121 research outputs found
Parallax-Shifted Microlensing Events from Ground-Based Observations of the Galactic Bulge
The parallax effect in ground-based microlensing (ML) observations consists
of a distortion to the standard ML light curve arising from the Earth's orbital
motion. In most cases, the resolution in current ML surveys is not accurate
enough to observe this effect, but parallax could conceivably be detected with
frequent followup observations of ML events in progress. We calculate the
expected fraction of events where parallax distortions will be detected by such
observations, adopting Galactic models consistent with the observed ML
timescale () distributions. We study the dependence of the rates for
parallax-shifted events on the sampling frequency and on the photometric
precision. For example, we find that for hourly observations with typical
photometric errors of 0.01 mag, 6\% of events where the lens is in the bulge,
and 31\% of events where the lens is in the disk, (or \% of events
overall) will give rise to a measurable parallax shift at the 95\% confidence
level. These fractions may be increased by improved photometric accuracy and
increased sampling frequency. Parallax measurements yield the reduced
transverse speed, , which gives both the relative transverse speed
and lens mass as functions of distance. We give examples of the accuracies with
which may be measured in typical parallax events. Using only the 3
standard ML parameters to fit ML light curves which may be shape-distorted by
parallax or blending, can result in incorrect inferred values for these
quantities. We find that the inferred timescales from such fits tend to shift
the event duration distribution by \% towards shorter for
events with disk lenses, but do not affect bulge lenses. In both cases, the
impact-parameter distribution is depressed slightly at the low and high ends.Comment: 25 pages, 7 Postscript figure
Vortices in vibrated granular rods
We report the experimental observation of novel vortex patterns in vertically
vibrated granular rods. Above a critical packing fraction, moving ordered
domains of nearly vertical rods spontaneously form and coexist with horizontal
rods. The domains of vertical rods coarsen in time to form large vortices. We
investigate the conditions under which the vortices occur by varying the number
of rods, vibration amplitude and frequency. The size of the vortices increases
with the number of rods. We characterize the growth of the ordered domains by
measuring the area fraction of the ordered regions as a function of time. A
{\em void filling} model is presented to describe the nucleation and growth of
the vertical domains. We track the ends of the vertical rods and obtain the
velocity fields of the vortices. The rotation speed of the rods is observed to
depend on the vibration velocity of the container and on the packing. To
investigate the impact of the direction of driving on the observed phenomena,
we performed experiments with the container vibrated horizontally. Although
vertical domains form, vortices are not observed. We therefore argue that the
motion is generated due to the interaction of the inclination of the rods with
the bottom of a vertically vibrated container. We also perform simple
experiments with a single row of rods in an annulus. These experiments directly
demonstrate that the rod motion is generated when the rods are inclined from
the vertical, and is always in the direction of the inclination.Comment: 6 pages, 10 figure, 2 movies at http://physics.clarku.edu/vortex uses
revtex
Cosmological Studies with Radio Galaxies and Supernovae
Physical sizes of extended radio galaxies can be employed as a cosmological
"standard ruler", using a previously developed method. Eleven new radio
galaxies are added to our previous sample of nineteen sources, forming a sample
of thirty objects with redshifts between 0 and 1.8. This sample of radio
galaxies are used to obtain the best fit cosmological parameters in a
quintessence model in a spatially flat universe, a cosmological constant model
that allows for non-zero space curvature, and a rolling scalar field model in a
spatially flat universe. Results obtained with radio galaxies are compared with
those obtained with different supernova samples, and with combined radio galaxy
and supernova samples. Results obtained with different samples are consistent,
suggesting that neither method is seriously affected by systematic errors. Best
fit radio galaxy and supernovae model parameters determined in the different
cosmological models are nearly identical, and are used to determine
dimensionless coordinate distances to supernovae and radio galaxies, and
distance moduli to the radio galaxies. The distance moduli to the radio
galaxies can be combined with supernovae samples to increase the number of
sources, particularly high-redshift sources, in the samples. The constraints
obtained here with the combined radio galaxy plus supernovae data set in the
rolling scalar field model are quite strong. The best fit parameter values
suggest a value of omega is less than about 0.35, and the model parameter alpha
is close to zero; that is, a cosmological constant provides a good description
of the data. We also obtain new constraints on the physics of engines that
power the large-scale radio emission.Comment: 32 pages. Accepted for publication in the Astrophysical Journa
