arXiv subjects

Angelica de Oliveira-Costa

Publications and source records attributed to Angelica de Oliveira-Costa.

At least 19 recordsLinked to original sources

arXiv2012

Low Frequency Imaging of Fields at High Galactic Latitude with the Murchison Widefield Array 32-Element Prototype

The Murchison Widefield Array (MWA) is a new low-frequency, wide field-of-view radio interferometer under development at the Murchison Radio-astronomy Observatory (MRO) in Western Australia. We have used a 32-element MWA prototype interferometer (MWA-32T) to observe two 50-degree diameter fields in the southern sky in the 110 MHz to 200 MHz band in order to evaluate the performance of the MWA-32T, to develop techniques for epoch of reionization experiments, and to make measurements of astronomical foregrounds. We developed a calibration and imaging pipeline for the MWA-32T, and used it to produce ~15' angular resolution maps of the two fields. We perform a blind source extraction using these confusion-limited images, and detect 655 sources at high significance with an additional 871 lower significance source candidates. We compare these sources with existing low-frequency radio surveys in order to assess the MWA-32T system performance, wide field analysis algorithms, and catalog quality. Our source catalog is found to agree well with existing low-frequency surveys in these regions of the sky and with statistical distributions of point sources derived from Northern Hemisphere surveys; it represents one of the deepest surveys to date of this sky field in the 110 MHz to 200 MHz band.

astro-ph.CO↗

arXiv2010

Clustering of Extragalactic Sources from 151 MHz to 232 MHz: Implications for Cosmological 21-cm Observations

In order to construct accurate point sources simulations at the frequencies relevant to 21-cm experiments, the angular correlation of radio sources must be taken into account. This paper presents a measurement of angular two-point correlation function, w(θ), at 232 MHz from the MIYUN survey - tentative measurements of w(θ) are also performed at 151 MHz. It is found that double power law with shape w(θ) = A θ^{-γ} fits the 232 MHz data well. For the angular lenght of 0.2 degrees < θ< 0.6 degrees, γ~ -1.12, and this value of slope is independent of the flux-density threshold; while for angular lenghts much greater than 0.6 degrees, γhas a shallower value of about -0.16. By comparing the results of this paper with previous measurements of w(θ), it is discussed how w(θ) changes with the change of frequency and completness limit.

astro-ph.CO↗

arXiv2009

Clustering at 74 MHz

In order to construct accurate point sources simulations at the frequencies relevant to 21 cm experiments, the angular correlation of radio sources must be taken into account. Using the 74 MHz VLSS survey, we measured the angular 2-point correlation function, w(θ). We obtain the first measurement of clustering at the low frequencies relevant to 21 cm tomography. We find that a single power law with shape w(θ) = A θ^{-γ} fits well the data. For a galactic cut of 10 degrees, with a data cut of δless than -10 degrees, and a flux limit of S = 770 mJy, we obtain a slope of γ= (-1.2 +/- 0.35). This value of γis consistent with that measured from other radio catalogues at the millimeter wavelengths. The amplitude of clustering has a length of 0.2 degrees - 0.6 degrees, and it is independent of the flux-density threshold.

astro-ph.CO↗

arXiv2008

A model of diffuse Galactic Radio Emission from 10 MHz to 100 GHz

Understanding diffuse Galactic radio emission is interesting both in its own right and for minimizing foreground contamination of cosmological measurements. Cosmic Microwave Background experiments have focused on frequencies > 10 GHz, whereas 21 cm tomography of the high redshift universe will mainly focus on < 0.2 GHz, for which less is currently known about Galactic emission. Motivated by this, we present a global sky model derived from all publicly available total power large-area radio surveys, digitized with optical character recognition when necessary and compiled into a uniform format, as well as the new Villa Elisa data extending the 1.4 GHz map to the entire sky. We quantify statistical and systematic uncertainties in these surveys by comparing them with various global multi-frequency model fits. We find that a principal component based model with only three components can fit the 11 most accurate data sets (at 10, 22, 45 & 408 MHz and 1.4, 2.3, 23, 33, 41, 61, 94 GHz) to an accuracy around 1%-10% depending on frequency and sky region. Both our data compilation and our software returning a predicted all-sky map at any frequency from 10 MHz to 100 GHz are publicly available at http://space.mit.edu/home/angelica/gsm .

