4,192 research outputs found
A Weak Gravitational Lensing Analysis of Abell 2390
We report on the detection of dark matter in the cluster Abell 2390 using the
weak gravitational distortion of background galaxies. We find that the cluster
light and total mass distributions are quite similar over an angular scale of
\simeq 7^\prime \;(1 \Mpc). The cluster galaxy and mass distributions are
centered on the cluster cD galaxy and exhibit elliptical isocontours in the
central \simeq 2^\prime \; (280 \kpc). The major axis of the ellipticity is
aligned with the direction defined by the cluster cD and a ``straight arc''
located to the northwest. We determined the radial
mass-to-light profile for this cluster and found a constant value of , which is consistent with other published
determinations. We also compared our weak lensing azimuthally averaged radial
mass profile with a spherical mass model proposed by the CNOC group on the
basis of their detailed dynamical study of the cluster. We find good agreement
between the two profiles, although there are weak indications that the CNOC
density profile may be falling more steeply for
(420\kpc).Comment: 14 pages, latex file. Postscript file and one additional figure are
available at
ftp://magicbean.berkeley.edu/pub/squires/a2390/massandlight.ps.g
Magnetic spectrum of the two-dimensional antiferromagnet La2CoO4 studied by inelastic neutron scattering
We report measurements of the magnetic excitation spectrum of the layered
antiferromagnet La2CoO4 by time-of-flight neutron inelastic scattering. In the
energy range probed in our experiments (0-250 meV) the magnetic spectrum
consists of spin-wave modes with strong in-plane dispersion extending up to 60
meV, and a nearly dispersionless peak at 190 meV. The spin-wave modes exhibit a
small (~1 meV) dispersion along the magnetic zone boundary. We show that the
magnetic spectrum can be described very well by a model of a Heisenberg
antiferromagnet that includes the full spin and orbital degrees of freedom of
Co2+ in an axially-distorted crystal field. The collective magnetic dynamics
are found to be controlled by dominant nearest-neighbour exchange interactions,
strong XY-like single-ion anisotropy and a substantial unquenched orbital
angular momentum.Comment: 8 pages, 7 figure
Slip-velocity of large neutrally-buoyant particles in turbulent flows
We discuss possible definitions for a stochastic slip velocity that describes
the relative motion between large particles and a turbulent flow. This
definition is necessary because the slip velocity used in the standard drag
model fails when particle size falls within the inertial subrange of ambient
turbulence. We propose two definitions, selected in part due to their
simplicity: they do not require filtration of the fluid phase velocity field,
nor do they require the construction of conditional averages on particle
locations. A key benefit of this simplicity is that the stochastic slip
velocity proposed here can be calculated equally well for laboratory, field,
and numerical experiments. The stochastic slip velocity allows the definition
of a Reynolds number that should indicate whether large particles in turbulent
flow behave (a) as passive tracers; (b) as a linear filter of the velocity
field; or (c) as a nonlinear filter to the velocity field. We calculate the
value of stochastic slip for ellipsoidal and spherical particles (the size of
the Taylor microscale) measured in laboratory homogeneous isotropic turbulence.
The resulting Reynolds number is significantly higher than 1 for both particle
shapes, and velocity statistics show that particle motion is a complex
non-linear function of the fluid velocity. We further investigate the nonlinear
relationship by comparing the probability distribution of fluctuating
velocities for particle and fluid phases
The Assembly of the Red Sequence at z ~ 1: The Color and Spectral Properties of Galaxies in the Cl1604 Supercluster
We investigate the properties of the 525 spectroscopically confirmed members of the Cl1604 supercluster at z ~ 0.9 as part of the Observations of Redshift Evolution in Large Scale Environments survey. In particular, we focus on the photometric, stellar mass, morphological, and spectral properties of the 305 member galaxies of the eight clusters and groups that comprise the Cl1604 supercluster. Using an extensive Keck Low-Resolution Imaging Spectrometer (LRIS)/DEep Imaging Multi-Object Spectrograph (DEIMOS) spectroscopic database in conjunction with ten-band ground-based, Spitzer, and Hubble Space Telescope imaging, we investigate the buildup of the red sequence in groups and clusters at high redshift. Nearly all of the brightest and most massive red-sequence galaxies present in the supercluster environment are found to lie within the bounds of the cluster and group systems, with a surprisingly large number of such galaxies present in low-mass group systems. Despite the prevalence of these red-sequence galaxies, we find that the average cluster galaxy has a spectrum indicative of a star-forming galaxy, with a star formation rate between those of z ~ 1 field galaxies and moderate-redshift cluster galaxies. The average group galaxy is even more active, exhibiting spectral properties indicative of a starburst. The presence of massive, red galaxies and the high fraction of starbursting galaxies present in the group environment suggest that significant processing is occurring in group environments at z ~ 1 and earlier. There is a deficit of low-luminosity red-sequence galaxies in all Cl1604 clusters and groups, suggesting that such galaxies transition to the red sequence at later times. Extremely massive (~10^(12)M_☉) red-sequence galaxies routinely observed in rich clusters at z ~ 0 are also absent from the Cl1604 clusters and groups. We suggest that such galaxies form at later times through merging processes. There are significant populations of transition galaxies at intermediate stellar masses (log(M_*)=10.25-10.75) present in the group and cluster environments, suggesting that this range is important for the buildup of the red-sequence mass function at z ~ 1. Through a comparison of the transitional populations present in the Cl1604 cluster and group systems, we find evidence that massive blue-cloud galaxies are quenched earliest in the most dynamically relaxed systems and at progressively later times in dynamically unrelaxed systems
