68,083 research outputs found
Resolving the virial discrepancy in clusters of galaxies with modified Newtonian dynamics
A sample of 197 X-ray emitting clusters of galaxies is considered in the
context of Milgrom's modified Newtonian dynamics (MOND). It is shown that the
gas mass, extrapolated via an assumed model to a fixed radius of 3 Mpc,
is correlated with the gas temperature as predicted by MOND (). The observed temperatures are generally consistent with the inferred
mass of hot gas; no substantial quantity of additional unseen matter is
required in the context of MOND. However, modified dynamics cannot resolve the
strong lensing discrepancy in those clusters where this phenomenon occurs. The
prediction is that additional baryonic matter may be detected in the central
regions of rich clusters.Comment: Submitted to A&A, 4 pages, 3 figures, A&A macro
Star-Formation in Low Radio Luminosity AGN from the Sloan Digital Sky Survey
We investigate faint radio emission from low- to high-luminosity Active
Galactic Nuclei (AGN) selected from the Sloan Digital Sky Survey (SDSS). Their
radio properties are inferred by co-adding large ensembles of radio image
cut-outs from the FIRST survey, as almost all of the sources are individually
undetected. We correlate the median radio flux densities against a range of
other sample properties, including median values for redshift, [OIII]
luminosity, emission line ratios, and the strength of the 4000A break. We
detect a strong trend for sources that are actively undergoing star-formation
to have excess radio emission beyond the ~10^28 ergs/s/Hz level found for
sources without any discernible star-formation. Furthermore, this additional
radio emission correlates well with the strength of the 4000A break in the
optical spectrum, and may be used to assess the age of the star-forming
component. We examine two subsamples, one containing the systems with emission
line ratios most like star-forming systems, and one with the sources that have
characteristic AGN ratios. This division also separates the mechanism
responsible for the radio emission (star-formation vs. AGN). For both cases we
find a strong, almost identical, correlation between [OIII] and radio
luminosity, with the AGN sample extending toward lower, and the star-formation
sample toward higher luminosities. A clearer separation between the two
subsamples is seen as function of the central velocity dispersion of the host
galaxy. For systems with similar redshifts and velocity dispersions, the
star-formation subsample is brighter than the AGN in the radio by an order of
magnitude. This underlines the notion that the radio emission in star-forming
systems can dominate the emission associated with the AGN.Comment: Accepted for publication in Astronomical Journal; 15 pages, 8 color
figure
Equivalence of two approaches for the inhomogeneous density in the canonical ensemble
In this article we show that the inhomogeneous density obtained from a
density-functional theory of classical fluids in the canonical ensemble (CE),
recently presented by White et al [Phys. Rev. Lett. 84 (2000) 1220], is
equivalent to first order to the result of the series expansion of the CE
inhomogeneous density introduced by Gonzalez et al [Phys. Rev. Lett. 79 (1997)
2466].Comment: 6 pages, RevTe
Spin anisotropy effects in dimer single molecule magnets
We present a model of equal spin dimer single molecule magnets. The
spins within each dimer interact via the Heisenberg and the most general set of
four quadratic anisotropic spin interactions with respective strengths and
, and with the magnetic induction . We solve the model
exactly for , and for antiferromagnetic Heisenberg couplings
(), present curves at low for these cases. Low-
curves for and electron paramagnetic susceptibility
for are also provided. For weak anisotropy
interactions, we employ a perturbative treatment, and show that the Hartree and
extended Hartree approximations lead to reliable analytic results at low
and large for these quantities and for the inelastic neutron scattering
cross-section . Our results are discussed with
regard to existing experiments on Fe dimer
single molecule magnets, and suggest that one of them contains a substantial
amount of single-ion anisotropy, without any sizeable global spin anisotropy.
We urge further experiments of the above types on single crystals of Fe and
on some [Mn] dimers, in order to elucidate the precise values
of the various microscopic interactions.Comment: 30 pages, 25 figures, submitted to Phys. Rev.
Thermodynamics of the anisotropic Heisenberg chain calculated by the density matrix renormalization group method
The density matrix renormalization group (DMRG) method is applied to the
anisotropic Heisenberg chain at finite temperatures. The free energy of the
system is obtained using the quantum transfer matrix which is iteratively
enlarged in the imaginary time direction. The magnetic susceptibility and the
specific heat are calculated down to T=0.01J and compared with the Bethe ansatz
results. The agreement including the logarithmic correction in the magnetic
susceptibility at the isotropic point is fairly good.Comment: 4 pages, 3 Postscript figures, REVTeX, to appear in J. Phys. Soc.
