2,134 research outputs found
Galaxy Morphology from NICMOS Parallel Imaging
We present high resolution NICMOS images of random fields obtained in
parallel to other HST observations. We present galaxy number counts reaching
H=24. The H-band galaxy counts show good agreement with the deepest I- and
K-band counts obtained from ground-based data. We present the distribution of
galaxies with morphological type to H<23. We find relatively fewer irregular
galaxies compared to an I-band sample from the Hubble Deep Field, which we
attribute to their blue color, rather than to morphological K-corrections. We
conclude that the irregulars are intrinsically faint blue galaxies at z<1.Comment: 13 pages, including 4 figures. Accepted for publication in ApJ
Letter
Constraining the unexplored period between reionization and the dark ages with observations of the global 21 cm signal
Observations of the frequency dependence of the global brightness temperature
of the redshifted 21 cm line of neutral hydrogen may be possible with single
dipole experiments. In this paper, we develop a Fisher matrix formalism for
calculating the sensitivity of such instruments to the 21 cm signal from
reionization and the dark ages. We show that rapid reionization histories with
duration delta z< 2 can be constrained, provided that local foregrounds can be
well modelled by low order polynomials. It is then shown that observations in
the range nu = 50 - 100 MHz can feasibly constrain the Lyman alpha and X-ray
emissivity of the first stars forming at z = 15 - 25, provided that systematic
temperature residuals can be controlled to less than 1 mK. Finally, we
demonstrate the difficulty of detecting the 21 cm signal from the dark ages
before star formation.Comment: 11 pages, 14 figures, submitted to PR
Evolution of the 21 cm signal throughout cosmic history
The potential use of the redshifted 21 cm line from neutral hydrogen for
probing the epoch of reionization is motivating the construction of several
low-frequency interferometers. There is also much interest in the possibility
of constraining the initial conditions from inflation and the nature of the
dark matter and dark energy by probing the power-spectrum of density
perturbations in three dimensions and on smaller scales than probed by the
microwave background anisotropies. Theoretical understanding of the 21 cm
signal has been fragmented into different regimes of physical interest. In this
paper, we make the first attempt to describe the full redshift evolution of the
21 cm signal between 0<z<300.
We include contributions to the 21 cm signal from fluctuations in the gas
density, temperature and neutral fraction, as well as the Lyman alpha flux, and
allow for a post-reionization signal from damped Ly alpha systems. Our
comprehensive analysis provides a useful foundation for optimizing the design
of future arrays whose goal is to separate the particle physics from the
astrophysics, either by probing the peculiar velocity distortion of the 21 cm
power spectrum, or by extending the 21 cm horizon to z > 25 before the first
galaxies had formed, or to z < 6 when the residual pockets of hydrogen trace
large scale structure.Comment: 15 pages, 11 figures, submitted to PR
Counts and Sizes of Galaxies in the Hubble Deep Field - South: Implications for the Next Generation Space Telescope
Science objectives for the Next Generation Space Telescope (NGST) include a
large component of galaxy surveys, both imaging and spectroscopy. The Hubble
Deep Field datasets include the deepest observations ever made in the
ultraviolet, optical and near infrared, reaching depths comparable to that
expected for NGST spectroscopy. We present the source counts, galaxy sizes and
isophotal filling factors of the HDF-South images. The observed integrated
galaxy counts reach >500 galaxies per square arcminute at AB<30. We extend
these counts to faint levels in the infrared using models. The trend previously
seen that fainter galaxies are smaller, continues to AB=29 in the high
resolution HDF-S STIS image, where galaxies have a typical half-light radius of
0.1 arcseconds. Extensive Monte Carlo simulations show that the small measured
sizes are not due to selection effects until >29mag. Using the HDF-S NICMOS
image, we show that galaxies are smaller in the near infrared than they are in
the optical. We analyze the isophotal filling factor of the HDF-S STIS image,
and show that this image is mostly empty sky even at the limits of galaxy
detection, a conclusion we expect to hold true for NGST spectroscopy. At the
surface brightness limits expected for NGST imaging, however, about a quarter
of the sky is occupied by the outer isophotes of AB<30 galaxies. We discuss the
implications of these data on several design concepts of the NGST near-infrared
spectrograph. We compare the effects of resolution and the confusion limit of
various designs, as well as the multiplexing advantages of either multi-object
or full-field spectroscopy. We argue that the optimal choice for NGST
spectroscopy of high redshift galaxies is a multi-object spectrograph (MOS)
with target selection by a micro electro mechanical system (MEMS) device.Comment: 27 pages including 10 figures, accepted for publication in the
Astronomical Journal, June 2000, abridged abstrac
Generic local distinguishability and completely entangled subspaces
A subspace of a multipartite Hilbert space is completely entangled if it
contains no product states. Such subspaces can be large with a known maximum
size, S, approaching the full dimension of the system, D. We show that almost
all subspaces with dimension less than or equal to S are completely entangled,
and then use this fact to prove that n random pure quantum states are
unambiguously locally distinguishable if and only if n does not exceed D-S.