An Efficient Search for Gravitationally-Lensed Radio Lobes
We performed an automated comparison of the FIRST radio survey with the APM
optical catalog to find radio lobes with optical counterparts. Based on an
initial survey covering ~3000 square degrees, we selected a sample of 33 lens
candidates for VLA confirmation. VLA and optical observations of these
candidates yielded two lens systems, one a new discovery (J0816+5003), and one
of which was previously known (J1549+3047). Two other candidates have radio
lobes with galaxies superposed, but lack evidence of multiple imaging. One of
our targets (J0958+2947) is a projected close pair of quasars (8'' separation
at redshifts 2.064 and 2.744). Our search method is highly efficient, with >5%
of our observing targets being lensed, compared to the usual success rate of
<1%. Using the whole FIRST survey, we expect to find 5--10 lenses in short
order using this approach, and the sample could increase to hundreds of lensed
lobes in the Northern sky, using deeper optical surveys and planned upgrades to
the VLA. Such a sample would be a powerful probe of galaxy structure and
evolution.Comment: Submitted to ApJ, 2000.07.28, revised 2000.09.12. Minor revisions and
new observations of best example. Eleven eps figures. Uses AASTeX/LaTeX,
psfig2.te
Minimal Angular Size of Distant Sources in Open, CDM, and Scalar Field Cosmologies
We propose a simple method for determining the redshift at which the
angular size of an extragalactic source with fixed proper diameter takes its
minimal value. A closed analytical expression, which is quite convenient for
numerical evaluation is derived. The method is exemplified with the following
FRW type expanding universes: the open matter dominated models
(), a critical density model with cosmological constant
(), and the class of scalar field cosmologies proposed
by Ratra and Peebles. The influence of systematic evolutionary effects is
briefly discussed.Comment: 8 pages, 1 postscript figures, uses revtex macro
The Millennium Galaxy Catalogue: the space density and surface brightness distribution(s) of galaxies
We recover the joint and individual space density and surface brightness
distribution(s) of galaxies from the Millennium Galaxy Catalogue. The MGC is a
local survey spanning 30.9 sq deg and probing approximately one--two mag/sq
arcsec deeper than either the Two-Degree Field Galaxy Redshift Survey (2dFGRS)
or the Sloan Digital Sky Survey (SDSS). The MGC contains 10,095 galaxies to
B_mgc < 20 mag with 96 per cent spectroscopic completeness. We implement a
joint luminosity-surface brightness step-wise maximum likelihood method to
recover the bivariate brightness distribution (BBD) inclusive of most selection
effects. Integrating the BBD over surface brightness we recover the following
Schechter function parameters: phi* = (0.0177 +/- 0.0015) h^3 Mpc^{-3}, M_{B}*
- 5 log h = (-19.60 +/- 0.04) mag and alpha =-1.13 +/- 0.02. Compared to the
2dFGRS (Norberg et al 2002) we find a consistent M* value but a slightly
flatter faint-end slope and a higher normalisation, resulting in a final
luminosity density j_{b_J} = (1.99 +/- 0.17) x 10^8 h L_{odot} Mpc^{-3}. The
MGC surface brightness distribution is a well bounded Gaussian at the M* point
with phi* = (3.5 +/- 0.1) x 10^{-2} h^3 Mpc^{-3}, mu^{e*} = (21.90 +/- 0.01)
mag/sq arcsec and sigma_{ln R_e} = 0.35 +/- 0.01. The characteristic surface
brightness for luminous systems is invariant to M_{B} - 5 log h ~ -19 mag
faintwards of which it moves to lower surface brightness. Higher resolution
(FWHM 26 mag/sq arcsec in the B-band)
observations of the local universe are now essential to probe to lower
luminosity and lower surface brightness levels. [abridged]Comment: Accepted for publication in MNRAS, 26 pages with 21 figures (some
degraded). A full pdf version, along with MGC data release, is available from
the MGC website at, http://www.eso.org/~jliske/mg
Simulating the Formation of the Local Galaxy Population
We simulate the formation and evolution of the local galaxy population
starting from initial conditions with a smoothed linear density field which
matches that derived from the IRAS 1.2 Jy galaxy survey. Our simulations track
the formation and evolution of all dark matter haloes more massive than 10e+11
solar masses out to a distance of 8000 km/s from the Milky Way. We implement
prescriptions similar to those of Kauffmann et al. (1999) to follow the
assembly and evolution of the galaxies within these haloes. We focus on two
variants of the CDM cosmology: an LCDM and a tCDM model. Galaxy formation in
each is adjusted to reproduce the I-band Tully-Fisher relation of Giovanelli et
al. (1997). We compare the present-day luminosity functions, colours,
morphology and spatial distribution of our simulated galaxies with those of the
real local population, in particular with the Updated Zwicky Catalog, with the
IRAS PSCz redshift survey, and with individual local clusters such as Coma,
Virgo and Perseus. We also use the simulations to study the clustering bias
between the dark matter and galaxies of differing type. Although some
significant discrepancies remain, our simulations recover the observed
intrinsic properties and the observed spatial distribution of local galaxies
reasonably well. They can thus be used to calibrate methods which use the
observed local galaxy population to estimate the cosmic density parameter or to
draw conclusions about the mechanisms of galaxy formation. To facilitate such
