2,462 research outputs found
Multi-band quantum ratchets
We investigate directed motion in non-adiabatically rocked ratchet systems
sustaining few bands below the barrier. Upon restricting the dynamics to the
lowest M bands, the total system-plus-bath Hamiltonian is mapped onto a
discrete tight-binding model containing all the information both on the intra-
and inter-well tunneling motion. A closed form for the current in the
incoherent tunneling regime is obtained. In effective single-band ratchets, no
current rectification occurs. We apply our theory to describe rectification
effects in vortex quantum ratchets devices. Current reversals upon variation of
the ac-field amplitude or frequency are predicted.Comment: Accepted for publication in Physical Review Letter
Redshifts in the Southern Abell Redshift Survey Clusters. I. The Data
The Southern Abell Redshift Survey contains 39 clusters of galaxies with
redshifts in the range 0.0 < z < 0.31 and a median redshift depth of z =
0.0845. SARS covers the region 0 21h (while
avoiding the LMC and SMC) with b > 40. Cluster locations were chosen from the
Abell and Abell-Corwin-Olowin catalogs while galaxy positions were selected
from the Automatic Plate Measuring Facility galaxy catalog with
extinction-corrected magnitudes in the range 15 <= b_j < 19. SARS utilized the
Las Campanas 2.5 m duPont telescope, observing either 65 or 128 objects
concurrently over a 1.5 sq deg field. New redshifts for 3440 galaxies are
reported in the fields of these 39 clusters of galaxies.Comment: 20 pages, 5 figures, accepted for publication in the Astronomical
Journal, Table 2 can be downloaded in its entirety from
http://trotsky.arc.nasa.gov/~mway/SARS1/sars1-table2.cs
The local space density of dwarf galaxies
We estimate the luminosity function of field galaxies over a range of ten
magnitudes (-22 < M_{B_J} < -12 for H_0 = 100 km/s/Mpc) by counting the number
of faint APM galaxies around Stromlo-APM redshift survey galaxies at known
distance. The faint end of the luminosity function rises steeply at M_{B_J}
\approx -15, implying that the space density of dwarf galaxies is at least two
times larger than predicted by a Schechter function with flat faint-end slope.
Such a high abundance of dwarf galaxies at low redshift can help explain the
observed number counts and redshift distributions of faint galaxies without
invoking exotic models for galaxy evolution.Comment: 20 pages, 5 included postscript figures, uses AAS LaTex macros.
Accepted for publication in the Astrophysical Journal. Two figures and
associated discussion added; results and conclusions unchange
The Galaxy Angular Correlation Functions and Power Spectrum from the Two Micron All Sky Survey
We calculate the angular correlation function of galaxies in the Two Micron
All Sky Survey. We minimize the possible contamination by stars, dust, seeing
and sky brightness by studying their cross correlation with galaxy density, and
limiting the galaxy sample accordingly. We measure the correlation function at
scales between 1-18 arcdegs using a half million galaxies. We find a best fit
power law to the correlation function has a slope of 0.76 and an amplitude of
0.11. However, there are statistically significant oscillations around this
power law. The largest oscillation occurs at about 0.8 degrees, corresponding
to 2.8 h^{-1} Mpc at the median redshift of our survey, as expected in halo
occupation distribution descriptions of galaxy clustering.
