 pages, 7 figures, more clusters analyzed in detail,
mass-concentration relation for 22 clusters presented, accepted for
publication in MNRA