4,337 research outputs found
Theoretical and experimental studies of the underlying processes and techniques of low pressure measurement Letter report, 1 Jul. 1967 - 31 May 1968
Ionization gage behavior in various gas mixtures and atmospheric composition
Using the filaments in the LCRS to test the LambdaCDM model
It has recently been established that the filaments seen in the Las Campanas
Redshift Survey (LCRS) are statistically significant at scales as large as 70
to 80 Mpc/h in the slice, and 50 to 70 Mpc/h in the five other
LCRS slices. The ability to produce such filamentary features is an important
test of any model for structure formation. We have tested the LCDM model with a
featureless, scale invariant primordial power spectrum by quantitatively
comparing the filamentarity in simulated LCRS slices with the actual data. The
filamentarity in an unbiased LCDM model, we find, is less than the LCRS.
Introducing a bias b=1.15, the model is in rough consistency with the data,
though in two of the slices the filamentarity falls below the data at a low
level of statistical significance. The filamentarity is very sensitive to the
bias parameter and a high value b=1.5, which enhances filamentarity at small
scales and suppresses it at large scales, is ruled out. A bump in the power
spectrum at k~0.05 Mpc/h is found to have no noticeable effect on the
filamentarity.Comment: 16 pages, 3 figures; Minor Changes, Accepted to Ap
Recommended from our members
The Town Lake Report, Volumes I and II
This report makes brief references to sediment and other trends seen in Waller Creek.EXECUTIVE SUMMARY: Town Lakeâs importance as a natural resource is growing in tandem with Austinâs rapid population. The lake is a source of drinking water for the City, and its greenbelt and open waters are widely used for recreation and as a focal-point for public events. In 1992, under the Clean Lakes program, a comprehensive report entitled the âTown Lake Studyâ (COA 1992a; COA 1992b; COA 1992c) was prepared. It examined the condition of the lake (Volume I), water quality control alternatives (Volume II) and a feasibility study (Volume III). This report updates the diagnostic study, Volume I (COA 1992a), including the current status of water quality with data analyzed through the year 2000. It also includes a summary of measures taken to reduce pollution from urban runoff since 1990.Waller Creek Working Grou
The Power Spectrum of Clusters of Galaxies and the Press-Schechter Approximation
We examine the power spectrum of clusters in the Press-Schechter (PS) theory
and in N-body simulations to see how the power spectrum of clusters is related
to the power spectrum of matter density fluctuations in the Universe. An
analytic model for the power spectrum of clusters for their given number
density is presented, both for real space and redshift space. We test this
model against results from N-body simulations and find that the agreement
between the analytic theory and the numerical results is good for wavelengths
Mpc. On smaller scales non-linear processes that are not
considered in the linear PS approximation influence the result. We also use our
analytic model to study the redshift-space power spectrum of clusters in cold
dark matter models with a cosmological constant (CDM) and with a
scale-invariant Harrison-Zel'dovich initial spectrum of density fluctuations.
We find that power spectra of clusters in these models are not consistent with
the observed power spectra of the APM and Abell-ACO clusters. One possible
explanation for the observed power spectra of clusters is an inflationary
scenario with a scalar field with the potential that has a localized steplike
feature. We use the PS theory to examine the power spectrum of clusters in this
model.Comment: 16 pages, 5 figures. Accepted by Ap
Clustering Analyses of 300,000 Photometrically Classified Quasars--I. Luminosity and Redshift Evolution in Quasar Bias
Using ~300,000 photometrically classified quasars, by far the largest quasar
sample ever used for such analyses, we study the redshift and luminosity
evolution of quasar clustering on scales of ~50 kpc/h to ~20 Mpc/h from
redshifts of z~0.75 to z~2.28. We parameterize our clustering amplitudes using
realistic dark matter models, and find that a LCDM power spectrum provides a
superb fit to our data with a redshift-averaged quasar bias of b_Q =
2.41+/-0.08 () for . This represents a better
fit than the best-fit power-law model (; ). We find b_Q increases with redshift.
This evolution is significant at >99.6% using our data set alone, increasing to
>99.9999% if stellar contamination is not explicitly parameterized. We measure
the quasar classification efficiency across our full sample as a = 95.6 +/-
^{4.4}_{1.9}%, a star-quasar separation comparable with the star-galaxy
separation in many photometric studies of galaxy clustering. We derive the mean
mass of the dark matter halos hosting quasars as MDMH=(5.2+/-0.6)x10^{12}
M_solar/h. At z~1.9 we find a deviation from luminosity-independent
quasar clustering; this suggests that increasing our sample size by a factor of
1.8 could begin to constrain any luminosity dependence in quasar bias at z~2.