This condition holds for almost all sets of states of all multipartite systems,
and reveals something surprising. The criterion is identical for separable and
for nonseparable states: entanglement makes no difference.Comment: 12 page
Global 21cm signal experiments: a designer's guide
[Abridged] The spatially averaged global spectrum of the redshifted 21cm line
has generated much experimental interest, for it is potentially a direct probe
of the Epoch of Reionization and the Dark Ages. Since the cosmological signal
here has a purely spectral signature, most proposed experiments have little
angular sensitivity. This is worrisome because with only spectra, the global
21cm signal can be difficult to distinguish from foregrounds such as Galactic
synchrotron radiation, as both are spectrally smooth and the latter is orders
of magnitude brighter. We establish a mathematical framework for global signal
data analysis in a way that removes foregrounds optimally, complementing
spectra with angular information. We explore various experimental design
trade-offs, and find that 1) with spectral-only methods, it is impossible to
mitigate errors that arise from uncertainties in foreground modeling; 2)
foreground contamination can be significantly reduced for experiments with fine
angular resolution; 3) most of the statistical significance in a positive
detection during the Dark Ages comes from a characteristic high-redshift trough
in the 21cm brightness temperature; and 4) Measurement errors decrease more
rapidly with integration time for instruments with fine angular resolution. We
show that if observations and algorithms are optimized based on these findings,
an instrument with a 5 degree beam can achieve highly significant detections
(greater than 5-sigma) of even extended (high Delta-z) reionization scenarios
after integrating for 500 hrs. This is in contrast to instruments without
angular resolution, which cannot detect gradual reionization. Abrupt ionization
histories can be detected at the level of 10-100's of sigma. The expected
errors are also low during the Dark Ages, with a 25-sigma detection of the
expected cosmological signal after only 100 hrs of integration.Comment: 34 pages, 30 figures. Replaced (v2) to match accepted PRD version
(minor pedagogical additions to text; methods, results, and conclusions
unchanged). Fixed two typos (v3); text, results, conclusions etc. completely
unchange
Microlensing and the Surface Brightness Profile of the Afterglow Image of GRB 000301C
The optical afterglow of Gamma-Ray Burst (GRB) 000301C exhibited a
significant, short-timescale deviation from the power-law flux decline expected
in the standard synchrotron shock model. Garnavich, Loeb & Stanek found that
this deviation was well-fit by an ad hoc model in which a thin ring of emission
is microlensed by an intervening star. We revisit the microlensing
interpretation of this variability, first by testing whether microlensing of
afterglow images with realistic surface brightness profiles (SBPs) can fit the
data, and second by directly inverting the observed light curve to obtain a
non-parametric measurement of the SBP. We find that microlensing of realistic
SBPs can reproduce the observed deviation, provided that the optical emission
arises from frequencies above the cooling break. Conversely, if the variability
is indeed caused by microlensing, the SBP must be significantly
limb-brightened. Specifically, greater than 60% of the flux must originate from
the outer 25% of the area of the afterglow image. The latter requirement is
satisfied by the best fit theoretical SBP. The underlying optical/infrared
afterglow lightcurve is consistent with a model in which a jet is propagating
into a uniform medium with the cooling break frequency below the optical band.Comment: 14 pages, including 3 figures and 1 Table, very minor changes.
Accepted to ApJ main journal. To appear in the November 1, 2001 issue (v561
CELLO: A fast algorithm for Covariance Estimation
We present CELLO (Covariance Estimation and Learning through Likelihood Optimization), an algorithm for predicting the covariances of measurements based on any available informative features. This algorithm is intended to improve the accuracy and reliability of on-line state estimation by providing a principled way to extend the conventional fixed-covariance Gaussian measurement model. We show that in experiments, CELLO learns to predict measurement covariances that agree with empirical covariances obtained by manually annotating sensor regimes. We also show that using the learned covariances during filtering provides substantial quantitative improvement to the overall state estimate. © 2013 IEEE.United States. National Aeronautics and Space AdministrationSiemens Corporate ResearchUnited States. Office of Naval Research. Multidisciplinary University Research InitiativeMicro Autonomous Consortium Systems and Technolog
Adenovirus-Mediated Gene Transfer of Viral Interleukin-10 Inhibits the Immune Response to Both Alloantigen and Adenoviral Antigen
Overview summary Adenoviral vectors are efficient for in vivo delivery of genes to a wide variety of tissue types, whereas the duration of expression is limited by the potent adenovirus-specific immune response directed to the infected cell. In this study, we demonstrate that adenovirus-mediated gene transfer and expression of viral interleukin-10 (vIL-10) not only prolongs murine cardiac allograft survival, but also inhibits the immune response toward adenoviral antigens, and thereby improves the persistence of the vector and extends transgene expression. These findings could be used to design a new generation of adenoviral vector that expresses both an immunosuppressive cytokine gene and another gene of interest. This strategy should have general application in many gene therapy settings other than transplantation. Nonetheless, although the efficacy of adenoviral vectors can be improved by incorporating immunosuppressive genes into the vector, there are also nonimmune mechanisms serving to limit vector gene expression.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/63260/1/hum.1997.8.11-1365.pd
Detecting Population III stars through observations of near-IR cosmic infrared background anisotropies
Following the successful mapping of the last scattering surface by WMAP and
balloon experiments, the epoch of the first stars, when Population III stars
formed, is emerging as the next cosmological frontier. It is not clear what
these stars' properties were, when they formed or how long their era lasted
before leading to the stars and galaxies we see today. We show that these
questions can be answered with the current and future measurements of the
near-IR cosmic infrared background (CIB). Theoretical arguments suggest that
Population III stars were very massive and short-lived stars that formed at
at rare peaks of the density field in the cold-dark-matter
Universe. Because Population III stars probably formed individually in small
mini-halos, they are not directly accessible to current telescopic studies. We
show that these stars left a strong and measurable signature via their
contribution to the CIB anisotropies for a wide range of their formation
scenarios. The excess in the recently measured near-IR CIB anisotropies over
that from normal galaxies can be explained by contribution from early
Population III stars. These results imply that Population III were indeed very
massive stars and their epoch started at and lasted past z\lsim
13. We show the importance of accurately measuring the CIB anisotropies
produced by Population III with future space-based missions.Comment: Ap.J., in press. (Replaced with accepted version
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