astro-ph↗

arXiv2006

CMB multipole measurements in the presence of foregrounds

Most analysis of Cosmic Microwave Background spherical harmonic coefficients a_lm has focused on estimating the power spectrum C_l=<|a_lm|^2> rather than the coefficients themselves. We present a minimum-variance method for measuring a_lm given anisotropic noise, incomplete sky coverage and foreground contamination, and apply it to the WMAP data. Our method is shown to constitute lossless data compression in the sense that the widely used quadratic estimators of the power spectrum C_l can be computed directly from our a_lm-estimators. As the Galactic cut is increased, the error bars Delta-a_lm on low multipoles go from being dominated by foregrounds to being dominated by sample variance from other multipoles, with the intervening minimum defining the optimal cut. Applying our method to the WMAP quadrupole and octopole, we find that their previously reported "axis of evil" alignment appears to be rather robust to Galactic cut and foreground contamination.

astro-ph↗

arXiv2004

The large-scale polarization of the microwave background and foreground

The DASI discovery of CMB polarization has opened a new chapter in cosmology. Most of the useful information about inflationary gravitational waves and reionization is on large angular scales where Galactic foreground contamination is the worst, so a key challenge is to model, quantify and remove polarized foregrounds. We use the POLAR experiment, COBE/DMR and radio surveys to provide the strongest limits to date on the TE cross power spectrum of the CMB on large angular scales and to quantify the polarized synchrotron radiation, which is likely to be the most challenging polarized contaminant for the MAP satellite. We find that the synchrotron E- and B-contributions are equal to within 10% from 408-820 MHz with a hint of E-domination at higher frequencies. We quantify Faraday Rotation and Depolarization effects in the two-dimensional (l,nu)-plane and show that they cause the synchrotron polarization percentage to drop both towards lower frequencies and towards lower multipoles.

astro-ph↗

arXiv2004

How accurately can suborbital experiments measure the CMB?

Great efforts are currently being channeled into ground- and balloon-based CMB experiments, mainly to explore anisotropy on small angular scales and polarization. To optimize instrumental design and assess experimental prospects, it is important to understand in detail the atmosphere-related systematic errors that limit the science achievable with new suborbital instruments. For this purpose, we spatially compare the 648 square degree ground- and balloon-based QMASK map with the atmosphere-free WMAP map, finding beautiful agreement on all angular scales where both are sensitive. This is a reassuring quantitative assessment of the power of the state-of-the-art FFT- and matrix-based mapmaking techniques that have been used for QMASK and virtually all subsequent experiments.

astro-ph↗

arXiv2004

The Cosmic Microwave Background and Its Polarization

The DASI discovery of CMB polarization, confirmed by WMAP, has opened a new chapter in cosmology. Most of the useful information about inflationary gravitational waves and reionization is on large angular scales where Galactic foreground contamination is the worst. The goal of the present review is to provide the state-of-the-art of the CMB polarization from a practical point of view, connecting real-world data to physical models. We present the physics of this polarized phenomena and illustrate how it depends of various cosmological parameters for standard adiabatic models. We also present all observational constraints to date and discuss how much we have learned about polarized foregrounds so far from the CMB studies. Finally, we comment on future prospects for the measurement of CMB polarization.

astro-ph↗

arXiv2003

The Quest for Microwave Foreground X

The WMAP team has produced a foreground map that can account for most of the low-frequency Galactic microwave emission in the WMAP maps, tentatively interpreting it as synchrotron emission. Finkbeiner and collaborators have challenged these conclusions, arguing that the WMAP team "synchrotron" template is in fact not dominated by synchrotron radiation, but by some dust-related Galactic emission process, perhaps spinning dust grains, making dramatically different predictions for its behavior at lower frequencies. By cross-correlating this "synchrotron" template with 10 and 15 GHz CMB observations, we find that its spectrum turns over in a manner consistent with spinning dust emission, falling about an order of magnitude below what the synchrotron interpretation would predict.