Our results agree with recent studies of quasar environments at z < 0.4, which
detected little luminosity dependence to quasar clustering on proper scales >50
kpc/h. At z < 1.6, our analysis suggests that b_Q is constant with luminosity
to within ~0.6, and that, for g < 21, angular quasar autocorrelation
measurements are unlikely to have sufficient statistical power at z < 1.6 to
detect any luminosity dependence in quasars' clustering.Comment: 13 pages, 9 figures, 2 tables; uses amulateapj; accepted to Ap
Future optical detectors based on Al superconducting tunnel junctions
Superconducting tunnel junctions are being developed for application as
photon detectors in astronomy. We present the latest results on the development
of very high quality, very low critical temperature junctions, fabricated out
of pure Al electrodes. The detectors are operated at 50 mK in an adiabatic
demagnetisation refrigerator. The contacts to the top and base electrodes of
these junctions are fabricated either out of Nb or Ta, which has strong
implications on the loss time of the quasiparticles. The Nb contacted junctions
show quasiparticle loss times varying between 5 and 80 usec, depending on the
device size. The bias range of the Nb-contacted junctions is limited to the
range 0-100 uV, because of the set-in of strong non-equilibrium quasiparticle
multiplication currents at higher bias voltages. The Ta-contacted junctions, on
the other hand, show quasiparticle loss times in excess of 200 usec. These long
loss times lead to very strong quasiparticle multiplication, which prevents the
stable biasing of the junctions even at very low bias voltages. Junction
fabrication and characterisation are described, as well as the response of the
detectors to monochromatic light with wavelengths varying from 250 to 1000 nm.
The energy resolution of the detectors is discussed.Comment: 12 pages, 10 figure
Synthesis of Single Phase Hg-1223 High Tc Superconducting Films With Multistep Electrolytic Process
We report the multistep electrolytic process for the synthesis of high Tc
single phase HgBa2Ca2Cu3O8+ (Hg-1223) superconducting films. The
process includes : i) deposition of BaCaCu precursor alloy, ii) oxidation of
BaCaCu films, iii) electrolytic intercalation of Hg in precursor BaCaCuO films
and iv) electrochemical oxidation and annealing of Hg-intercalated BaCaCuO
films to convert into Hg1Ba2Ca2Cu3O8+ (Hg-1223). Films were
characterized by thermo-gravimetric analysis (TGA) and differential thermal
analysis (DTA), X-ray diffraction (XRD) and scanning electron microscopy (SEM).
The electrolytic intercalation of Hg in BaCaCuO precursor is proved to be a
novel alternative to high temperature-high pressure mercuration process. The
films are single phase Hg-1223 with Tc = 121.5 K and Jc = 4.3 x 104 A/cm2.Comment: 17 Pages, 10 Figures. Submitted to Superconductor Science and
Technolog
The mass function
We present the mass functions for different mass estimators for a range of
cosmological models. We pay particular attention to how universal the mass
function is, and how it depends on the cosmology, halo identification and mass
estimator chosen. We investigate quantitatively how well we can relate observed
masses to theoretical mass functions.Comment: 14 pages, 12 figures, to appear in ApJ
High-Redshift Galaxies in Cold Dark Matter Models
We use hydrodynamic cosmological simulations to predict the star formation
properties of high-redshift galaxies (z=2-6) in five variants of the
inflationary cold dark matter scenario, paying particular attention to z=3, the
redshift of the largest "Lyman-break galaxy" (LBG) samples. Because we link the
star formation timescale to the local gas density, the rate at which a galaxy
forms stars is governed mainly by the rate at which it accretes cooled gas from
the surrounding medium. At z=3, star formation in most of the simulated
galaxies is steady on 200 Myr timescales, and the instantaneous star formation
rate (SFR) is correlated with total stellar mass. However, there is enough
scatter in this correlation that a sample selected above a given SFR threshold
may contain galaxies with a fairly wide range of masses. The redshift history
and global density of star formation in the simulations depend mainly on the
amplitude of mass fluctuations in the underlying cosmological model. The three
models whose mass fluctuation amplitudes agree with recent analyses of the
Lyman-alpha forest also reproduce the observed luminosity function of LBGs
reasonably well, though the dynamic range of the comparison is small and the
theoretical and observational uncertainties are large. The models with higher
and lower amplitudes appear to predict too much and too little star formation,
respectively, though they are not clearly ruled out. The intermediate amplitude
models predict SFR ~ 30-40 Msun/yr for galaxies with a surface density 1 per
arcmin^2 per unit redshift at z=3. They predict much higher surface densities
at lower SFR, and significant numbers of galaxies with SFR > 10 Msun/yr at z >=
5.Comment: Submitted to ApJ. 31 pages including 10 ps figures. Full resolution
version of Fig 2 available at
http://www.astronomy.ohio-state.edu/~dhw/Sph/zgal.fig2.ps.g
Ultra-high energy cosmic rays may come from clustered sources
Clustering of cosmic-ray sources affects the flux observed beyond the cutoff
imposed by the cosmic microwave background and may be important in interpreting
the AGASA, Fly's Eye, and HiRes data. The standard deviation, sigma, in the
predicted number, N, of events above 10^{20} eV is sigma/N = 0.9(r_0/10
Mpc)^{0.9}, where r_0 is the unknown scale length of the correlation function
(r_0 = 10 Mpc for field galaxies). Future experiments will allow the
determination of r_0 through the detection of anisotropies in arrival
directions of ~ 10^{20} eV cosmic-rays over angular scales of Theta ~ r_0/30
Mpc.Comment: Accepted for publication in Astrophysical Journa
- âŠ