616 research outputs found
Evidence for Quasar Activity Triggered by Galaxy Mergers in HST Observations of Dust-reddened Quasars
We present Hubble ACS images of thirteen dust reddened Type-1 quasars
selected from the FIRST/2MASS Red Quasar Survey. These quasars have high
intrinsic luminosities after correction for dust obscuration (-23.5 > M_B >
-26.2 from K-magnitude). The images show strong evidence of recent or ongoing
interaction in eleven of the thirteen cases, even before the quasar nucleus is
subtracted. None of the host galaxies are well fit by a simple elliptical
profile. The fraction of quasars showing interaction is significantly higher
than the 30% seen in samples of host galaxies of normal, unobscured quasars.
There is a weak correlation between the amount of dust reddening and the
magnitude of interaction in the host galaxy, measured using the Gini
coefficient and the Concentration index. Although few host galaxy studies of
normal quasars are matched to ours in intrinsic quasar luminosity, no evidence
has been found for a strong dependence of merger activity on host luminosity in
samples of the host galaxies of normal quasars. We thus believe that the high
merger fraction in our sample is related to their obscured nature, with a
significant amount of reddening occurring in the host galaxy. The red quasar
phenomenon seems to have an evolutionary explanation, with the young quasar
spending the early part of its lifetime enshrouded in an interacting galaxy.
This might be further indication of a link between AGN and starburst galaxies.Comment: 18 pages, 6 low resolution figures, accepted for publication in Ap
A Model To Measure Supination And Pronation Of The Foot Over Different Levels Of Physiological Stress Using An In-Shoe Force Monitoring System
The purpose of this study was to create a diagnostic model of supination and pronation of the foot using vertical ground reaction forces. A size adjustable capacitive transducer retaining 960 individual pressure cells was used to assess orthopaedic parameters of gait cyale timing and vertical ground reaction forces.
A pilot sample of five males were used for this model. The subjects were exposed to six experimental conditions. These being; a) a walk, b) jog, c) walk on treadmill, and d) three levels of perceived exertion (mild, moderate and hard running). Perceived exertion was measured with the Borg (RPE) scale. All subjects were measured in the same brand of athletic shoe to control for intershoe differences. The ptonation/supination model was determined by medial/lateral force and timing measurements of the calcaneus, forefoot prominence, arch, the first and fifth metatarsal heads and the toe off (end of gait cycle). Results indicated bi-Iateral differences in the medial and lateral force measurements of the calcaneus. Timing in this area was slightly different.
Medial to lateral timing pronation was evidenced in the treadmill walking and moderate running condition. As well, the loading of the first and fifth metatarsal heads as a percentage of the gait cycle did not change over the running conditions. The preliminary results of five subjects provides for limited support of a vertical ground reaction model to assess pronation and supination. Further research with techniques such as high speed photography will allow for clarification of this model
On the Proof of Dark Matter, the Law of Gravity and the Mass of Neutrinos
We develop a new method to predict the density associated with weak lensing
maps of (un)relaxed clusters in a range of theories interpolating between GR
and MOND (General Relativity and Modified Newtonian Dynamics). We apply it to
fit the lensing map of the bullet merging cluster 1E0657-56, in order to
constrain more robustly the nature and amount of collisionless matter in
clusters {\it beyond} the usual assumption of spherical equilibrium
(Pointecouteau & Silk 2005) and the validity of GR on cluster scales (Clowe et
al. 2006). Strengthening the proposal of previous authors we show that the
bullet cluster is dominated by a collisionless -- most probably non-baryonic --
component in GR as well as in MOND, a result consistent with the dynamics of
many X-ray clusters. Our findings add to the number of known pathologies for a
purely baryonic MOND, including its inability to fit the latest data from the
Wilkinson Microwave Anisotropy Probe. A plausible resolution of all these
issues and standard issues of Cold Dark Matter with galaxy rotation curves is
the "marriage" of MOND with ordinary hot neutrinos of 2eV. This prediction is
just within the GR-independent maximum of neutrino mass from current
-decay experiments, and is falsifiable by the Karlsruhe Tritium Neutrino
(KATRIN) experiment by 2009. Issues of consistency with strong lensing arcs and
the large relative velocity of the two clusters comprising the bullet cluster
are also addressed.Comment: 4 pages, 1 figure, accepted for publication in ApJL. Added a simple
model of the bullet cluster's high velocity in TeVeS, and discussions of
sterile neutrinos and of non-uniqueness of the lensing deprojectio
A Note on Gravitational Brownian Motion
Chandrasekhar's theory of stellar encounters predicts a dependence of the
Brownian motion of a massive particle on the velocity distribution of the
perturbing stars. One consequence is that the expectation value of the massive
object's kinetic energy can be different from that of the perturbers. This
effect is shown to be modest however, and substantially smaller than claimed in
a recent study based on a more approximate treatment of the encounter
equations.Comment: 7 pages, 3 figure
DYNAMIC MEASUREMENT OF FORCE WITHIN THE SHOE DURING CONDITIONS OF PERCEIVED EXERTION
INTRODUCTION
Athletics and recreation are becoming a very large component of modem day society. Now, more than ever, more people are becoming very active and involved in recreational pursuits that include aerobics and running/jogging. The resultant increase in activity has lead to a noted increase in injuries (Nigg, 1985; Mckenzie et al., 1985). James et al (cited in Cavanagh, 1990), in a study of 180 injured runners identified three categories of running injury problem areas. These areas being:
a) training errors;
b)anatomic factors (biomechanics); and
c) shoes and surfaces.
