arXiv subjects

Asantha Cooray

Publications and source records attributed to Asantha Cooray.

At least 235 records · Page 13Linked to original sources

arXiv2008

Contribution to Unresolved Infrared Fluctuations from Dwarf Galaxies at Redshifts of 2-3

In order to understand the origin of clustered anisotropies detected in Spitzer images between 3.6 and 8 microns, we stack the Spitzer IRAC/Great Observatories Origins Deep Survey (GOODS) images at pixel locations corresponding to faint, z_{AB}~27 mag, optical sources with no obvious IR counterparts. We obtain a strong detection of the sources with a stacked median flux at 3.6 microns of 130+/-5 nJy above the background. The wealth of multi-wavelength imaging data in GOODS enables a similar stacking analysis to be undertaken at various wavelengths between the ultraviolet and near-infrared bands. We obtain strong stacked detections of these optically faint sources over the entire wavelength range which places constraints on the average properties of these sources. We find that the flux spectrum of the median, stacked source is consistent with a L<0.03 L_{*,UV} galaxy with a 90% confidence interval for the redshift of 1.9-2.7. These sources produce a 3.6 micron absolute background intensity between 0.1 and 0.35 nW/m^2/sr and the clustered IR light could account for ~30-50% of fluctuation power in the IR background at 4 arcminute angular scales. Although the exact redshift distribution of these sources is unknown, these galaxies appear to contain 5-20% of the co-moving stellar mass density at z~2.5.

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arXiv2008

Cosmic Shear from Scalar-Induced Gravitational Waves

Weak gravitational lensing by foreground density perturbations generates a gradient mode in the shear of background images. In contrast, cosmological tensor perturbations induce a non-zero curl mode associated with image rotations. In this note, we study the lensing signatures of both primordial gravitational waves from inflation and second-order gravitational waves generated from the observed spectrum of primordial density fluctuations. We derive the curl mode for galaxy lensing surveys at redshifts of 1 to 3 and for lensing of the cosmic microwave background (CMB) at a redshift of 1100. We find that the curl mode angular power spectrum associated with secondary tensor modes for galaxy lensing surveys dominates over the corresponding signal generated by primary gravitational waves from inflation. However, both tensor contributions to the shear curl mode spectrum are below the projected noise levels of upcoming galaxy and CMB lensing surveys and therefore are unlikely to be detectable.

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arXiv2008

The inflationary gravitational-wave background and measurements of the scalar spectral index

Inflation predicts a stochastic background of gravitational waves over a broad range of frequencies, from those accessible with cosmic microwave background (CMB) measurements, to those accessible directly with gravitational-wave detectors, like NASA's Big-Bang Observer (BBO), currently under study. In a previous paper [Phys. Rev. D73, 023504 (2006)] we connected CMB constraints to the amplitude and tensor spectral index of the inflationary gravitational-wave background (IGWB) at BBO frequencies for four classes of models of inflation by directly solving the inflationary equations of motion. Here we extend that analysis by including results obtained in the WMAP third-year data release as well as by considering two additional classes of inflationary models. As often noted in the literature, the recent indication that the primordial density power-spectrum has a red spectral index implies (with some caveats) that the amplitude of the IGWB may be large enough to be observable in the CMB polarization. Here we also explore the implications for the direct detection of the IGWB.

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arXiv2008

Large Scale Structure as a Probe of Gravitational Slip

A new time-dependent, scale-independent parameter, \varpi, is employed in a phenomenological model of the deviation from General Relativity in which the Newtonian and longitudinal gravitational potentials slip apart on cosmological scales as dark energy, assumed to be arising from a new theory of gravitation, appears to dominate the universe. A comparison is presented between \varpi and other parameterized post-Friedmannian models in the literature. The effect of \varpi on the cosmic microwave background anisotropy spectrum, the growth of large scale structure, the galaxy weak-lensing correlation function, and cross-correlations of cosmic microwave background anisotropy with galaxy clustering are illustrated. Cosmological models with conventional maximum likelihood parameters are shown to find agreement with a narrow range of gravitational slip.

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arXiv2008

Lensing and Supernovae: Quantifying The Bias on the Dark Energy Equation of State

The gravitational magnification and demagnification of Type Ia supernovae (SNe) modify their positions on the Hubble diagram, shifting the distance estimates from the underlying luminosity-distance relation. This can introduce a systematic uncertainty in the dark energy equation of state (EOS) estimated from SNe, although this systematic is expected to average away for sufficiently large data sets. Using mock SN samples over the redshift range $0 < z \leq 1.7$ we quantify the lensing bias. We find that the bias on the dark energy EOS is less than half a percent for large datasets ($\gtrsim$ 2,000 SNe). However, if highly magnified events (SNe deviating by more than 2.5$σ$) are systematically removed from the analysis, the bias increases to $\sim$ 0.8%. Given that the EOS parameters measured from such a sample have a 1$σ$ uncertainty of 10%, the systematic bias related to lensing in SN data out to $z \sim 1.7$ can be safely ignored in future cosmological measurements.