We invert the angular correlation function using Singular Value Decomposition
to measure the three-dimensional power spectrum and find that it too is in good
agreement with previous measurements. A dip seen in the power spectrum at small
wavenumber k is statistically consistent with CDM-type power spectra. A fit of
CDM-type power spectra to k < 0.2 h Mpc^{-1} give constraints of
\Gamma_{eff}=0.116 and \sigma_8=0.96. This suggest a K_s-band linear bias of
1.1+/-0.2. This \Gamma_{eff} is different from the WMAP CMB derived value. On
small scales the power-law shape of our power spectrum is shallower than that
derived for the SDSS. These facts together imply a biasing of these different
galaxies that might be nonlinear, that might be either waveband or luminosity
dependent, and that might have a nonlocal origin.Comment: 14 pages, 20 figures, to be published in ApJ January 20th, revision
included two new figures, version with high resolution figures can be found
here http::ww
Star Formation, Metallicity and Dust Properties Derived from the SAPM Galaxy Survey Spectra
We have derived star formation rates (SFRs), gas-phase oxygen abundances and
effective dust absorption optical depths for a sample of galaxies drawn from
the Stromlo-APM redshift survey using the new Charlot and Longhetti (2001;
CL01) models, which provide a physically consistent description of the effects
of stars, gas and dust on the integrated spectra of galaxies. Our sample
consists of 705 galaxies with measurements of the fluxes and equivalent widths
of Halpha, [OII], and one or both of [NII] and [SII]. For a subset of the
galaxies, 60 and 100 micron IRAS fluxes are available. We compare the star
formation rates derived using the models with those derived using standard
estimators based on the Halpha, the [OII] and the far-infrared luminosities of
the galaxies. The CL01 SFR estimates agree well with those derived from the
IRAS fluxes, but are typically a factor of ~3 higher than those derived from
the Halpha or the [OII] fluxes, even after the usual mean attenuation
correction of A_Halpha=1 mag is applied to the data. We show that the reason
for this discrepancy is that the standard Halpha estimator neglects the
absorption of ionizing photons by dust in HII regions and the contamination of
Halpha emission by stellar absorption. We also use our sample to study
variations in star formation and metallicity as a function of galaxy absolute
bJ magnitude. For this sample, the star formation rate per unit bJ luminosity
is independent of magnitude. The gas-phase oxygen abundance does increase with
bJ luminosity, although the scatter in metallicity at fixed magnitude is large.Comment: 17 pages, 8 figures, accepted for publication in MNRA
Large-scale structure and matter in the universe
This paper summarizes the physical mechanisms that encode the type and
quantity of cosmological matter in the properties of large-scale structure, and
reviews the application of such tests to current datasets. The key lengths of
the horizon size at matter-radiation equality and at last scattering determine
the total matter density and its ratio to the relativistic density; acoustic
oscillations can diagnose whether the matter is collisionless, and small-scale
structure or its absence can limit the mass of any dark-matter relic particle.
The most stringent constraints come from combining data on present-day galaxy
clustering with data on CMB anisotropies. Such an analysis breaks the
degeneracies inherent in either dataset alone, and proves that the universe is
very close to flat. The matter content is accurately consistent with pure Cold
Dark Matter, with about 25% of the critical density, and fluctuations that are
scalar-only, adiabatic and scale-invariant. It is demonstrated that these
conclusions cannot be evaded by adjusting either the equation of state of the
vacuum, or the total relativistic density.Comment: 17 Pages. Review paper from the January 2003 Royal Society Discussion
Meeting, "The search for dark matter and dark energy in the universe
Radio Sources in the 2dF Galaxy Redshift Survey. I. Radio Source Populations
We present the first results from a study of the radio continuum properties
of galaxies in the 2dF Galaxy Redshift Survey, based on thirty 2dF fields
covering a total area of about 100 square degrees. About 1.5% of galaxies with
b(J) < 19.4 mag are detected as radio continuum sources in the NRAO VLA Sky
Survey (NVSS). Of these, roughly 40% are star-forming galaxies and 60% are
active galaxies (mostly low-power radio galaxies and a few Seyferts). The
combination of 2dFGRS and NVSS will eventually yield a homogeneous set of
around 4000 radio-galaxy spectra, which will be a powerful tool for studying
