6,476 research outputs found
Reduced Fine-Tuning in Supersymmetry with R-parity violation
Both electroweak precision measurements and simple supersymmetric extensions
of the standard model prefer a mass of the Higgs boson less than the
experimental lower limit of 114 GeV. We show that supersymmetric models with R
parity violation and baryon number violation have a significant range of
parameter space in which the Higgs dominantly decays to six jets. These decays
are much more weakly constrained by current LEP analyses and would allow for a
Higgs mass near that of the . In general, lighter scalar quark and other
superpartner masses are allowed and the fine-tuning typically required to
generate the measured scale of electroweak symmetry breaking is ameliorated.
The Higgs would potentially be discovered at hadron colliders via the
appearance of new displaced vertices. The lightest neutralino could be
discovered by a scan of vertex-less events LEP I data.Comment: 5 pages, 2 figures. Significant detail added to the arguments
regarding LEP limits - made more quantitative. Better figures used, plotting
more physical quantities. Typos corrected and references updated. Conclusions
unchange
The Ellipticity and Orientation of Clusters of Galaxies from N-Body Experiments
In this study we use simulations of 128 particles to study the
ellipticity and orientation of clusters of galaxies in N-body simulations of
differing power-law initial spectra (P(k) \propto k^n ,n = +1, 0, -1, -2\Omega_0 = 0.2nD < 15 h^{-1}n-$dependent way.Comment: 22 pages, requires aaspp4.sty, flushrt.sty, and epsf.sty Revised
manuscript, accepted for publication in Ap
Alignments of the Dominant Galaxies in Poor Clusters
We have examined the orientations of brightest cluster galaxies (BCGs) in
poor MKW and AWM clusters and find that, like their counterparts in richer
Abell clusters, poor cluster BCGs exhibit a strong propensity to be aligned
with the principal axes of their host clusters as well as the surrounding
distribution of nearby (< 20/h Mpc) Abell clusters. The processes responsible
for dominant galaxy alignments are therefore independent of cluster richness.
We argue that these alignments most likely arise from anisotropic infall of
material into clusters along large-scale filaments.Comment: 8 pages, 5 figure
The rotation curves of dwarf galaxies: a problem for Cold Dark Matter?
We address the issue of accuracy in recovering density profiles from
observations of rotation curves of galaxies. We ``observe'' and analyze our
models in much the same way as observers do the real galaxies. We find that the
tilted ring model analysis produces an underestimate of the central rotational
velocity. In some cases the galaxy halo density profile seems to have a flat
core, while in reality it does not. We identify three effects, which explain
the systematic biases: (1) inclination (2), small bulge, and (3) bar. The
presence of even a small non-rotating bulge component reduces the rotation
velocity. In the case of a disk with a bar, the underestimate of the circular
velocity is larger due to a combination of non-circular motions and random
velocities. Signatures of bars can be difficult to detect in the surface
brightness profiles of the model galaxies. The variation of inclination angle
and isophote position angle with radius are more reliable indicators of bar
presence than the surface brightness profiles. The systematic biases in the
central ~ 1 kpc of galaxies are not large. Each effect separately gives
typically a few kms error, but the effects add up. In some cases the error in
circular velocity was a factor of two, but typically we get about 20 percent.
The result is the false inference that the density profile of the halo flattens
in the central parts. Our observations of real galaxies show that for a large
fraction of galaxies the velocity of gas rotation (as measured by emission
lines) is very close to the rotation of stellar component (as measured by
absorption lines). This implies that the systematic effects discussed in this
paper are also applicable both for the stars and emission-line gas.Comment: ApJ, in press, 30 pages, Latex, 21 .eps figure
The Origin of the Brightest Cluster Galaxies
Most clusters and groups of galaxies contain a giant elliptical galaxy in
their centres which far outshines and outweighs normal ellipticals. The origin
of these brightest cluster galaxies is intimately related to the collapse and
formation of the cluster. Using an N-body simulation of a cluster of galaxies
in a hierarchical cosmological model, we show that galaxy merging naturally
produces a massive, central galaxy with surface brightness and velocity
dispersion profiles similar to observed BCG's. To enhance the resolution of the
simulation, 100 dark halos at are replaced with self-consistent
disk+bulge+halo galaxy models following a Tully-Fisher relation using 100000
particles for the 20 largest galaxies and 10000 particles for the remaining
ones. This technique allows us to analyze the stellar and dark matter
components independently. The central galaxy forms through the merger of
several massive galaxies along a filament early in the cluster's history.