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arXiv2008

The trispectrum of 21-cm background anisotropies as a probe of primordial non-Gaussianity

The 21-cm anisotropies from the neutral hydrogen distribution prior to the era of reionization is a sensitive probe of primordial non-Gaussianity. Unlike the case with cosmic microwave background, 21-cm anisotropies provide multi-redshift information with frequency selection and is not damped at arcminute angular scales. We discuss the angular trispectrum of the 21-cm background anisotropies and discuss how the trispectrum signal generated by the primordial non-Gaussianity can be measured with the three-to-one correlator and the corresponding angular power spectrum. We also discuss the separation of primordial non-Gaussian information in the trispectrum with that generated by the subsequent non-linear gravitational evolution of the density field. While with the angular bispectrum of 21-cm anisotropies one can limit the second order corrections to the primordial fluctuations below f_NL< 1, using the trispectrum information we suggest that the third order coupling term, f_2 or g_NL, can be constrained to be arounde 10 with future 21-cm observations over the redshift interval of 50 to 100.

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arXiv2008

Impact of Secondary non-Gaussianities on the Search for Primordial Non-Gaussianity with CMB Maps

When constraining the primordial non-Gaussianity parameter f_NL with cosmic microwave background anisotropy maps, the bias resulting from the covariance between primordial non-Gaussianity and secondary non-Gaussianities to the estimator of f_NL is generally assumed to be negligible. We show that this assumption may not hold when attempting to measure the primordial non-Gaussianity out to angular scales below a few tens arcminutes with an experiment like Planck, especially if the primordial non-Gaussianity parameter is around the minimum detectability level with f_NL between 5 and 10. In future, it will be necessary to jointly estimate the combined primordial and secondary contributions to the CMB bispectrum and establish f_NL by properly accounting for the confusion from secondary non-Gaussianities.

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arXiv2007

The Warm Spitzer Mission: Prospects for Studies of the Distant Universe

IRAC excels at detecting distant objects. Due to a combination of the shapes of the spectral energy distributions of galaxies and the low background achieved from space, IRAC reaches greater depth in comparable exposure time at 3.6 and 4.5 micron than any ground- or space-based facility currently can at 2.2 micron. Furthermore, the longer wavelengths probed by IRAC enable studies of the rest-frame optical and near-infrared light of galaxies and AGN to much higher redshift than is possible from the ground. This white paper explores the merits of different survey strategies for studying the distant universe during the warm mission. A three-tiered approach serves a wide range of science goals and uses the spacecraft effectively: 1) an ultra-deep survey of ~0.04 square degrees to a depth of ~250 hrs (in conjunction with an HST/WFC3 program), to study the Universe at 7 6 and characterize the relation between the build-up of dark matter halos and their constituent galaxies at 2<z<6, and 3) a 500 square degree survey to the SWIRE depth of 120 s, to systematically study large scale structure at 1<z<2 and characterize high redshift AGN. One or more of these programs could conceivably be implemented by the SSC, following the example of the Hubble Deep Field campaigns. As priorities in this field continuously shift it is also crucial that a fraction of the exposure time remains unassigned, thus enabling science that will reflect the frontiers of 2010 and beyond rather than those of 2007.

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arXiv2007

Anisotropy Studies of the Unresolved Far-infrared Background

Dusty, starforming galaxies and active galactic nuclei that contribute to the integrated background intensity at far-infrared wavelengths trace the large-scale structure. Below the point source detection limit, correlations in the large-scale structure lead to clustered anisotropies in the unresolved component of the far-infrared background (FIRB). The angular power spectrum of the FIRB anisotropies could be measured in large-area surveys with the Spectral and Photometric Imaging Receiver (SPIRE) on the upcoming Herschel observatory. To study statistical properties of these anisotropies, the confusion from foreground Galactic dust emission needs to be reduced even in the ``cleanest'' regions of the sky.The multi-frequency coverage of SPIRE allows the foreground dust to be partly separated from the extragalactic background composed of dusty starforming galaxies as well as faint normal galaxies. The separation improves for fields with sizes greater than a few hundred square degrees and when combined with Planck data. We show that an area of about $\sim$ 400 degrees$^2$ observed for about 1000 hours with Herschel-SPIRE and complemented by Planck provides maximal information on the anisotropy power spectrum. We discuss the scientific studies that can be done with measurements of the unresolved FIRB anisotropies including a determination of the large scale bias and the small-scale halo occupation distribution of FIRB sources with fluxes below the point-source detection level.