Two thirds of these injuries were accounted for by training errors (increased mileage or increased intensity). Training errors concerning sudden increases in mileage or intensity tend to subject the body to new or greater than expected physiological stressors. Voloshin and Wosk (1981), have investigated the relationship between heel strike shock wave transmission and joint degeneration in walking subjects. Taken one step further, the implications of damage to the muscle-skeletal system by running is noted by Cavanagh (1990). This combined with the Nigg et al. (1983) data that suggests a relationship between the hardness of the athletic (running) surface and the incidence of injury has serious implications for the recreational runner/athlete. Clarke et al. (1985) have highlighted the possible injurious force involved in tibial accelerations that are the result of the runner taking longer strides. Since most people retain a constant stride frequency, as velocity increases, the athlete tends to increase their stride length. The resultant increase in stride length increases impact forces at the joint. This combined with the increase of ground reaction forces with higher running speeds (Munro et al., 1987) identifies high impact forces as a major factor to be considered when investigating the causal nature of running injuries. Research into the type of shoe and ground reaction forces has been equivocal. Nigg and Bahlsen (1988) have stated that shoes with the hardest mid soles elicit the lowest maximal vertical forces. Conversely other research has indicated that shoe hardness is related to higher (vertical) loading rates. Listed above are a number of factors involved in the prediction of running injuries.
Further research is needed to discover the causative factors involved in etiology of sport medicine running injuries. An analysis of running shoes, in particular the vertical ground reaction forces, may account for dynamic patterns of gait. Running at different levels of perceived exertion may elicit clues as to the biomechanics patterns that may be injurious to runners. For example, the gait of a runner at the beginning of the run may be markedly different from the gait at the end of the run. Variables such as intensity and distance will greatly affect the athlete's form as they become more tired.
Thus, the number of running injuries (2/3 of Clarke's population) as a result of improper training may be the function of bad form (gait mechanics). An analysis under different levels of exertion will identify patterns of pressure with the foot that may have implications for the construction and design of athletic footwear as well as training methods for runners.
This study was an attempt to understand the dynamic of in-shoe vertical ground reaction forces within the shoe under differing levels of perceived exertion. Research in the area of running shoe forces may lead to the development of a better product that will decrease the rate and type of running injuries
Supermassive Black Holes and the Evolution of Galaxies
Black holes, an extreme consequence of the mathematics of General Relativity,
have long been suspected of being the prime movers of quasars, which emit more
energy than any other objects in the Universe. Recent evidence indicates that
supermassive black holes, which are probably quasar remnants, reside at the
centers of most galaxies. As our knowledge of the demographics of these relics
of a violent earlier Universe improve, we see tantalizing clues that they
participated intimately in the formation of galaxies and have strongly
influenced their present-day structure.Comment: 20 pages, - This is a near-duplicate of the paper in Nature 395, A14,
1998 (Oct 1
Accretion of a satellite onto a spherical galaxy. II. Binary evolution and orbital decay
We study the dynamical evolution of a satellite orbiting outside of a
companion spherical galaxy. The satellite is subject to a back-reaction force
resulting from the density fluctuations excited in the primary stellar system.