astro-ph↗

arXiv2003

The significance of the largest scale CMB fluctuations in WMAP

We investigate anomalies reported in the Cosmic Microwave Background maps from the Wilkinson Microwave Anisotropy Probe (WMAP) satellite on very large angular scales and discuss possible interpretations. Three independent anomalies involve the quadrupole and octopole: 1. The cosmic quadrupole on its own is anomalous at the 1-in-20 level by being low (the cut-sky quadrupole measured by the WMAP team is more strikingly low, apparently due to a coincidence in the orientation of our Galaxy of no cosmological significance); 2. The cosmic octopole on its own is anomalous at the 1-in-20 level by being very planar; 3. The alignment between the quadrupole and octopole is anomalous at the 1-in-60 level. Although the a priori chance of all three occurring is 1 in 24000, the multitude of alternative anomalies one could have looked for dilutes the significance of such a posteriori statistics. The simplest small universe model where the universe has toroidal topology with one small dimension of order half the horizon scale, in the direction towards Virgo, could explain the three items above. However, we rule this model out using two topological tests: the S-statistic and the matched circle test.

astro-ph↗

arXiv2003

A high resolution foreground cleaned CMB map from WMAP

We perform an independent foreground analysis of the WMAP maps to produce a cleaned CMB map (available online) useful for cross-correlation with, e.g., galaxy and X-ray maps. We use a variant of the Tegmark & Efstathiou (1996) technique that is completely blind, making no assumptions about the CMB power spectrum, the foregrounds, WMAP detector noise or external templates. Compared with the foreground-cleaned internal linear combination map produced by the WMAP team, our map has the advantage of containing less non-CMB power (from foregrounds and detector noise) outside the Galactic plane. The difference is most important on the the angular scale of the first acoustic peak and below, since our cleaned map is at the highest (13') rather than lowest (49') WMAP resolution. We also produce a Wiener filtered CMB map, representing our best guess as to what the CMB sky actually looks like, as well as CMB-free maps at the five WMAP frequencies useful for foreground studies. We argue that our CMB map is clean enough that the lowest multipoles can be measured without any galaxy cut, and obtain a quadrupole value that is slightly less low than that from the cut-sky WMAP team analysis. This can be understood from a map of the CMB quadrupole, which shows much of its power falling within the Galaxy cut region, seemingly coincidentally. Intriguingly, both the quadrupole and the octopole are seen to have power suppressed along a particular spatial axis, which lines up between the two, roughly towards (l,b) \~ (-110,60) in Virgo.

astro-ph↗

arXiv2003

CMB Polarization at Large Angular Scales: Data Analysis of the POLAR Experiment

The coming flood of CMB polarization experiments, spurred by the recent detection of CMB polarization by DASI and WMAP, will be confronted by many new analysis tasks specific to polarization. For the analysis of CMB polarization data sets, the devil is truly in the details. With this in mind, we present the details of the data analysis for the POLAR experiment, which recently led to the tightest upper limits on the polarization of the Cosmic Microwave Background Radiation at large angular scales. We discuss the data selection process, mapmaking algorithms, offset removal, and the likelihood analysis which were used to find upper limits on the polarization. Stated using the modern convention for reporting CMB Stokes parameters, these limits are 5.0 \uK on both E-type and B-type polarization at 95% confidence. Finally, we discuss simulations used to test our analysis techniques and to probe the fundamental limitations of the experiment.

astro-ph↗

arXiv2003

The Large-Scale Polarization of the Microwave Foreground

Most of the useful information about inflationary gravitational waves and reionization is on large angular scales where Galactic foreground contamination is the worst, so a key challenge is to model, quantify and remove polarized foregrounds. We use the Leiden radio surveys to quantify the polarized synchrotron radiation at large angular scales, which is likely to be the most challenging polarized contaminant for the WMAP satellite. We find that the synchrotron E- and B-contributions are equal to within 10% from 408-820MHz with a hint of E-domination at higher frequencies. We quantify Faraday Rotation & Depolarization effects and show that they cause the synchrotron polarization percentage to drop both towards lower frequencies and towards lower multipoles.