Galactic cannibalism of smaller galaxies through dynamical friction over a
Hubble time only accounts for a small fraction of the accreted mass. The galaxy
is a flattened, triaxial object whose long axis aligns with the primordial
filament and the long axis of the cluster galaxy distribution agreeing with
observed trends for galaxy-cluster alignment.Comment: Revised and accepted in ApJ, 25 pages, 10 figures, online version
available at http://www.cita.utoronto.ca/~dubinski/bcg
Recommended from our members
Meteorological and chemical factors controlling ozone formation in Seoul during MAPS-Seoul 2015
To understand the chemical mechanisms of controlling factors in ozone (O3) formation in early summer in Seoul, a comprehensive study encompassing measurement and modeling was conducted under the Megacity Air Pollution Study-Seoul (MAPS-Seoul) campaign. From May 18 to June 12, 2015, O3 and peroxyacetyl nitrate (PAN) were measured, along with their precursors, including NOx and volatile organic compounds (VOCs), at the Korea Institute of Science and Technology, located in northeast Seoul. VOCs were sampled in a canister twice a day (at 09:30 and 15:00) and analyzed via gas chromatography. The meteorological conditions and chemical regimes of the air masses were clearly distinguished during the study period. In May, NOx concentrations were higher with more pronounced diurnal cycles of precursors and O3 under constant westerly winds. By contrast, stagnant conditions developed in June, which reduced the inflow of primary emissions from the downtown area but increased the influence from the neighboring forest under high temperatures. As a result, the ratio of O3 to odd oxygen was higher in June, indicating a less efficient removal of O3 by NOx. In the same context, the air mass was chemically more aged with a higher NO2/NOx ratio and enhanced OH reactivity of oxygenated and biogenic VOCs in June. The overall measurement results suggest that O3 formation is slightly more sensitive to VOCs than to NOx in Seoul during this season, when O3 concentrations are the highest of the year
Einstein Cluster Alignments Revisited
We have examined whether the major axes of rich galaxy clusters tend to point
toward their nearest neighboring cluster. We have used the data of Ulmer,
McMillan, and Kowalski, who used position angles based on X-ray morphology. We
also studied a subset of this sample with updated positions and distances from
the MX Northern Abell Cluster Survey (for rich clusters () with well
known redshifts). A Kolmogorov-Smirnov (KS) test showed no significant signal
for nonrandom angles on any scale Mpc. However, refining the
null hypothesis with the Wilcoxon rank-sum test, we found a high confidence
signal for alignment. Confidence levels increase to a high of 99.997% as only
near neighbors which are very close are considered. We conclude there is a
strong alignment signal in the data, consistent with gravitational instability
acting on Gaussian perturbations.Comment: Minor revisions. To be published in Ap
Trend in ice moistening the stratosphere â constraints from isotope data of water and methane
Water plays a major role in the chemistry and radiative budget of the stratosphere. Air enters the stratosphere predominantly in the tropics, where the very low temperatures around the tropopause constrain water vapour mixing ratios to a few parts per million. Observations of stratospheric water vapour show a large positive long-term trend, which can not be explained by change in tropopause temperatures. Trends in the partitioning between vapour and ice of water entering the stratosphere have been suggested to resolve this conundrum. We present measurements of stratospheric H_(2)O, HDO, CH_4 and CH_(3)D in the period 1991â2007 to evaluate this hypothesis. Because of fractionation processes during phase changes, the hydrogen isotopic composition of H_(2)O is a sensitive indicator of changes in the partitioning of vapour and ice. We find that the seasonal variations of H_(2)O are mirrored in the variation of the ratio of HDO to H_(2)O with a slope of the correlation consistent with water entering the stratosphere mainly as vapour. The variability in the fractionation over the entire observation period is well explained by variations in H_(2)O. The isotopic data allow concluding that the trend in ice arising from particulate water is no more than (0.01±0.13) ppmv/decade in the observation period. Our observations suggest that between 1991 and 2007 the contribution from changes in particulate water transported through the tropopause plays only a minor role in altering in the amount of water entering the stratosphere
Two-Temperature Intracluster Medium in Merging Clusters of Galaxies
We investigate the evolution of intracluster medium during a cluster merger,
explicitly considering the relaxation process between the ions and electrons by
N-body and hydrodynamical simulations. When two subclusters collide each other,
a bow shock is formed between the centers of two substructures and propagate in
both directions along the collision axis. The shock primarily heats the ions
because the kinetic energy of an ion entering the shock is larger than that of
an electron by the ratio of masses. In the post-shock region the energy is
transported from the ions to electrons via Coulomb coupling. However, since the
energy exchange timescale depends both on the gas density and temperature,
distribution of electron temperature becomes more complex than that of the
plasma mean temperature, especially in the expanding phase. After the collision
of two subclusters, gas outflow occurs not only along the collision axis but
also in its perpendicular direction. The gas which is originally located in the
central part of the subclusters moves both in the parallel and perpendicular
directions. Since the equilibrium timescale of the gas along these directions
is relatively short, temperature difference between ions and electrons is
larger in the directions tilted by the angles of with respect to
the collision axis. The electron temperature could be significantly lower that
the plasma mean temperature by at most. The significance of our
results in the interpretation of X-ray observations is briefly discussed.Comment: 20 pages, 11 figures, Accepted for publication in Ap
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