We evaluate this force using the linear response theory developed in Colpi and
Pallavicini (1997). The force is computed in the reference frame comoving with
the primary galaxy and is expanded in multipoles. To capture the relevant
features of the physical process determining the evolution of the detached
binary, we introduce in the Hamiltonian the harmonic potential as interaction
potential among stars. The dynamics of the satellite is computed
self-consistently. We determine the conditions for tidal capture of a satellite
from an asymptotic free state. If the binary comes to existence as a bound
pair, stability against orbital decay is lost near resonance. The time scale of
binary coalescence is computed as a function of the eccentricity and mass
ratio. In a comparison with Weinberg's perturbative technique we demonstrate
that pinning the center of mass of the galaxy would induce a much larger torque
on the satellite.Comment: 13 pages, Tex,+ 10 .ps figures Submitted to The Astrophysical Journa
Constraints on the acceleration of the solar system from high-precision timing
Many astronomers have speculated that the solar system contains undiscovered
massive planets or a distant stellar companion. The acceleration of the solar
system barycenter can constrain the mass and position of the putative
companion. In this paper we use the most recent timing data on accurate
astronomical clocks (millisecond pulsars, pulsars in binary systems and
pulsating white dwarfs) to constrain this acceleration. No evidence for
non-zero acceleration has been found; the typical sensitivity achieved by our
method is a/c=a few times 10^{-19} s^{-1}, comparable to the acceleration due
to a Jupiter-mass planet at 200 AU. The acceleration method is limited by the
uncertainties in the distances and by the timing precision for pulsars in
binary systems, and by the intrinsic distribution of the period derivatives for
millisecond pulsars. Timing data provide stronger constraints than residuals in
the motions of comets or planets if the distance to the companion exceeds a few
hundred AU. The acceleration method is also more sensitive to the presence of a
distant companion (> 300-400 AU) than existing optical and infrared surveys. We
outline the differences between the effects of the peculiar acceleration of the
solar system and the background of gravitational waves on high-precision
timing.Comment: 28 pages including 7 figures; submitted to AJ on April 22, 200
Anomalously Weak Dynamical Friction in Halos
A bar rotating in a pressure-supported halo generally loses angular momentum
and slows down due to dynamical friction. Valenzuela & Klypin report a
counter-example of a bar that rotates in a dense halo with little friction for
several Gyr, and argue that their result invalidates the claim by Debattista &
Sellwood that fast bars in real galaxies require a low halo density. We show
that it is possible for friction to cease for a while should the pattern speed
of the bar fluctuate upward. The reduced friction is due to an anomalous
gradient in the phase-space density of particles at the principal resonance
created by the earlier evolution. The result obtained by Valenzuela & Klypin is
probably an artifact of their adaptive mesh refinement method, but anyway could
not persist in a real galaxy. The conclusion by Debattista & Sellwood still
stands.Comment: To appear in "Island Universes - Structure and Evolution of Disk
Galaxies" ed. R. S. de Jong, 8 pages, 4 figures, .cls and .sty files include
Hybrid nature of 0846+51W1: a BL Lac object with a narrow line Seyfert 1 nucleus
We have found a NLS1 nucleus in the extensively studied eruptive BL Lac,
0846+51W1, out of a large sample of NLS1 compiled from the spectroscopic
dataset of SDSS DR1. Its optical spectrum can be well decomposed into three
components, a power law component from the relativistic jet, a stellar
component from the host galaxy, and a component from a typical NLS1 nucleus.
The emission line properties of 0846+51W1, FWHM(Hbeta) ~ 1710 km s^-1 and
[OIII]5007/Hbeta ~ 0.32 when it was in faint state, fulfil the conventional
definition of NLS1. Strong FeII emission is detected in the SDSS spectrum,
which is also typical of NLS1s. We try to estimate its central black hole mass
using various techniques and find that 0846+51W1 is very likely emitting at a
few times 10% L_Edd. We speculate that Seyfert-like nuclei, including NLS1s,
might be concealed in a significant fraction of BL Lacs but have not been
sufficiently explored due to the fact that, by definition, the optical-UV
continuum of such kind of objects are often overwhelmed by the synchrotron
emission.Comment: ChJAA accepte
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