20,139 research outputs found
Cosmological Parameters from the Comparison of the 2MASS Gravity Field with Peculiar Velocity Surveys
We compare the peculiar velocity field within 65 Mpc predicted from
2MASS photometry and public redshift data to three independent peculiar
velocity surveys based on type Ia supernovae, surface brightness fluctuations
in ellipticals, and Tully-Fisher distances to spirals. The three peculiar
velocity samples are each in good agreement with the predicted velocities and
produce consistent results for \beta_{K}=\Omega\sbr{m}^{0.6}/b_{K}. Taken
together the best fit . We explore the effects of
morphology on the determination of by splitting the 2MASS sample into
E+S0 and S+Irr density fields and find both samples are equally good tracers of
the underlying dark matter distribution, but that early-types are more
clustered by a relative factor b\sbr{E}/b\sbr{S} \sim 1.6. The density
fluctuations of 2MASS galaxies in Mpc spheres in the local volume is
found to be \sigma\sbr{8,K} = 0.9. From this result and our value of
, we find \sigma_8 (\Omega\sbr{m}/0.3)^{0.6} = 0.91\pm0.12. This
is in excellent agreement with results from the IRAS redshift surveys, as well
as other cosmological probes. Combining the 2MASS and IRAS peculiar velocity
results yields \sigma_8 (\Omega\sbr{m}^/0.3)^{0.6} = 0.85\pm0.05.Comment: 11 pages, ApJ accepte
A Test for Large-Scale Systematic Errors in Maps of Galactic Reddening
Accurate maps of Galactic reddening are important for a number of
applications, such as mapping the peculiar velocity field in the nearby
Universe. Of particular concern are systematic errors which vary slowly as a
function of position on the sky, as these would induce spurious bulk flow. We
have compared the reddenings of Burstein & Heiles (BH) and those of Schlegel,
Finkbeiner & Davis (SFD) to independent estimates of the reddening, for
Galactic latitudes |b| > 10. Our primary source of Galactic reddening estimates
comes from comparing the difference between the observed B-V colors of
early-type galaxies, and the predicted B-V color determined from the B-V--Mg_2
relation. We have fitted a dipole to the residuals in order to look for
large-scale systematic deviations. There is marginal evidence for a dipolar
residual in the comparison between the SFD maps and the observed early-type
galaxy reddenings. If this is due to an error in the SFD maps, then it can be
corrected with a small (13%) multiplicative dipole term. We argue, however,
that this difference is more likely to be due to a small (0.01 mag.) systematic
error in the measured B-V colors of the early-type galaxies. This
interpretation is supported by a smaller, independent data set (globular
cluster and RR Lyrae stars), which yields a result inconsistent with the
early-type galaxy residual dipole. BH reddenings are found to have no
significant systematic residuals, apart from the known problem in the region
230 < l < 310, -20 < b < 20.Comment: 8 pages, PASP, in press (Jan 1999
The Peculiar Velocities of Local Type Ia Supernovae and their Impact on Cosmology
We quantify the effect of supernova Type Ia peculiar velocities on the
derivation of cosmological parameters. The published distant and local Ia SNe
used for the Supernova Legacy Survey first-year cosmology report form the
sample for this study. While previous work has assumed that the local SNe are
at rest in the CMB frame (the No Flow assumption), we test this assumption by
applying peculiar velocity corrections to the local SNe using three different
flow models. The models are based on the IRAS PSCz galaxy redshift survey, have
varying beta = Omega_m^0.6/b, and reproduce the Local Group motion in the CMB
frame. These datasets are then fit for w, Omega_m, and Omega_Lambda using
flatness or LambdaCDM and a BAO prior. The chi^2 statistic is used to examine
the effect of the velocity corrections on the quality of the fits. The most
favored model is the beta=0.5 model, which produces a fit significantly better
than the No Flow assumption, consistent with previous peculiar velocity
studies. By comparing the No Flow assumption with the favored models we derive
the largest potential systematic error in w caused by ignoring peculiar
velocities to be Delta w = +0.04. For Omega_Lambda, the potential error is
Delta Omega_Lambda = -0.04 and for Omega_m, the potential error is Delta
Omega_m < +0.01. The favored flow model (beta=0.5) produces the following
cosmological parameters: w = -1.08 (+0.09,-0.08), Omega_m = 0.27 (+0.02,-0.02)
assuming a flat cosmology, and Omega_Lambda = 0.80 (+0.08,-0.07) and Omega_m =
0.27 (+0.02,-0.02) for a w = -1 (LambdaCDM) cosmology.Comment: 4 pages, 2 figures, 1 table, accepted for publication in ApJ Letter
Galaxy Distances in the Nearby Universe: Corrections For Peculiar Motions
By correcting the redshift--dependent distances for peculiar motions through
a number of peculiar velocity field models, we recover the true distances of a
wide, all-sky sample of nearby galaxies (~ 6400 galaxies with velocities
cz<5500 km/s), which is complete up to the blue magnitude B=14 mag. Relying on
catalogs of galaxy groups, we treat ~2700 objects as members of galaxy groups
and the remaining objects as field galaxies.