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arXiv2007

Red Density Perturbations and Inflationary Gravitational Waves

We study the implications of recent indications for a red spectrum of primordial density perturbations for the detection of inflationary gravitational waves (IGWs) with forthcoming cosmic microwave background experiments. We find that if inflation occurs with a single field with an inflaton potential minimized at V=0, then Planck will be able to detect IGWs at better than 2$σ$ confidence level, unless the inflaton potential is a power law with a very weak power. The proposed satellite missions of the Cosmic Vision and Inflation Probe programs will be able to detect IGWs from all the models we have surveyed at better than 5$σ$ confidence level. We provide an example of what is required if the IGW background is to remain undetected even by these latter experiments.

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arXiv2007

Narrowing Constraints with Type Ia Supernovae: Converging on a Cosmological Constant

We apply a parameterization-independent approach to fitting the dark energy equation-of-state (EOS). Utilizing the latest type Ia supernova data, combined with results from the cosmic microwave background and baryon acoustic oscillations, we find that the dark energy is consistent with a cosmological constant. We establish independent estimates of the evolution of the dark energy EOS by diagonalizing the covariance matrix. We find three independent constraints, which taken together imply that the equation of state is more negative than -0.2 at the 68% confidence level in the redshift range 0 1.2 and rolls to a value of -1 today, and ``thawing'' models where the EOS is near -1 at high redshifts, but rapidly evolves to values greater than -0.85 at z < 0.2. Finally, we propose a parameterization-independent figure-of-merit, to help assess the ability of future surveys to constrain dark energy. While previous figures-of-merit presume specific dark energy parameterizations, we suggest a binning approach to evaluate dark energy constraints with a minimum number of assumptions.

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arXiv2007

On the origin of the cosmic microwave background anisotropies

Suggestions have been made that the microwave background observed by COBE and WMAP and dubbed Cosmic Microwave Background (CMB) may have an origin within our own Galaxy or Earth. To consider the signal that may be correlated with Earth, a correlate-by-eye exercise was attempted by overlaying the CMB map from Wilkinson Microwave Anisotropy Probe on a topographical map of Earth. Remarkably, several hot spots in the CMB map are found to be well aligned with either large cities on Earth or regions of high altitude. To further study the correlations between Earth and CMB, we performed a complicated cross-correlation analysis in the multipole space. The overall correlations are detected at more than 5 sigma confidence level. These results can be naively interpreted to suggest that large angular scale fluctuations in CMB are generated on Earth by a process that traces the altitude relative to a mean radius. Simply extending our analysis, we suggest that cross-correlations between CMB and any other map of a Solar system body, image of a person, or an image of an animal will be detected at some statistical significance. It is unclear how Occam's razor can be applied in such a situation to identify which sources are responsible for CMB fluctuations.

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arXiv2007

Constraints on a New Post-General Relativity Cosmological Parameter

A new cosmological variable is introduced which characterizes the degree of departure from Einstein's General Relativity (GR) with a cosmological constant. The new parameter, \varpi, is the cosmological analog of γ, the parametrized post-Newtonian variable which measures the amount of spacetime curvature per unit mass. In the cosmological context, \varpi measures the difference between the Newtonian and longitudinal potentials in response to the same matter sources, as occurs in certain scalar-tensor theories of gravity. Equivalently, \varpi measures the scalar shear fluctuation in a dark energy component. In the context of a "vanilla" LCDM background cosmology, a non-zero \varpi signals a departure from GR or a fluctuating cosmological constant. Using a phenomenological model for the time evolution \varpi=\varpi_0 ρ_{DE}/ρ_{M} which depends on the ratio of energy density in the cosmological constant to the matter density at each epoch, it is shown that the observed cosmic microwave background (CMB) temperature anisotropies limit the overall normalization constant to be -0.4 < \varpi_0 < 0.1 at the 95% confidence level. Existing measurements of the cross-correlations of the CMB with large-scale structure further limit \varpi_0 > -0.2 at the 95% CL. In the future, integrated Sachs-Wolfe and weak lensing measurements can more tightly constrain \varpi_0, providing a valuable clue to the nature of dark energy and the validity of GR.

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arXiv2007

Searching for primordial black hole dark matter with pulsar timing arrays

We discuss the possibility of detecting the presence of primordial black holes (PBHs), such as those that might account for galactic dark matter, using modification of pulsar timing residuals when PBHs pass within ~1000 AU and impart impulse accelerations to the Earth. With this technique, PBHs with masses around 10^{25} g (~0.1 lunar mass) can be detected. Currently, the constraints on the abundance of such dark matter candidates are weak. A 30 year-long monitoring campaign with the proposed Square Kilometer Array (SKA) can rule out a PBH fraction more than ~1/10 in the solar neighborhood in the form of dark matter with mass ~10^{25} g.

