15,075 research outputs found
Cosmology with dropout selection: Straw-man surveys and CMB lensing
We seek to prove the means, motive and opportunity of 2 < z < 5 dropout
galaxies for large-scale structure. Together with low-z tracers, these samples
would map practically every linear mode and facilitate a tomographic
decomposition of the CMB lensing kernel over an unprecedented volume, thereby
yielding a proxy for (the time evolution of) matter density fluctuations that
provides compelling tests of horizon-scale General Relativity, neutrino masses
and Inflation-- viz., curvature, running of the spectral index and a
scale-dependent halo bias induced by (local) primordial non-Gaussianity.
Focusing on color-color selection, we estimate the completeness,
contamination, and spectroscopic survey speed of tailored Lyman-break galaxy
(LBG) samples. We forecast the potential of CMB lensing cross-correlation,
clustering redshifts and Redshift-Space Distortions (RSD) analyses. In
particular, we estimate: the depth dependence of interlopers based on CFHTLS
data and propagate this to biases in cosmology; new inferences of (non-linear)
halo bias at these redshifts and depths using legacy data; detailed forecasts
of LBG spectra as would be observed by DESI, PFS, and their successors. We
further assess the relative competitiveness of potential spectroscopic
facilities based on an intuitive figure-of-merit and define a modernisation of
traditional selections to the photometric system of LSST where necessary.
We confirm these science cases to be compelling for achievable facilities in
the next decade by defining a LBG sample of increasing Lyman-alpha equivalent
width with redshift, which delivers both percent-level RSD constraints on the
growth rate at high-z and measurements of CMB lensing cross-correlation at z=3
and 4 with a significance measured in the hundreds. Finally, we discuss the
limitations and avenues for improvement beyond this initial exploration
(abridged).Comment: 59 pages, comments welcom
Discriminant Function Analysis in Marine Ecology: Some Oversights and their Solutions
Marine ecologists commonly use discriminant function analysis (DFA) to evaluate the similarity of distinct populations and to classify individuals of unknown origin to known populations. However, investigators using DFA must account for (1) the possibility of correct classification due to chance alone, and (2) the influence of prior probabilities of group membership on classification results. A search of the recent otolith chemistry literature showed that these two concerns are sometimes ignored, so we used simulated data sets to explore the potential pitfalls of such oversights. We found that when estimating reclassification success for a training data set, small sample sizes or unbalanced sampling designs can produce remarkably high reclassification success rates by chance alone, especially when prior probabilities are estimated from sample size. When using a training data set to classify unknown individuals, maximum likelihood estimation of mixture proportions and group membership afforded up to 20% improvement over DFA with uninformative priors when groups contributed to the sample unequally. Given these results, we recommend the use of (1) randomization tests to estimate the probability that reclassification success is better than random, and (2) maximum likelihood estimation of mixture proportions in place of uninformative priors
The Physical Conditions and Dynamics of the Interstellar Medium in the Nucleus of M83: Observations of CO and CI
This paper presents CI, CO J=4-3, and CO J=3-2 maps of the barred spiral
galaxy M83 taken at the James Clerk Maxwell Telescope. Observations indicate a
double peaked structure which is consistent with gas inflow along the bar
collecting at the inner Lindblad resonance. This structure suggests that
nuclear starbursts can occur even in galaxies where this inflow/collection
occurs, in contrast to previous studies of barred spiral galaxies. However, the
observations also suggest that the double peaked emission may be the result of
a rotating molecular ring oriented nearly perpendicular to the main disk of the
galaxy. The CO J=4-3 data indicate the presence of warm gas in the nucleus that
is not apparent in the lower-J CO observations, which suggests that CO J=1-0
emission may not be a reliable tracer of molecular gas in starburst galaxies.
The twelve CI/CO J=4-3 line ratios in the inner 24'' x 24'' are uniform at the
2 sigma level, which indicates that the CO J=4-3 emission is originating in the
same hot photon-dominated regions as the CI emission. The CO J=4-3/J=3-2 line
ratios vary significantly within the nucleus with the higher line ratios
occurring away from peaks of emission along an arc of active star forming
regions. These high line ratios (>1) likely indicate optically thin gas created
by the high temperatures caused by star forming regions in the nucleus of this
starburst galaxy.Comment: 15 pages with 10 figures. To appear in the August 10 1998 issue of
The Astrophysical Journa
Transcriptional repression by ApiAP2 factors is central to chronic toxoplasmosis
Tachyzoite to bradyzoite development in Toxoplasma is marked by major changes in gene expression resulting in a parasite that expresses a new repertoire of surface antigens hidden inside a modified parasitophorous vacuole called the tissue cyst. The factors that control this important life cycle transition are not well understood. Here we describe an important transcriptional repressor mechanism controlling bradyzoite differentiation that operates in the tachyzoite stage. The ApiAP2 factor, AP2IV-4, is a nuclear factor dynamically expressed in late S phase through mitosis/cytokinesis of the tachyzoite cell cycle. Remarkably, deletion of the AP2IV-4 locus resulted in the expression of a subset of bradyzoite-specific proteins in replicating tachyzoites that included tissue cyst wall components BPK1, MCP4, CST1 and the surface antigen SRS9. In the murine animal model, the mis-timing of bradyzoite antigens in tachyzoites lacking AP2IV-4 caused a potent inflammatory monocyte immune response that effectively eliminated this parasite and prevented tissue cyst formation in mouse brain tissue. Altogether, these results indicate that suppression of bradyzoite antigens by AP2IV-4 during acute infection is required for Toxoplasma to successfully establish a chronic infection in the immune-competent host
The 90 GHz radiometric imaging