We model the peculiar velocity field using: i) a cluster dipole
reconstruction scheme; ii) a multi--attractor model fitted to the Mark II and
Mark III catalogs of galaxy peculiar velocities. According to Mark III data the
Great Attractor has a smaller influence on local dynamics than previously
believed, whereas the Perseus-Pisces and Shapley superclusters acquire a
specific dynamical role. Remarkably, the Shapley structure, which is found to
account for nearly half the peculiar motion of the Local Group, is placed by
Mark III data closer to the zone of avoidance with respect to its optical
position.
Our multi--attractor model based on Mark III data favors a cosmological
density parameter Omega ~ 0.5 (irrespective of a biasing factor of order
unity). Differences among distance estimates are less pronounced in the ~ 2000
- 4000 km/s distance range than at larger or smaller distances. In the last
regions these differences have a serious impact on the 3D maps of the galaxy
distribution and on the local galaxy density --- on small scales.Comment: 24 pages including (9 eps figures and 7 tables). Figures 1,2,3,4 are
available only upon request. Accepted by Ap
Partnership in practice
This paper examines human resource management practices adopted in a group of eight case study firms and their tendencies towards versus away from partnership. The analysis is based on data collected during interviews with 124 employees (75 in organisations tending towards partnership and 49 in organisations tending away from partnership) and senior managers, conducted in 1997-1998 for the Job Insecurity and Work Intensification Survey (JIWIS). Drawing on the perspectives of senior managers and employees, we examine the tendency of firms towards and away from partnership in employment relations; and in keeping with the JIWIS methodology (Burchell et.al., 2001) we combine quantitative and qualitative evidence in our analysis. Specifically, we are interested in what partnership looks like in these different contexts, the reasons it is pursued (or not), the degree to which companies have been successful in achieving their partnership objectives (from the perspective of both management and employees), and the conditions that have either facilitated or impeded partnership in relationships with employees
Nearby Optical Galaxies: Selection of the Sample and Identification of Groups
In this paper we describe the Nearby Optical Galaxy (NOG) sample, which is a
complete, distance-limited (6000 km/s) and magnitude-limited
(B14) sample of 7000 optical galaxies. The sample covers 2/3 (8.27
sr) of the sky () and appears to have a good completeness in
redshift (98%). We select the sample on the basis of homogenized corrected
total blue magnitudes in order to minimize systematic effects in galaxy
sampling.
We identify the groups in this sample by means of both the hierarchical and
the percolation {\it friends of friends} methods. The resulting catalogs of
loose groups appear to be similar and are among the largest catalogs of groups
presently available. Most of the NOG galaxies (60%) are found to be
members of galaxy pairs (580 pairs for a total of 15% of objects)
or groups with at least three members (500 groups for a total of
45% of objects). About 40% of galaxies are left ungrouped (field
galaxies). We illustrate the main features of the NOG galaxy distribution.
