3,105 research outputs found
The multi-frequency angular power spectrum of the epoch of reionization 21 cm signal
Observations of redshifted 21cm radiation from HI at high redshifts is an
important future probe of reionization. We consider the Multi-frequency Angular
Power Spectrum (MAPS) to quantify the statistics of the HI signal as a joint
function of the angular multipole l and frequency separation \Delta\nu. The
signal at two different frequencies is expected to get decorrelated as
\Delta\nu is increased, and quantifying this decorrelation is particularly
important in deciding the frequency resolution for future HI observations. This
is also expected to play a very crucial role in extracting the signal from
foregrounds as the signal is expected to decorrelate much faster than the
foregrounds (which are largely continuum sources) with increasing \Delta\nu. In
this paper we develop formulae relating the MAPS to different components of the
three dimensional HI power spectrum taking into account HI peculiar velocities.
We show that the flat-sky approximation provides a very good representation
over the angular scales of interest, and a final expression which is very
simple to calculate and interpret. We present results considering two models
for the HI distribution, namely, (i) DM: where the HI traces the dark matter
and (ii) PR: where the effects of patchy reionization are incorporated through
two parameters. We find that while the DM signal is largely featureless, the PR
signal peaks at the angular scales of the individual bubbles, and the signal is
considerably enhanced for large bubble size. For most cases of interest at l
\sim 100 the signal is uncorrelated beyond \Delta\nu \sim 1 MHz or even less,
whereas it occurs around \sim 0.1 MHz at l \sim 10^3. The \Delta\nu dependence
also carries an imprint of the bubble size and the bias, and is expected to be
an important probe of the reionization scenario (abridged).Comment: Accepted for publication in MNRAS. Revised to match the accepted
versio
Emission from Bow Shocks of Beamed Gamma-Ray Bursts
Beamed gamma-ray burst (GRB) sources produce a bow shock in their gaseous
environment. The emitted flux from this bow shock may dominate over the direct
emission from the jet for lines of sight which are outside the angular radius
of the jet emission, theta. The event rate for these lines of sight is
increased by a factor of 260*(theta/5_degrees)^{-2}. For typical GRB
parameters, we find that the bow shock emission from a jet with half-angle of
about 5 degrees is visible out to tens of Mpc in the radio and hundreds of Mpc
in the X-rays. If GRBs are linked to supernovae, studies of peculiar supernovae
in the local universe should reveal this non-thermal bow shock emission for
weeks to months following the explosion.Comment: ApJ, submitted, 15 pages, 3 figure
Influence of leaf trichomes on predatory mite ( Typhlodromus pyri ) abundance in grape varieties
Non-glandular leaf trichomes positively influence the abundance of many phytoseiid mites. We characterized the influence of grape leaf trichomes (domatia, hairs, and bristles) on Typhlodromus pyri Scheuten abundance over two years in a common garden planting of many grape varieties and 2years of sampling in a commercial vineyard. In general, a lack of trichomes was associated with much lower predator numbers and in the case of Dechaunac, a cultivar with almost no trichomes, very few T. pyri were found. Phytoseiid abundance was best predicted by a model where domatia and hair had an additive effect (r 2=0.815). Over two years of sampling at a commercial vineyard there were T. pyri present on all of the 5 cultivars except Dechaunac. At the same time, European red mite prey were present on Dechaunac alone. These results suggest that on grape cultivars lacking leaf trichomes, T. pyri likely will not attain sufficient densities to provide biological control of European red mite, despite presence of the mite food source. The relationship between leaf trichomes and phytoseiid abundance that is observed at the scale of single vines in a garden planting appears to also be manifest at the scale of a commercial vineyard. Because persistence of predatory mites in or nearby the habitats of prey mites is important for effective mite biological control, leaf trichomes, through their influence on phytoseiid persistence, may be critical for successful mite biological control in some system
Cosmological Magnetic Fields from Primordial Helical Seeds
Most early Universe scenarios predict negligible magnetic fields on
cosmological scales if they are unprocessed during subsequent expansion of the
Universe. We present a new numerical treatment of the evolution of primordial
fields and apply it to weakly helical seeds as they occur in certain early
Universe scenarios. We find that initial helicities not much larger than the
baryon to photon number can lead to fields of about 10^{-13} Gauss with
coherence scales slightly below a kilo-parsec today.Comment: 4 revtex pages, 2 postscript figures include
Is a Classical Language Adequate in Assessing the Detectability of the Redshifted 21cm Signal from the Early Universe?
