5,654 research outputs found
Maximum-Likelihood Comparisons of Tully-Fisher and Redshift Data: Constraints on Omega and Biasing
We compare Tully-Fisher (TF) data for 838 galaxies within cz=3000 km/sec from
the Mark III catalog to the peculiar velocity and density fields predicted from
the 1.2 Jy IRAS redshift survey. Our goal is to test the relation between the
galaxy density and velocity fields predicted by gravitational instability
theory and linear biasing, and thereby to estimate where is the linear bias parameter for IRAS galaxies.
Adopting the IRAS velocity and density fields as a prior model, we maximize the
likelihood of the raw TF observables, taking into account the full range of
selection effects and properly treating triple-valued zones in the
redshift-distance relation. Extensive tests with realistic simulated galaxy
catalogs demonstrate that the method produces unbiased estimates of
and its error. When we apply the method to the real data, we model the presence
of a small but significant velocity quadrupole residual (~3.3% of Hubble flow),
which we argue is due to density fluctuations incompletely sampled by IRAS. The
method then yields a maximum likelihood estimate
(1-sigma error). We discuss the constraints on and biasing that follow
if we assume a COBE-normalized CDM power spectrum. Our model also yields the
1-D noise noise in the velocity field, including IRAS prediction errors, which
we find to be be 125 +/- 20 km/sec.Comment: 53 pages, 20 encapsulated figures, two tables. Submitted to the
Astrophysical Journal. Also available at http://astro.stanford.edu/jeff
Slowing heavy, ground-state molecules using an alternating gradient decelerator
Cold supersonic beams of molecules can be slowed down using a switched
sequence of electrostatic field gradients. The energy to be removed is
proportional to the mass of the molecules. Here we report deceleration of YbF,
which is 7 times heavier than any molecule previously decelerated. We use an
alternating gradient structure to decelerate and focus the molecules in their
ground state. We show that the decelerator exhibits the axial and transverse
stability required to bring these molecules to rest. Our work significantly
extends the range of molecules amenable to this powerful method of cooling and
trapping.Comment: 4 pages, 5 figure
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
The CDM growth rate of structure revisited
We re-examine the growth index of the concordance cosmology in the
light of the latest 6dF and {\em WiggleZ} data. In particular, we investigate
five different models for the growth index , by comparing their
cosmological evolution using observational data of the growth rate of structure
formation at different redshifts. Performing a joint likelihood analysis of the
recent supernovae type Ia data, the Cosmic Microwave Background shift
parameter, Baryonic Acoustic Oscillations and the growth rate data, we
determine the free parameters of the parametrizations and we
statistically quantify their ability to represent the observations. We find
that the addition of the 6dF and {\em WiggleZ} growth data in the likelihood
analysis improves significantly the statistical results. As an example,
considering a constant growth index we find and
.Comment: 8 pages, 5 figures, Accepted for publication by International J. of
Modern Physics D (IJMPD). arXiv admin note: substantial text overlap with
arXiv:1203.672
The Interstellar Rubidium Isotope Ratio toward Rho Ophiuchi A
The isotope ratio, 85Rb/87Rb, places constraints on models of the
nucleosynthesis of heavy elements, but there is no precise determination of the
ratio for material beyond the Solar System. We report the first measurement of
the interstellar Rb isotope ratio. Our measurement of the Rb I line at 7800 A
for the diffuse gas toward rho Oph A yields a value of 1.21 +/- 0.30 (1-sigma)
that differs significantly from the meteoritic value of 2.59. The Rb/K
elemental abundance ratio for the cloud also is lower than that seen in
meteorites. Comparison of the 85Rb/K and 87Rb/K ratios with meteoritic values
indicates that the interstellar 85Rb abundance in this direction is lower than
the Solar System abundance. We attribute the lower abundance to a reduced
contribution from the r-process. Interstellar abundances for Kr, Cd, and Sn are
consistent with much less r-process synthesis for the solar neighborhood
compared to the amount inferred for the Solar System.Comment: 12 pages with 2 figures and 1 table; will appear in ApJ Letter
Superparaelectric phase in the ensemble of non-interacting ferroelectric nanoparticles
For the first time we predict the conditions of superparaelectric phase
appearance in the ensemble of non-interacting spherical ferroelectric
nanoparticles. The superparaelectricity in nanoparticle was defined by analogy
with superparamagnetism, obtained earlier in small nanoparticles made of
paramagnetic material. Calculations of correlation radius, energetic barriers
of polarization reorientation and polarization response to external electric
field, were performed within Landau-Ginzburg phenomenological approach for
perovskites Pb(Zr,Ti)O3, BiFeO3 and uniaxial ferroelectrics rochelle salt and
triglycine sulfate.Comment: 28 pages, 7 figures, 3 Appendices, to be submitted to Phys. Rev.
First high-resolution look at the quiet Sun with ALMA at 3 mm
We present an overview of high resolution quiet Sun observations, from disk
center to the limb, obtained with the Atacama Large mm and sub-mm Array (ALMA)
at 3 mm. Seven quiet Sun regions were observed with resolution of up to 2.5" by
4.5". We produced both average and snapshot images by self-calibrating the ALMA
visibilities and combining the interferometric images with full disk solar
images. The images show well the chromospheric network, which, based on the
unique segregation method we used, is brighter than the average over the fields
of view of the observed regions by K while the intranetwork is less
bright by K, with a slight decrease of the network/intranetwork
contrast toward the limb. At 3 mm the network is very similar to the 1600 \AA\
images, with somewhat larger size. We detected for the first time spicular
structures, rising up to 15" above the limb with a width down to the image
resolution and brightness temperature of 1800 K above the local
background. No trace of spicules, either in emission or absorption, was found
on the disk. Our results highlight ALMA's potential for the study of the quiet
chromosphere.Comment: Astronomy and Astrophysics (Letters), in pres
Epidemiology of Cigarette and Smokeless Tobacco Use among South Asian Immigrants in the Northeastern United States
As the most preventable cause of death in the world today, understanding tobacco use among one of the fastest growing ethnic/racial groups is warranted. We explore cigarette and smokeless tobacco (SLT) use among South Asians in NJ and the Northeast using the Tobacco Use Supplement to the Current Population Survey. Overall, tobacco use rates among South Asians were similar or lower than the population. However, in NJ, South Asian males had the highest SLT rate (2.7%) and in the Northeast, White (AOR = 5.8, 95% CI = 3.7–9.4) and South Asian males (AOR = 4.0, 95% CI = 1.5–10.6) had significantly higher odds of current SLT use relative to non-White males. Tobacco use among South Asians was not homogeneous; Pakistanis are overrepresented among cigarette smokers while Indians are overrepresented among SLT users. Given the differential tobacco use among and within South Asian, disaggregating data to understand tobacco use behaviors is necessary to develop effective interventions for tobacco cessation
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