work, we publically release our z=0 galaxy catalogues, together with the
underlying mass distribution.Comment: 25 pages, 20 figures, submitted to MNRAS. High resolution copies of
figures 1 and 3, halo and galaxy catalogues can be found at
http://www.mpa-garching.mpg.de/NumCos/CR/index.htm
Some Observational Consequences of Brane World Cosmologies
The presence of dark energy in the Universe is inferred directly and
indirectly from a large body of observational evidence. The simplest and most
theoretically appealing possibility is the vacuum energy density (cosmological
constant). However, although in agreement with current observations, such a
possibility exacerbates the well known cosmological constant problem, requiring
a natural explanation for its small, but nonzero, value. In this paper we focus
our attention on another dark energy candidate, one arising from gravitational
\emph{leakage} into extra dimensions. We investigate observational constraints
from current measurements of angular size of high- compact radio-sources on
accelerated models based on this large scale modification of gravity. The
predicted age of the Universe in the context of these models is briefly
discussed. We argue that future observations will enable a more accurate test
of these cosmologies and, possibly, show that such models constitute a viable
possibility for the dark energy problem.Comment: 6 pages, 4 figures, to appear in Phys. Rev. D (minor revisions
Dynamical properties of a large young disk galaxy at z=2.03
Context. The study of high redshift Tully-Fisher relations (TFRs) is limited
by the use of long slit spectrographs, rest frame B band and star formation
selected galaxies. Aims. We try to circumvent these issues by using integral
field spectroscopy (SINFONI), by studying the rest frame K band and stellar
mass TFR, and by selecting targets without a bias to strongly star forming
galaxies. In this paper, we demonstrate our methods on our best case. This
galaxy, F257, at z=2.03, was selecte from a sample of candidate high redshift
large disk galaxies in the Hubble Deep Field South that were selected with
photometric and morphological criteria. Methods. We used SINFONI at the VLT to
obtain an integral field spectrum of the Halpha line and hence a velocity field
and rotation curve. We also use UBVIJHK+IRAC band photometry to determine a
stellar photometric mass. Results. We find that F257 is indistinguishable from
local late type galaxies in many respects: it has a regular velocity field,
increasing velocity disperion towards its center, its rotation curve flattens
at 1-2 disk scale lengths, it has the same specific angular momentum as local
disks, its properties are consistent with the local K band TFR. Although mainly
rotationally supported, its gas component is dynamically heated with respect to
local galaxies (V/sigma_z ~ 4) and it is offset from the local stellar mass TFR
at the 2sigma level. But, this offset depends on the SED modeling parameters.
In particular, for a 2-component star formation history (SFH), F257 is in
agreement with the local stellar mass TFR. F257 is then a nearly (~75%) maximum
disk. The dynamical properties of F257 are more like those of local galaxies
than those of any other galaxy at similar redshift observed to date. However,
the gas-to-stellar mass ratio is unusally large: 2.5.Comment: Accepted for publication in A&A, 15 pages, 14 figure
The effects of a three-week use of lumbosacral orthoses on trunk muscle activity and on the muscular response to trunk perturbations
<p>Abstract</p> <p>Background</p> <p>The effects of lumbosacral orthoses (LSOs) on neuromuscular control of the trunk are not known. There is a concern that wearing LSOs for a long period may adversely alter muscle control, making individuals more susceptible to injury if they discontinue wearing the LSOs. The purpose of this study was to document neuromuscular changes in healthy subjects during a 3-week period while they regularly wore a LSO.</p> <p>Methods</p> <p>Fourteen subjects wore LSOs 3 hrs a day for 3 weeks. Trunk muscle activity prior to and following a quick force release (trunk perturbation) was measured with EMG in 3 sessions on days 0, 7, and 21. A longitudinal, repeated-measures, factorial design was used. Muscle reflex response to trunk perturbations, spine compression force, as well as effective trunk stiffness and damping were dependent variables. The LSO, direction of perturbation, and testing session were the independent variables.</p> <p>Results</p> <p>The LSO significantly (<it>P </it>< 0.001) increased the effective trunk stiffness by 160 Nm/rad (27%) across all directions and testing sessions. The number of antagonist muscles that responded with an onset activity was significantly reduced after 7 days of wearing the LSO, but this difference disappeared on day 21 and is likely not clinically relevant. The average number of agonist muscles switching off following the quick force release was significantly greater with the LSO, compared to without the LSO (<it>P </it>= 0.003).</p> <p>Conclusions</p> <p>The LSO increased trunk stiffness and resulted in a greater number of agonist muscles shutting-off in response to a quick force release. However, these effects did not result in detrimental changes to the neuromuscular function of trunk muscles after 3 weeks of wearing a LSO 3 hours a day by healthy subjects.</p
- âŠ