astro-ph↗

arXiv2002

E/B decomposition of finite pixelized CMB maps

Separation of the E and B components of a microwave background polarization map or a weak lensing map is an essential step in extracting science from it, but when the map covers only part of the sky and/or is pixelized, this decomposition cannot be done perfectly. We present a method for decomposing an arbitrary sky map into a sum of three orthogonal components that we term ``pure E'', ``pure B'' and ``ambiguous''. This method is useful both for providing intuition for experimental design and for analyzing data sets in practice. We show how to find orthonormal bases for all three components in terms of bilaplacian eigenfunctions. The number of ambiguous modes is proportional to the length of the map boundary so fairly round maps are preferred. For real-world data analysis, we present a simple matrix eigenvalue method for calculating nearly pure E and B modes in pixelized maps. We find that the dominant source of leakage between E and B is aliasing of small-scale power caused by the pixelization. This problem can be eliminated by heavily oversampling the map, but is exacerbated by the fact that the E power spectrum is expected to be much larger than the B power spectrum and extremely blue. We found that a factor of 2 to 3 more pixels are needed in a polarization map to achieve the same level of contamination by aliased power than in a temperature map.

astro-ph↗

arXiv2002

First attempt at measuring the CMB cross-polarization

We compute upper limits on CMB cross-polarization by cross-correlating the PIQUE and Saskatoon experiments. We also discuss theoretical and practical issues relevant to measuring cross-polarization and illustrate them with simulations of the upcoming BOOMERanG 2002 experiment. We present a method that separates all six polarization power spectra (TT, EE, BB, TE, TB, EB) without any other "leakage" than the familiar EE-BB mixing caused by incomplete sky coverage. Since E and B get mixed, one might expect leakage between TE and TB, between EE and EB and between BB and EB - our method eliminates this by preserving the parity symmetry under which TB and EB are odd and the other four power spectra are even.

astro-ph↗

arXiv2002

The CMB power spectrum at l=30-200 from QMASK

We measure the cosmic microwave background (CMB) power spectrum on angular scales l~30-200 (1-6 degrees) from the QMASK map, which combines the data from the QMAP and Saskatoon experiments. Since the accuracy of recent measurements leftward of the first acoustic peak is limited by sample-variance, the large area of the QMASK map (648 square degrees) allows us to place among the sharpest constraints to date in this range, in good agreement with BOOMERanG and (on the largest scales) COBE/DMR. By band-pass-filtering the QMAP and Saskatoon maps, we are able to spatially compare them scale-by-scale to check for beam- and pointing-related systematic errors.

astro-ph↗

arXiv2001

A New Spin on Galactic Dust

We present a new puzzle involving Galactic microwave emission and attempt to resolve it. On one hand, a cross-correlation analysis of the WHAM H-alpha map with the Tenerife 10 and 15 GHz maps shows that the well-known DIRBE correlated microwave emission cannot be dominated by free-free emission. On the other hand, recent high resolution observations in the 8-10 GHz range with the Green Bank 140 ft telescope by Finkbeiner et al. failed to find the corresponding 8 sigma signal that would be expected in the simplest spinning dust models. So what physical mechanism is causing this ubiquitous dust-correlated emission? We argue for a model predicting that spinning dust is the culprit after all, but that the corresponding small grains are well correlated with the larger grains seen at 100 micron only on large angular scales. In support of this grain segregation model, we find the best spinning dust template to involve higher frequency maps in the range 12-60 micron, where emission from transiently heated small grains is important. Upcoming CMB experiments such as ground-based interferometers, MAP and Planck LFI with high resolution at low frequencies should allow a definitive test of this model.

astro-ph↗

arXiv2001

How to measure CMB polarization power spectra without losing information

We present a method for measuring CMB polarization power spectra given incomplete sky coverage and test it with simulated examples such as Boomerang 2001 and MAP. By augmenting the quadratic estimator method with an additional step, we find that the E and B power spectra can be effectively disentangled on angular scales substantially smaller than the width of the sky patch in the narrowest direction. We find that the basic quadratic and maximum-likelihood methods display a unneccesary sensitivity to systematic errors when T-E cross-correlation is involved, and show how this problem can be eliminated at negligible cost in increased error bars. We also test numerically the widely used approximation that sample variance scales inversely with sky coverage, and find it to be an excellent approximation on scales substantially smaller than the sky patch.

astro-ph↗