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arXiv2007

Search for Gravitational Waves in the CMB After WMAP3: Foreground Confusion and The Optimal Frequency Coverage for Foreground Minimization

B-modes of Cosmic Microwave Background (CMB) polarization can be created by a primordial gravitational wave background. If this background was created by Inflation, then the amplitude of the polarization signal is proportional the energy density of the universe during inflation. The primordial signal will be contaminated by polarized foregrounds including dust and synchrotron emission within the galaxy. In light of the WMAP polarization maps, we consider the ability of several hypothetical CMB polarization experiments to separate primordial CMB B-mode signal from galactic foregrounds. We also study the optimization of a CMB experiment with a fixed number of detectors in the focal plane to determine how the detectors should be distributed in different frequency bands to minimize foreground confusion. We show that the optimal configuration requires observations in at least 5 channels spread over the frequency range between 30 GHz and 500 GHz with substantial coverage around 150 GHz. If a low-resolution space experiment using 1000 detectors to reach a noise level of about 1000 nK$^2$ concentrates on roughly 66% of the sky with the least foreground contamination the minimum detectable level of the tensor-to-scalar ratio would be about 0.002 at the 99% confidence level for an optical depth of 0.1.

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arXiv2007

Cross-correlation studies as a probe of reionization physics

The process of reionization is now believed to have proceeded in an orchestrated manner beginning with UV photons emitted by high redshift galaxies containing a large fraction of Population III stars carving out ionised regions around them. The physics during this era can be studied with a combination of redshifted 21-cm spin-flip transition tracing neutral hydrogen gas, IR emission from massive primordial stars that trace the global star-formation rate during reionization, and the imprint of hot-electrons in first supernovae remnants Compton-cooling off of cosmic microwave background (CMB) radiation through the Sunyaev-Zel'dovich effect. While these individual effects and their observable signatures have been advocated as probes of reionization history, here we show how cross-correlation studies between these signals can be used to further understand physics during reionization. Cross-correlation studies are advantageous since the measurable statistics do not suffer in the same manner from foregrounds and systematic effects as is the case of auto-correlation function measurements. We discuss the prospects for detecting various cross-correlation statistics using present and next generation experiments and the information related to reionization captured by them.

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arXiv2006

IR Background Anisotropies in Spitzer GOODS images and constraints on first galaxies

We describe the angular power spectrum of unresolved 3.6 micron IR light in Spitzer GOODS fields. The amplitude of the anisotropy spectrum decreases with decreasing flux threshold to which resolved sources are removed from images. When all pixels brighter than a Vega magnitude of 24.6 are removed, the amplitude of the power spectrum at arcminute angular scales can be described with an extra component of z>8 sources with a IRB contribution around 0.4 nW m^-2 sr-1. The shape of the power spectrum, however, is more consistent with that expected for unresolved, faint galaxies at lower redshifts with Vega magnitudes fainter than 23 with a total 3.6 micron intensity between 0.1 to 0.8 nW m^-2 sr^-1. We confirm this assumption by showing that large-scale power decreases rapidly when the unresolved clustering spectrum is measured from a processed HDF-N IRAC image where locations of faint ACS sources with no IR counterparts were also masked. Based on resolved counts and unresolved fluctuations, we find that, at most, about 7.0 nW m^-2 sr^-1 can be ascribed to galaxies.

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arXiv2006

Clustering of the IR Background Light with Spitzer: Contribution from Resolved Sources

We describe the angular power spectrum of resolved sources at 3.6 microns (L-band) in Spitzer imaging data of the GOODS HDF-N, the GOODS CDF-S, and the NDWFS Bootes field in several source magnitude bins. We also measure angular power spectra of resolved sources in the Bootes field at K_S and J-bands using ground-based IR imaging data. In the three bands, J, K_S, and L, we detect the clustering of galaxies on top of the shot-noise power spectrum at multipoles between ell ~ 10^2 and 10^5. The angular power spectra range from the large, linear scales to small, non-linear scales of galaxy clustering, and in some magnitude ranges, show departure from a power-law clustering spectrum. We consider a halo model to describe clustering measurements and to establish the halo occup ation number parameters of IR bright galaxies at redshifts around one. We also extend our clustering results and completeness-corrected faint source number counts in GOODS fields to understand the underlying nature of unresolved sources responsible for IR background (IRB) anisotropies that were detected in deep Spitzer images. While these unresolved fluctuations were measured at sub-arcminute angular scales, if a high-redshift diffuse component associated with first galaxies exists in the IRB, then it's clustering properties are best studied with shallow, wide-field images that allow a measurement of the clustering spectrum from a few degrees to arcminute angular scales.

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