5,691 research outputs found
Large scale structure in the HI Parkes All-Sky Survey: Filling the Voids with HI galaxies?
We estimate the two-point correlation function in redshift space of the
recently compiled HIPASS neutral hydrogen (HI) sources catalogue, which if
modeled as a power law, , the best-fitting
parameters for the HI selected galaxies are found to be Mpc with . Fixing the slope to its universal
value , we obtain Mpc. Comparing the
measured two point correlation function with the predictions of the concordance
cosmological model, we find that at the present epoch the HI selected galaxies
are anti-biased with respect to the underlying matter fluctuation field with
their bias value being . Furthermore, dividing the HI
galaxies into two richness subsamples we find that the low mass HI galaxies
have a very low present bias factor (), while the high mass
HI galaxies trace the underlying matter distribution as the optical galaxies
(). Using our derived present-day HI galaxy bias we estimate
their redshift space distortion parameter, and correct accordingly the
correlation function for peculiar motions. The resulting real-space correlation
length is Mpc and Mpc for the low and high mass HI galaxies, respectively. The
low-mass HI galaxies appear to have the lowest correlation length among all
extragalactic populations studied to-date. Also, we have correlated the
IRAS-PSCz reconstructed density field, smoothed over scales of 5 Mpc,
with the positions of the HI galaxies, to find that indeed the HI galaxies are
typically found in negative overdensity regions (\delta\rho/\rho_{\rm PSCz}
\mincir 0).Comment: 9 pages, 8 figures, MNRAS in pres
Current-induced spin polarization in InGaAs and GaAs epilayers with varying doping densities
The current-induced spin polarization and momentum-dependent spin-orbit field
were measured in InGaAs epilayers with varying indium
concentrations and silicon doping densities. Samples with higher indium
concentrations and carrier concentrations and lower mobilities were found to
have larger electrical spin generation efficiencies. Furthermore,
current-induced spin polarization was detected in GaAs epilayers despite the
absence of measurable spin-orbit fields, indicating that the extrinsic
contributions to the spin polarization mechanism must be considered.
Theoretical calculations based on a model that includes extrinsic contributions
to the spin dephasing and the spin Hall effect, in addition to the intrinsic
Rashba and Dresselhaus spin-orbit coupling, are found to qualitatively agree
with the experimental results.Comment: 16 pages, 8 figure
Anisotropy of the Microwave Sky at 90 GHz: Results from Python II
We report on additional observations of degree scale anisotropy at 90~GHz
from the Amundsen-Scott South Pole Station in Antarctica. Observations during
the first season with the Python instrument yielded a statistically significant
sky signal; in this paper we report the confirmation of that signal with data
taken in the second year, and on results from an interleaving set of fields.Comment: 10 pages, plus 2 figures. Postscript and uufiles versions available
via anonymous ftp at ftp://astro.uchicago.edu/pub/astro/ruhl/pyI
A study of the mechanisms of corrosion inhibition of AA2024-T3 by vanadates using the split cell technique
The mechanisms of corrosion inhibition of AA2024-T3 by vanadates were studied in this work using the split cell technique and polarization curves. The electrochemical behavior of clear solutions containing metavanadates and orange solutions containing decavanadates was clearly distinctive. Injection of metavanadates to the cathode side of the different split cell setups greatly reduced the galvanic current, indicating a potent inhibition of the oxygen reduction kinetics. The galvanic current never exhibited a transient current peak, suggesting that metavanadates inhibit AA2024-T3 corrosion by a mechanism that does not involve electrochemical reduction. Injection of metavanadate to the anode side of the different split cells had no effect on the galvanic current. Injection of orange decavanadate to the cathode side of the AA2024-T3 split cell resulted in a large current peak, associated with the electrochemical reduction of decavanadate. However, decavanadates did not impart significant corrosion protection.This work was partially funded by AFOSR under award F 49620-02-0321, Major J. Gresham, PhD, contract monitor. JK’s travel expenses were paid by NATO under grant PST.CLG.979370
Parameter Estimation from Improved Measurements of the Cosmic Microwave Background from QUaD
We evaluate the contribution of cosmic microwave background (CMB) polarization spectra to cosmological parameter constraints. We produce cosmological parameters using high-quality CMB polarization data from the ground-based QUaD experiment and demonstrate for the majority of parameters that there is significant improvement on the constraints obtained from satellite CMB polarization data. We split a multi-experiment CMB data set into temperature and polarization subsets and show that the best-fit confidence regions for the ΛCDM six-parameter cosmological model are consistent with each other, and that polarization data reduces the confidence regions on all parameters. We provide the best limits on parameters from QUaD EE/BB polarization data and we find best-fit parameters from the multi-experiment CMB data set using the optimal pivot scale of k_p = 0.013 Mpc^(–1) to be {h^2Ω_c, h^2Ω_b, H_0, A_s, n_s, τ} = {0.113, 0.0224, 70.6, 2.29 × 10^(–9), 0.960, 0.086}
A massive, distant proto-cluster at z=2.47 caught in a phase of rapid formation?
Numerical simulations of cosmological structure formation show that the
Universe's most massive clusters, and the galaxies living in those clusters,
assemble rapidly at early times (2.5 < z < 4). While more than twenty
proto-clusters have been observed at z > 2 based on associations of 5-40
galaxies around rare sources, the observational evidence for rapid cluster
formation is weak. Here we report observations of an asymmetric, filamentary
structure at z = 2.47 containing seven starbursting, submillimeter-luminous
galaxies and five additional AGN within a comoving volume of 15000 Mpc.
As the expected lifetime of both the luminous AGN and starburst phase of a
galaxy is ~100 Myr, we conclude that these sources were likely triggered in
rapid succession by environmental factors, or, alternatively, the duration of
these cosmologically rare phenomena is much longer than prior direct
measurements suggest. The stellar mass already built up in the structure is
and we estimate that the cluster mass will exceed that
of the Coma supercluster at . The filamentary structure is in line
with hierarchical growth simulations which predict that the peak of cluster
activity occurs rapidly at z > 2.Comment: 7 pages, 3 figures, 2 tables, accepted in ApJL (small revisions from
previous version
DASI First Results: A Measurement of the Cosmic Microwave Background Angular Power Spectrum
We present measurements of anisotropy in the Cosmic Microwave Background
(CMB) from the first season of observations with the Degree Angular Scale
Interferometer (DASI). The instrument was deployed at the South Pole in the
austral summer 1999--2000, and made observations throughout the following
austral winter. We have measured the angular power spectrum of the CMB in the
range 100<l<900 with high signal-to-noise. In this paper we review the
formalism used in the analysis, in particular the use of constraint matrices to
project out contaminants such as ground and point source signals, and to test
for correlations with diffuse foreground templates. We find no evidence of
foregrounds other than point sources in the data, and find a maximum likelihood
temperature spectral index beta = -0.1 +/- 0.2 (1 sigma), consistent with CMB.
We detect a first peak in the power spectrum at l approx 200, in agreement with
previous experiments. In addition, we detect a peak in the power spectrum at l
approx 550 and power of similar magnitude at l approx 800 which are consistent
with the second and third harmonic peaks predicted by adiabatic inflationary
cosmological models.Comment: 8 pages, 1 figure, minor changes in response to referee comment
- …