Compared to previous optical and IRAS galaxy samples, the NOG provides a denser
sampling of the galaxy distribution in the nearby universe. Given its large sky
coverage, the identification of groups, and its high-density sampling, the NOG
is suited for the analysis of the galaxy density field of the nearby universe,
especially on small scales.Comment: 47 pages including 6 figures. Accepted for publication in Ap
OMEGA AND BIASING FROM OPTICAL GALAXIES VERSUS POTENT MASS
The mass density field in the local universe, recovered by the POTENT method
from peculiar velocities of 3000 galaxies, is compared with the density
field of optically-selected galaxies. Both density fields are smoothed with a
Gaussian filter of radius 12 Mpc. Under the assumptions of
gravitational instability and a linear biasing parameter b\sbo between
optical galaxies and mass, we obtain \beta\sbo \equiv \om^{0.6}/b\sbo = 0.74
\pm 0.13. This result is obtained from a regression of POTENT mass density on
optical density after correcting the mass density field for systematic biases
in the velocity data and POTENT method. The error quoted is just the
formal error estimated from the observed scatter in the density--density
scatterplot; it does not include the uncertainty due to cosmic scatter in the
mean density or in the biasing relation. We do not attempt a formal analysis of
the goodness of fit, but the scatter about the fit is consistent with our
estimates of the uncertainties.Comment: Final revised version (minor typos corrected). 13 pages, gzipped tar
file containing LaTeX and figures. The Postscript file is available at
ftp://dust0.dur.ac.uk/pub/mjh/potopt/potopt.ps.Z or (gzipped) at
ftp://xxx.lanl.gov/astro-ph/ps/9501/9501074.ps.gz or via WWW at
http://xxx.lanl.gov/ps/astro-ph/9501074 or as separate LaTeX text and
encapsulated Postscript figures in a compressed tar'd file at
ftp://dust0.dur.ac.uk/pub/mjh/potopt/latex/potopt.tar.
Nonaxisymmetric, multi-region relaxed magnetohydrodynamic equilibrium solutions
We describe a magnetohydrodynamic (MHD) constrained energy functional for
equilibrium calculations that combines the topological constraints of ideal MHD
with elements of Taylor relaxation.
Extremizing states allow for partially chaotic magnetic fields and
non-trivial pressure profiles supported by a discrete set of ideal interfaces
with irrational rotational transforms.
Numerical solutions are computed using the Stepped Pressure Equilibrium Code,
SPEC, and benchmarks and convergence calculations are presented.Comment: Submitted to Plasma Physics and Controlled Fusion for publication
with a cluster of papers associated with workshop: Stability and Nonlinear
Dynamics of Plasmas, October 31, 2009 Atlanta, GA on occasion of 65th
birthday of R.L. Dewar. V2 is revised for referee
Optical properties of ion beam textured metals
Copper, silicon, aluminum, titanium and 316 stainless steel were textured by 1000 eV xenon ions from an 8 cm diameter electron bombardment ion source. Simultaneously sputter-deposited tantalum was used to facilitate the development of the surface microstructure. Scanning electron microscopy of the ion textured surfaces revealed two types of microstructure. Copper, silicon, and aluminum developed a cone structure with an average peak-to-peak distance ranging from 1 micron for silicon to 6 microns for aluminum. Titanium and 316 stainless steel developed a serpentine ridge structure. The average peak-to-peak distance for both of these materials was 0.5 micron. Spectral reflectance was measured using an integrating sphere and a holraum reflectometer. Total reflectance for air mass 0 and 2, solar absorptance and total emittance normalized for a 425 K black body were calculated from the reflectance measurements
The Velocity Field from Type Ia Supernovae Matches the Gravity Field from Galaxy Surveys
We compare the peculiar velocities of nearby SNe Ia with those predicted by
the gravity fields of full sky galaxy catalogs. The method provides a powerful
test of the gravitational instability paradigm and strong constraints on the
density parameter beta = Omega^0.6/b. For 24 SNe Ia within 10,000 km/s we find
the observed SNe Ia peculiar velocities are well modeled by the predictions
derived from the 1.2 Jy IRAS survey and the Optical Redshift Survey (ORS). Our
best is 0.4 from IRAS, and 0.3 from the ORS, with beta>0.7 and
beta<0.15 ruled out at 95% confidence levels from the IRAS comparison.
Bootstrap resampling tests show these results to be robust in the mean and in
its error. The precision of this technique will improve as additional nearby
SNe Ia are discovered and monitored.Comment: 16 pages (LaTex), 3 postscript figure
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