A 90-GHz (3 mm wavelength) radiometer with a noise output fluctuation of 0.22 K (RMS), with a scanning antenna beam mirror, and the data processing system are described. Real-time radiometric imaging of terrain and man-made objects are shown. Flying at an altitude of 1500 ft a radiometer antenna with a 2 degrees halfpower beamwidth can distinguish landforms, waterways, roads, runways, bridges, ships at sea and their wakes, aircraft on runways, and athletic fields. A flight taken at an altitude of 3000 ft with approximately 2000 ft of clouds below the radiometer demonstrates the ability to distinguish bridges, rivers, marshland and other landforms even though the clouds are optically opaque. The radiometric images of a few representative scenes along with photographs of the corresponding scenes are presented to demonstrate the resolution of the imager system
454-Pyrosequencing: A Molecular Battiscope for Freshwater Viral Ecology
Viruses, the most abundant biological entities on the planet, are capable of infecting organisms from all three branches of life, although the majority infect bacteria where the greatest degree of cellular diversity lies. However, the characterization and assessment of viral diversity in natural environments is only beginning to become a possibility. Through the development of a novel technique for the harvest of viral DNA and the application of 454 pyrosequencing, a snapshot of the diversity of the DNA viruses harvested from a standing pond on a cattle farm has been obtained. A high abundance of viral genotypes (785) were present within the virome. The absolute numbers of lambdoid and Shiga toxin (Stx) encoding phages detected suggested that the depth of sequencing had enabled recovery of only ca. 8% of the total virus population, numbers that agreed within less than an order of magnitude with predictions made by rarefaction analysis. The most abundant viral genotypes in the pond were bacteriophages (93.7%). The predominant viral genotypes infecting higher life forms found in association with the farm were pathogens that cause disease in cattle and humans, e.g. members of the Herpesviridae. The techniques and analysis described here provide a fresh approach to the monitoring of viral populations in the aquatic environment, with the potential to become integral to the development of risk analysis tools for monitoring the dissemination of viral agents of animal, plant and human diseases
Detection of weak lensing by a cluster of galaxies at z=0.83
We report the detection of weak gravitational lensing of faint, distant
background galaxies by the rich, X-ray luminous cluster of galaxies MS1054-03
at z=0.83. This is the first measurement of weak lensing by a bona fide cluster
at such a high redshift. We detect tangential shear at the 5% - 10% level over
a range of radii 50'' < r < 250'' centered on the optical position of the
cluster. Two-dimensional mass reconstruction using galaxies with 21.5 < I <
25.5 shows a strong peak which coincides with the peak of the smoothed cluster
light distribution. Splitting this sample by magnitude (at I = 23.5) and color
(at R-I = 0.7), we find that the brighter and redder subsamples are only very
weakly distorted, indicating that the faint blue galaxies (FBG's), which
dominate the shear signal, are relatively more distant. The derived cluster
mass is quite sensitive to the N(z) for the FBG's. At one extreme, if all the
FBG's are at z_s = 3, then the mass within a Mpc aperture is \h1 , and the mass-to-light ratio is in solar units. For the derived mass is 70\%
higher and . If follows the no evolution model (in
shape) then , and if all the FBG's lie at z_s\la 1 the
required exceeds . These data provide clear evidence that large,
dense mass concentrations existed at early epochs; that they can be weighed
efficiently by weak lensing observations; and that most of the FBG's are at
high redshift.Comment: Submitted to ApJ, 15 pages (incl 8 figs, 3 of which are plates).
Plate images not included, but are available from
ftp://hubble.ifa.hawaii.edu/pub/ger/ms1054/ms1054_fig[1,3,5].ps.
Molecular Hydrogen and Paschen-alpha Emission in Cooling Flow Galaxies
We present near-infrared spectra obtained to search for Pa-alpha and
molecular hydrogen lines in edge-darkened (FR I-type) radio galaxies with
bright Halpha emission in the redshift range 0.0535<z<0.15. We find that all
three galaxies in our sample (PKS 0745-191, PKS 1346+26, & PKS2322-12) which
are associated with strong cooling flows also have strong Pa-alpha and H_2
(1-0) S(1) through S(5) emission, while other radio galaxies do not. Together
with earlier observations this confirms claims that cooling flow galaxies are
copious emitters of molecular hydrogen with large H_2 (1-0) S(3)/Pa-alpha
ratios in the range 0.5 to 2. The emission is centrally concentrated within the
inner few kiloparsec and could come from warm (T ~ 1000-1500 K) molecular
material which is being deposited by the cooling flow. We speculate that the
H_2 emission could be related to the interaction between the jets and this
molecular gas.Comment: ApJ Letters, in press, AAS LaTex, preprint also available at
http://www.astro.umd.edu/~hfalcke/publications.html#nirga
The Density Matrix Renormalization Group for finite Fermi systems
The Density Matrix Renormalization Group (DMRG) was introduced by Steven
White in 1992 as a method for accurately describing the properties of
one-dimensional quantum lattices. The method, as originally introduced, was
based on the iterative inclusion of sites on a real-space lattice. Based on its
enormous success in that domain, it was subsequently proposed that the DMRG
could be modified for use on finite Fermi systems, through the replacement of
real-space lattice sites by an appropriately ordered set of single-particle
levels. Since then, there has been an enormous amount of work on the subject,
ranging from efforts to clarify the optimal means of implementing the algorithm
to extensive applications in a variety of fields. In this article, we review
these recent developments. Following a description of the real-space DMRG
method, we discuss the key steps that were undertaken to modify it for use on
finite Fermi systems and then describe its applications to Quantum Chemistry,
ultrasmall superconducting grains, finite nuclei and two-dimensional electron
systems. We also describe a recent development which permits symmetries to be
taken into account consistently throughout the DMRG algorithm. We close with an
outlook for future applications of the method.Comment: 48 pages, 17 figures Corrections made to equation 19 and table
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