The classical radiometer equation is commonly used to calculate the
detectability of the 21cm emission by diffuse cosmic hydrogen at high
redshifts. However, the classical description is only valid in the regime where
the occupation number of the photons in phase space is much larger than unity
and they collectively behave as a classical electromagnetic field. At redshifts
z<20, the spin temperature of the intergalactic gas is dictated by the
radiation from galaxies and the brightness temperature of the emitting gas is
in the range of mK, independently from the existence of the cosmic microwave
background. In regions where the observed brightness temperature of the 21cm
signal is smaller than the observed photon energy, of 68/(1+z) mK, the
occupation number of the signal photons is smaller than unity. Neverethless,
the radiometer equation can still be used in this regime because the weak
signal is accompanied by a flood of foreground photons with a high occupation
number (involving the synchrotron Galactic emission and the cosmic microwave
background). As the signal photons are not individually distinguishable, the
combined signal+foreground population of photons has a high occupation number,
thus justifying the use of the radiometer equation.Comment: 4 pages, Accepted for publication in JCA
Dwarf Galaxy Formation Was Suppressed By Cosmic Reionization
A large number of faint galaxies, born less than a billion years after the
big bang, have recently been discovered. The fluctuations in the distribution
of these galaxies contributed to a scatter in the ionization fraction of cosmic
hydrogen on scales of tens of Mpc, as observed along the lines of sight to the
earliest known quasars. Theoretical simulations predict that the formation of
dwarf galaxies should have been suppressed after cosmic hydrogen was reionized,
leading to a drop in the cosmic star formation rate. Here we present evidence
for this suppression. We show that the post-reionization galaxies which
produced most of the ionizing radiation at a redshift z~5.5, must have had a
mass in excess of ~10^{10.6+/-0.4} solar masses or else the aforementioned
scatter would have been smaller than observed. This limiting mass is two orders
of magnitude larger than the galaxy mass that is thought to have dominated the
reionization of cosmic hydrogen (~10^8 solar masses). We predict that future
surveys with space-based infrared telescopes will detect a population of
smaller galaxies that reionized the Universe at an earlier time, prior to the
epoch of dwarf galaxy suppression.Comment: 19 pages, 3 figures. Accepted for publication in Nature; press
embargo until publishe
Enhanced Microlensing by Stars Around the Black Hole in the Galactic Center
The effect of stars on the lensing properties of the supermassive black hole
in the Galactic Center is similar to the effect of planets on microlensing by a
star. We show that the dense stellar cluster around SgrA* increases by factors
of a few the probability of high-magnification lensing events of a distant
background source by the black hole. Conversely, the gravitational shear of the
black hole changes and enhances the microlensing properties of the individual
stars. The effect is largest when the source image lies near the Einstein
radius of the black hole 1.75"+/-0.20" for a source at infinity). We estimate
that the probability of observing at least one distant background star which is
magnified by a factor >5 in any infrared snapshot of the inner ~ 2" of the
Galactic Center is 1% with a K-band detection threshold of 20 mag. The largest
source of uncertainty in this estimate is the luminosity function of the
background stars. The gravitational shear of the black hole lengthens the
duration of high-magnification events near the Einstein radius up to a few
months, and introduces a large variety of lightcurve shapes that are different
from those of isolated microlenses. Identification of such events by image
subtraction can be used to probe the mass function, density and velocity
distributions of faint stars near the black hole, which are not detectable
otherwise.Comment: 16 pages, 4 figures, ApJ in press. Lensing probability for GC
significantly decreased due to revised background sources mode
Non-destructive classification and quality evaluation of proso millet cultivars using NIR hyperspectral imaging with machine learning
