6,851 research outputs found
A Method for 21cm Power Spectrum Estimation in the Presence of Foregrounds
21cm tomography promises to be a powerful tool for estimating cosmological
parameters, constraining the epoch of reionization, and probing the so-called
dark ages. However, realizing this promise will require the extraction of a
cosmological power spectrum from beneath overwhelmingly large sources of
foreground contamination. In this paper, we develop a unified matrix-based
framework for foreground subtraction and power spectrum estimation, which
allows us to quantify the errors and biases that arise in the power spectrum as
a result of foreground subtraction. We find that existing line-of-sight
foreground subtraction proposals can lead to substantial mode-mixing as well as
residual noise and foreground biases, whereas our proposed inverse variance
foreground subtraction eliminates noise and foreground biases, gives smaller
error bars, and produces less correlated measurements of the power spectrum. We
also numerically confirm the intuitive belief in the literature that 21cm
foreground subtraction is best done using frequency rather than angular
information.Comment: 24 pages, 11 figures; replaced with accepted PRD version (minor
editorial changes to text; methods, results, and conclusions unchanged
Constraining cosmology and ionization history with combined 21 cm power spectrum and global signal measurements
Improvements in current instruments and the advent of next-generation
instruments will soon push observational 21 cm cosmology into a new era, with
high significance measurements of both the power spectrum and the mean
("global") signal of the 21 cm brightness temperature. In this paper we use the
recently commenced Hydrogen Epoch of Reionization Array as a worked example to
provide forecasts on astrophysical and cosmological parameter constraints. In
doing so we improve upon previous forecasts in a number of ways. First, we
provide updated forecasts using the latest best-fit cosmological parameters
from the Planck satellite, exploring the impact of different Planck datasets on
21 cm experiments. We also show that despite the exquisite constraints that
other probes have placed on cosmological parameters, the remaining
uncertainties are still large enough to have a non-negligible impact on
upcoming 21 cm data analyses. While this complicates high-precision constraints
on reionization models, it provides an avenue for 21 cm reionization
measurements to constrain cosmology. We additionally forecast HERA's ability to
measure the ionization history using a combination of power spectrum
measurements and semi-analytic simulations. Finally, we consider ways in which
21 cm global signal and power spectrum measurements can be combined, and
propose a method by which power spectrum results can be used to train a compact
parameterization of the global signal. This parameterization reduces the number
of parameters needed to describe the global signal, increasing the likelihood
of a high significance measurement.Comment: 16 pages, 8 figures. Revised to match accepted MNRAS version:
expanded discussion of covariances between astrophysics and cosmology in
Section 2.2, including two new figures; short discussion relating to KL modes
added to Section 4; final results unchange
A conjugate prior for discrete hierarchical log-linear models
In Bayesian analysis of multi-way contingency tables, the selection of a
prior distribution for either the log-linear parameters or the cell
probabilities parameters is a major challenge. In this paper, we define a
flexible family of conjugate priors for the wide class of discrete hierarchical
log-linear models, which includes the class of graphical models. These priors
are defined as the Diaconis--Ylvisaker conjugate priors on the log-linear
parameters subject to "baseline constraints" under multinomial sampling. We
also derive the induced prior on the cell probabilities and show that the
induced prior is a generalization of the hyper Dirichlet prior. We show that
this prior has several desirable properties and illustrate its usefulness by
identifying the most probable decomposable, graphical and hierarchical
log-linear models for a six-way contingency table.Comment: Published in at http://dx.doi.org/10.1214/08-AOS669 the Annals of
Statistics (http://www.imstat.org/aos/) by the Institute of Mathematical
Statistics (http://www.imstat.org
On the Jacquet Conjecture on the Local Converse Problem for p-adic GL_n
Based on previous results of Jiang, Nien and the third author, we prove that any two minimax unitarizable supercuspidals of GL_N that have the same depth and central character admit a special pair of Whittaker functions. This result gives a new reduction towards a final proof of Jacquet's conjecture on the local converse problem for GL_N. As a corollary of our result, we prove Jacquet's conjecture for GL_N, when N is prime
Affirmative Action & Negative Action: How Jian Li\u27s Case Can Benefit Asian Americans
In October 2006, Asian American student Jian D filed a civil rights complaint against Princeton University claiming that Princeton\u27s affirmative action policies were discriminatory. Li argues that affirmative action gives preferences to non-Asian minorities at the expense of Asian students. Li\u27s case aligns the interests of Asian Americans with Whites who challenge affirmative action and suggests that such policies are inherently discriminatory because they exclude students based on race and sacrifice merit. This Article argues that Li\u27s exclusion is not due to affirmative action but is likely due to negative action, the unfavorable treatment of Asian Americans relative to Whites. Affirmative action is not discriminatory because it considers a multitude of factors, including race, to achieve a diverse student population. Nor does affirmative action sacrifice merit; rather, it redefines merit in a way that can benefit students of all racial groups. On the other hand, negative action is discriminatory and prevalent. Whether it takes the form of legacies, admission limits or racial group comparisons, negative action discriminates against Asian Americans based on their race and contributes to existing inequalities in admissions. Framing Li\u27s case as a claim against negative action instead of affirmative action is a more accurate analysis that attacks ongoing discrimination in admissions, but preserves affirmative action\u27s benefit for all racial groups
Precision Calibration of Radio Interferometers Using Redundant Baselines
Growing interest in 21 cm tomography has led to the design and construction
of broadband radio interferometers with low noise, moderate angular resolution,
high spectral resolution, and wide fields of view. With characteristics
somewhat different from traditional radio instruments, these interferometers
may require new calibration techniques in order to reach their design
sensitivities. Self-calibration or redundant calibration techniques that allow
an instrument to be calibrated off complicated sky emission structures are
ideal. In particular, the large number of redundant baselines possessed by
these new instruments makes redundant calibration an especially attractive
option. In this paper, we explore the errors and biases in existing redundant
calibration schemes through simulations, and show how statistical biases can be
eliminated. We also develop a general calibration formalism that includes both
redundant baseline methods and basic point source calibration methods as
special cases, and show how slight deviations from perfect redundancy and
coplanarity can be taken into account.Comment: 18 pages, 13 figures; Replaced to match accepted MNRAS versio
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