Jpn. Vol.66 No.8 (1997
Stripes in Doped Antiferromagnets: Single-Particle Spectral Weight
Recent photoemission (ARPES) experiments on cuprate superconductors provide
important guidelines for a theory of electronic excitations in the stripe
phase. Using a cluster perturbation theory, where short-distance effects are
accounted for by exact cluster diagonalization and long-distance effects by
perturbation (in the hopping), we calculate the single-particle Green's
function for a striped t-J model. The data obtained quantitatively reproduce
salient (ARPES-) features and may serve to rule out "bond-centered" in favor of
"site-centered" stripes.Comment: final version as appeared in PRL; (c) 2000 The American Physical
Society; 4 pages, 4 figure
Deconvolution of ASCA X-ray data: II. Radial temperature and metallicity profiles for 106 galaxy clusters
In Paper-I we presented a methodology to recover the spatial variations of
properties of the intracluster gas from ASCA X-ray satellite observations of
galaxy clusters. We verified the correctness of this procedure by applying it
to simulated cluster datasets which we had subjected to the various
contaminants common in ASCA data. In this paper we present the results which we
obtain when we apply this method to real galaxy cluster observations. We
determine broad-band temperature and cooling-flow mass-deposition rates for the
106 clusters in our sample, and obtain temperature, abundance and emissivity
profiles (i.e. at least two annular bins) for 98 of these clusters. We find
that 90 percent of these temperature profiles are consistent with isothermality
at the 3-sigma confidence level. This conflicts with the prevalence of
steeply-declining cluster temperature profiles found by Markevitch et al.
(1998) from a sample of 30 clusters. In Paper-III (in preparation) we utilise
our temperature and emissivity profiles to determine radial hydrostatic-mass
properties for a subsample of the clusters presented in this paper.Comment: MNRAS, accpeted. Postscript copy of paper and individual postscript
files for plots in Appendix B can be obtained from:
http://www-xray.ast.cam.ac.uk/~da
Diagonal Ladders: A New Class of Models for Strongly Coupled Electron Systems
We introduce a class of models defined on ladders with a diagonal structure
generated by plaquettes. The case corresponds to the necklace
ladder and has remarkable properties which are studied using DMRG and recurrent
variational ansatzes. The AF Heisenberg model on this ladder is equivalent to
the alternating spin-1/spin-1/2 AFH chain which is known to have a
ferrimagnetic ground state (GS). For doping 1/3 the GS is a fully doped (1,1)
stripe with the holes located mostly along the principal diagonal while the
minor diagonals are occupied by spin singlets. This state can be seen as a Mott
insulator of localized Cooper pairs on the plaquettes. A physical picture of
our results is provided by a model of plaquettes coupled diagonally
with a hopping parameter . In the limit we recover the
original model on the necklace ladder while for weak hopping parameter
the model is easily solvable. The GS in the strong hopping regime is
essentially an "on link" Gutzwiller projection of the weak hopping GS. We
generalize the model to diagonal ladders with and the 2D
square lattice. We use in our construction concepts familiar in Statistical
Mechanics as medial graphs and Bratelli diagrams.Comment: REVTEX file, 22 pages (twocolumn), 35 figures inserted in text. 12
Table
The Structure of Dark Matter Haloes in Dwarf Galaxies
Recent observations indicate that dark matter haloes have flat central
density profiles. Cosmological simulations with non-baryonic dark matter
predict however self similar haloes with central density cusps. This
contradiction has lead to the conclusion that dark matter must be baryonic.
Here it is shown that the dark matter haloes of dwarf spiral galaxies represent
a one parameter family with self similar density profiles. The observed global
halo parameters are coupled with each other through simple scaling relations
which can be explained by the standard cold dark matter model if one assumes
that all the haloes formed from density fluctuations with the same primordial
amplitude. We find that the finite central halo densities correlate with the
other global parameters. This result rules out scenarios where the flat halo
cores formed subsequently through violent dynamical processes in the baryonic
component. These cores instead provide important information on the origin and
nature of dark matter in dwarf galaxies.Comment: uuencoded Z-compressed postscript file, 10 pages, 3 figures included,
to appear in ApJ Letter
- …