Spin Waves in Striped Phases
In many antiferromagnetic, quasi-two-dimensional materials, doping with holes
leads to "stripe" phases, in which the holes congregate along antiphase domain
walls in the otherwise antiferromagnetic texture. Using a suitably parametrized
two-dimensional Heisenberg model on a square lattice, we study the spin wave
spectra of well-ordered spin stripes, comparing bond-centered antiphase domain
walls to site-centered antiphase domain walls for a range of spacings between
the stripes and for stripes both aligned with the lattice ("vertical") and
oriented along the diagonals of the lattice ("diagonal"). Our results establish
that there are qualitative differences between the expected neutron scattering
responses for the bond-centered and site-centered cases. In particular,
bond-centered stripes of odd spacing generically exhibit more elastic peaks
than their site-centered counterparts. For inelastic scattering, we find that
bond-centered stripes produce more spin wave bands than site-centered stripes
of the same spacing and that bond-centered stripes produce rather isotropic low
energy spin wave cones for a large range of parameters, despite local
microscopic anisotropy. We find that extra scattering intensity due to the
crossing of spin wave modes (which may be linked to the "resonance peak" in the
cuprates) is more likely for diagonal stripes, whether site- or bond-centered,
whereas spin wave bands generically repel, rather than cross, when stripes are
vertical.Comment: 12 pages, 12 figures, for some high-res.pics, see
http://physics.bu.edu/~yaodx/spinwave/spinw.htm
The Dark Matter, Gas and Galaxy Distributions in Abell 2218: A Weak Gravitational Lensing and X-ray Analysis
We report on the detection of dark matter in the cluster Abell 2218 using the
weak gravitational distortion of background galaxies. We find a highly
significant, coherent detection of the distortion in the images of the
background galaxies. The inferred 2D mass distribution has a peak that is
coincident with the optical and X-ray centroid. The qualitative distributions
of the cluster light, the X-ray emission and the dark matter are similar and
the projected total mass, gas, and light surface densities are consistent with
a profile at distance of from the cluster cD
galaxy. Using the weak lensing technique, we determine a lower bound for the
total mass in A2218 of ~h~M within
a fiducial aperture of radius 0.4~hMpc. The associated cluster
mass-to-light ratio is ~h~. The mass
estimated by the weak lensing method is consistent with that inferred from the
X-ray data under the assumption of hydrostatic equilibrium and we derive an
upper bound for the gas-to-total mass ratio at 400~hkpc of
h.Comment: 25 pages, Latex, no figures. Complete version with figures available
by anon ftp at ftp://ftp.cita.utoronto.ca/ftp/cita/squires/a221
Weak Lensing Detection of Cl 1604+4304 at z = 0.90
We present a weak lensing analysis of the high-redshift cluster Cl 1604+4304.
At z=0.90, this is the highest-redshift cluster yet detected with weak lensing.
It is also one of a sample of high-redshift, optically-selected clusters whose
X-ray temperatures are lower than expected based on their velocity dispersions.
Both the gas temperature and galaxy velocity dispersion are proxies for its
mass, which can be determined more directly by a lensing analysis. Modeling the
cluster as a singular isothermal sphere, we find that the mass contained within
projected radius R is 3.69+-1.47 * (R/500 kpc) 10^14 M_odot. This corresponds
to an inferred velocity dispersion of 1004+-199 km/s, which agrees well with
the measured velocity dispersion of 989+98-76 km/s (Gal & Lubin 2004). These
numbers are higher than the 575+110-85 km/s inferred from Cl 1604+4304 X-ray
temperature, however all three velocity dispersion estimates are consistent
within ~ 1.9 sigma.Comment: Revised version accepted for publication in AJ (January 2005). 2
added figures (6 figures total
Dispersive magnetic excitations in the S=1 antiferromagnet BaMnO
We present powder inelastic neutron scattering measurements of the S=1
dimerized antiferromagnet BaMnO. The K magnetic spectrum
exhibits a spin-gap of meV and a dispersive spectrum with
a bandwidth of approximately 1.5 meV. Comparison to coupled dimer models
describe the dispersion and scattering intensity accurately and determine the
exchange constants in BaMnO. The wave vector dependent scattering
intensity confirms the proposed S=1 dimer bond. Temperature dependent
measurements of the magnetic excitations indicate the presence of both
singlet-triplet and thermally activated triplet-quintet excitations.Comment: 8 pages, 8 figures, Submitted to Physical Review B, Resubmited
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