Millet is a small-seeded cereal crop with big potential and remarkable characteristics such as high drought resistance, short growing time, low water footprint, and the ability to grow in acidic soil. There is a need to develop nondestructive methods for differentiation and evaluation of the quality attributes of different of proso millet cultivars grown in the U.S. Current methods of cultivar classification are either subjective or destructive, time consuming, not allowing for the whole population to be tested, and requiring trained operators and special equipment. In this study, the feasibility of using near-infrared (NIR) hyperspectral imaging (900-1700 nm) to predict the quality attributes of proso millet (Panicum miliaceum L.) seeds as well to classify its different cultivars was demonstrated. Ten different cultivars of proso millet variety, which are the most popular in the US, investigated in this study included Cerise, Cope, Earlybird, Huntsman, Minco, Plateau, Rise, Snowbird, Sunrise, and Sunup. To reduce the large dimensionality of the hyperspectral imaging, principal component analysis (PCA) was applied, and the first two principal components were used as imaging features for building the classification models. The Classification performance showed a test accuracy rates as high as 99% for classifying the different cultivars of proso millet using gradient tree boosting ensemble machine learning algorithm. Moreover, using the partial least squares regression (PLSR) the coefficient of determination (R2) for quality prediction of proso millet seeds were 0.87, 0.80, 0.83, 0.93, and 0.92 for moisture content, crude protein, crude fat, ash, and carbohydrate, respectively. The overall results indicate that NIR hyperspectral imaging could be used to non-destructively classify and predict the quality of proso millet seeds
Gravitational Lensing of the X-Ray Background by Clusters of Galaxies
Gravitational lensing by clusters of galaxies affects the cosmic X-ray
background (XRB) by altering the observed density and flux distribution of
background X-ray sources. At faint detection flux thresholds, the resolved
X-ray sources appear brighter and diluted, while the unresolved component of
the XRB appears dimmer and more anisotropic, due to lensing. The diffuse X-ray
intensity in the outer halos of clusters might be lower than the sky-averaged
XRB, after the subtraction of resolved sources. Detection of the lensing signal
with a wide-field X-ray telescope could probe the mass distribution of a
cluster out to its virialization boundary. In particular, we show that the
lensing signature imprinted on the resolved component of the XRB by the cluster
A1689, should be difficult but possible to detect out to 8' at the 2-4 sigma
level, after 10^6 seconds of observation with the forthcoming AXAF satellite.
The lensing signal is fairly insensitive to the lens redshift in the range
0.1<z<0.6. The amplitude of the lensing signal is however sensitive to the
faint end slope of the number-flux relation for unresolved X-ray sources, and
can thus help constrain models of the XRB. A search for X-ray arcs or arclets
could identify the fraction of all faint sources which originate from extended
emission of distant galaxies. The probability for a 3 sigma detection of an
arclet which is stretched by a factor of about 3 after a 10^6 seconds
observation of A1689 with AXAF, is roughly comparable to the fraction of all
background X-ray sources that have an intrinsic size of order 1''.Comment: 41 LaTeX pages, 11 postscript figures, 1 table, in AASTeX v4.0
format. To appear in ApJ, April 1, 1997, Vol. 47
Gamma-Ray Background from Structure Formation in the Intergalactic Medium
The universe is filled with a diffuse and isotropic extragalactic background
of gamma-ray radiation, containing roughly equal energy flux per decade in
photon energy between 3 MeV-100 GeV. The origin of this background is one of
the unsolved puzzles in cosmology. Less than a quarter of the gamma-ray flux
can be attributed to unresolved discrete sources, but the remainder appears to
constitute a truly diffuse background whose origin has hitherto been
mysterious. Here we show that the shock waves induced by gravity during the
formation of large-scale structure in the intergalactic medium, produce a
population of highly-relativistic electrons with a maximum Lorentz factor above
10^7. These electrons scatter a small fraction of the microwave background
photons in the present-day universe up to gamma-ray energies, thereby providing
the gamma-ray background. The predicted diffuse flux agrees with the observed
background over more than four decades in photon energy, and implies a mean
cosmological density of baryons which is consistent with Big-Bang
nucleosynthesis.Comment: 7 pages, 1 figure. Accepted for publication in Nature. (Press embargo
until published.
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