the distriibution and evolution of both AGN and starburst galaxies out to
redshift z=0.3.Comment: 14 pages, 7 figures, accepted for publication in PAS
The Apparent and Intrinsic Shape of the APM Galaxy Clusters
We estimate the distribution of intrinsic shapes of APM galaxy clusters from
the distribution of their apparent shapes. We measure the projected cluster
ellipticities using two alternative methods. The first method is based on
moments of the discrete galaxy distribution while the second is based on
moments of the smoothed galaxy distribution. We study the performance of both
methods using Monte Carlo cluster simulations covering the range of APM cluster
distances and including a random distribution of background galaxies. We find
that the first method suffers from severe systematic biases, whereas the second
is more reliable. After excluding clusters dominated by substructure and
quantifying the systematic biases in our estimated shape parameters, we recover
a corrected distribution of projected ellipticities. We use the non-parametric
kernel method to estimate the smooth apparent ellipticity distribution, and
numerically invert a set of integral equations to recover the corresponding
distribution of intrinsic ellipticities under the assumption that the clusters
are either oblate or prolate spheroids. The prolate spheroidal model fits the
APM cluster data best.Comment: 8 pages, including 7 figures, accepted for publication in MNRA
Probing dark energy with steerable wavelets through correlation of WMAP and NVSS local morphological measures
Using local morphological measures on the sphere defined through a steerable
wavelet analysis, we examine the three-year WMAP and the NVSS data for
correlation induced by the integrated Sachs-Wolfe (ISW) effect. The steerable
wavelet constructed from the second derivative of a Gaussian allows one to
define three local morphological measures, namely the signed-intensity,
orientation and elongation of local features. Detections of correlation between
the WMAP and NVSS data are made with each of these morphological measures. The
most significant detection is obtained in the correlation of the
signed-intensity of local features at a significance of 99.9%. By inspecting
signed-intensity sky maps, it is possible for the first time to see the
correlation between the WMAP and NVSS data by eye. Foreground contamination and
instrumental systematics in the WMAP data are ruled out as the source of all
significant detections of correlation. Our results provide new insight on the
ISW effect by probing the morphological nature of the correlation induced
between the cosmic microwave background and large scale structure of the
Universe. Given the current constraints on the flatness of the Universe, our
detection of the ISW effect again provides direct and independent evidence for
dark energy. Moreover, this new morphological analysis may be used in future to
help us to better understand the nature of dark energy.Comment: 12 pages, 10 figures, replaced to match version accepted by MNRA
Extragalactic Foregrounds of the Cosmic Microwave Background: Prospects for the MAP Mission
(Abridged) While the major contribution to the Cosmic Microwave Background
(CMB) anisotropies are the sought-after primordial fluctuations produced at the
surface of last scattering, other effects produce secondary fluctuations at
lower redshifts. Here, we study the extragalactic foregrounds of the CMB in the
context of the upcoming MAP mission. We first survey the major extragalactic
foregrounds and show that discrete sources, the Sunyaev-Zel'dovich (SZ) effect,
and gravitational lensing are the most dominant ones for MAP. We then show that
MAP will detect (>5 sigma) about 46 discrete sources and 10 SZ clusters
directly with 94 GHz fluxes above 2 Jy. The mean SZ fluxes of fainter clusters
can be probed by cross-correlating MAP with cluster positions extracted from
existing catalogs. For instance, a MAP-XBACs cross-correlation will be
sensitive to clusters with S(94GHz)>200mJy, and will thus provide a test of
their virialization state and a measurement of their gas fraction. Finally, we
consider probing the hot gas on supercluster scales by cross-correlating the
CMB with galaxy catalogs. Assuming that galaxies trace the gas, we show that a
cross-correlation between MAP and the APM catalog should yield a marginal
detection, or at least a four-fold improvement on the COBE upper limits for the
rms Compton y-parameter.Comment: 27 LaTeX pages, including 5 ps figures and 2 tables. To appear in
ApJ. Minor revisions to match accepted version. Color figures and further
links available at http://www.astro.princeton.edu/~refreg
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