arXiv subjects

Albert Stebbins

Publications and source records attributed to Albert Stebbins.

At least 55 records · Page 3Linked to original sources

arXiv2010

A ground-based 21cm Baryon acoustic oscillation survey

Baryon acoustic oscillations (BAO) provide a robust standard ruler with which to measure the acceleration of the Universe. The BAO feature has so far been detected in optical galaxy surveys. Intensity mapping of neutral hydrogen emission with a ground-based radio telescope provides another promising window for measuring BAO at redshifts of order unity for relatively low cost. While the cylindrical radio telescope (CRT) proposed for these measurements will have excellent redshift resolution, it will suffer from poor angular resolution (a few arcminutes at best). We investigate the effect of angular resolution on the standard ruler test with BAO, using the Dark Energy Task Force Figure of Merit as a benchmark. We then extend the analysis to include variations in the parameters characterizing the telescope and the underlying physics. Finally, we optimize the survey parameters (holding total cost fixed) and present an example of a CRT BAO survey that is competitive with Stage III dark energy experiments. The tools developed here form the backbone of a publicly available code that can be used to obtain estimates of cost and Figure of Merit for any set of parameters.

astro-ph.CO↗

arXiv2009

The Sloan Nearby Cluster Weak Lensing Survey

We describe and present initial results of a weak lensing survey of nearby ($\rm{z}\lesssim0.1$) galaxy clusters in the Sloan Digital Sky Survey (SDSS). In this first study, galaxy clusters are selected from the SDSS spectroscopic galaxy cluster catalogs of \citet{miller05} and \citet{berlind06}. We report a total of seven individual low redshift cluster weak lensing measurements which include: A2048, A1767, A2244, A1066, A2199, and two clusters specifically identified with the C4 algorithm. Our program of weak lensing of nearby galaxy clusters in the SDSS will eventually reach $\sim 200$ clusters, making it the largest weak lensing survey of individual galaxy clusters to date.

astro-ph.CO↗

arXiv2009

21 cm Intensity Mapping

Using the 21 cm line, observed all-sky and across the redshift range from 0 to 5, the large scale structure of the Universe can be mapped in three dimensions. This can be accomplished by studying specific intensity with resolution ~ 10 Mpc, rather than via the usual galaxy redshift survey. The data set can be analyzed to determine Baryon Acoustic Oscillation wavelengths, in order to address the question: 'What is the nature of Dark Energy?' In addition, the study of Large Scale Structure across this range addresses the questions: 'How does Gravity effect very large objects?' and 'What is the composition our Universe?' The same data set can be used to search for and catalog time variable and transient radio sources.

astro-ph.IM↗

arXiv2008

The PAMELA and ATIC Excesses From a Nearby Clump of Neutralino Dark Matter

In this letter, we suggest that a nearby clump of 600-1000 GeV neutralinos may be responsible for the excesses recently observed in the cosmic ray positron and electron spectra by the PAMELA and ATIC experiments. Although neutralino dark matter annihilating throughout the halo of the Milky Way is predicted to produce a softer spectrum than is observed, and violate constraints from cosmic ray antiproton measurements, a large nearby (within 1-2 kiloparsecs of the Solar System) clump of annihilating neutralinos can lead to a spectrum which is consistent with PAMELA and ATIC, while also producing an acceptable antiproton flux. Furthermore, the presence of a large dark matter clump can potentially accommodate the very large annihilation rate required to produce the PAMELA and ATIC signals.

hep-ph↗

arXiv2008

A new method of measuring the cluster peculiar velocity power spectrum

We propose to use spatial correlations of the kinetic Sunyaev-Zeldovich (KSZ) flux as an estimator of the peculiar velocity power spectrum. In contrast with conventional techniques, our new method does not require measurements of the thermal SZ signal or the X-ray temperature. Moreover, this method has the special advantage that the expected systematic errors are always sub-dominant to statistical errors on all scales and redshifts of interest. We show that future large sky coverage KSZ surveys may allow a peculiar velocity power spectrum estimates of an accuracy reaching ~10%.

astro-ph↗

arXiv2007

The Mass Of The Coma Cluster From Weak Lensing In The Sloan Digital Sky Survey

We present a weak lensing analysis of the Coma Cluster using the Sloan Digital Sky Survey (SDSS) Data Release Five. Complete imaging of a ~ 200 square degree region is used to measure the tangential shear of this cluster. The shear is fit to an NFW model and we find a virial radius of r_{200}=1.99_{-0.22}^{+0.21}h^{-1}Mpc which corresponds to a virial mass of M_{200}=1.88_{-0.56}^{+0.65}\times10^{15}h^{-1}M_{\odot}. We additionally compare our weak lensing measurement to the virial mass derived using dynamical techniques, and find they are in agreement. This is the lowest redshift, largest angle weak lensing measurement of an individual cluster to date.

astro-ph↗

arXiv2007

CMB Spectral Distortions from the Scattering of Temperature Anisotropies

Thomson scattering of CMBR temperature anisotropies will cause the spectrum of the CMBR to differ from blackbody even when one resolves all anisotropies. A formalism for computing the anisotropic and inhomogeneous spectral distortions of intensity and polarization is derived in terms of Lorentz invariant central moments of the temperature distribution. The formalism is non-perturbative, requiring neither small anisotropies nor small metric or matter inhomogeneities; but it does assume cold electrons. The low order moments are not coupled to the higher order moments allowing one to truncate the equations without any loss of accuracy. This formalism is applied to a standard Lambda-CDM cosmology after reionization where the temperature anisotropies are dominated by the Doppler effect for the bulk motion of the gas with respect to the CMBR frame. The resultant spectral distortion is parameterized by u ~ 3e-8, where in this case u is observationally degenerate with the Sunyaev-Zel'dovich (SZ) y parameter. In comparison the expected thermal SZ y-distortion from the hot IGM is expected to be >30 times larger. However at z >5 the effect described here would have been the dominant source of spectral distortions. The effect could be much larger in non-standard cosmologies.

astro-ph↗

arXiv2005

Dark Energy Studies: Challenges to Computational Cosmology

The ability to test the nature of dark mass-energy components in the universe through large-scale structure studies hinges on accurate predictions of sky survey expectations within a given world model. Numerical simulations predict key survey signatures with varying degrees of confidence, limited mainly by the complex astrophysics of galaxy formation. As surveys grow in size and scale, systematic uncertainties in theoretical modeling can become dominant. Dark energy studies will challenge the computational cosmology community to critically assess current techniques, develop new approaches to maximize accuracy, and establish new tools and practices to efficiently employ globally networked computing resources.

astro-ph↗

arXiv2005

Constraining Dark Energy with the Dark Energy Survey: Theoretical Challenges

The Dark Energy Survey (DES) will use a new imaging camera on the Blanco 4-m telescope at CTIO to image 5000 square degrees of sky in the South Galactic Cap in four optical bands, and to carry out repeat imaging over a smaller area to identify and measure lightcurves of Type Ia supernovae. The main imaging area overlaps the planned Sunyaev-Zel'dovich survey of the South Pole Telescope. The idea behind DES is to use four distinct and largely independent methods to probe the properties of dark energy: baryon oscillations of the power spectrum, abundance and spatial distribution of clusters, weak gravitational lensing, and Type Ia supernovae. This white paper outlines, in broad terms, some of the theoretical issues associated with the first three of these probes (the issues for supernovae are mostly different in character), and with the general task of characterizing dark energy and distinguishing it from alternative explanations for cosmic acceleration. A companion white paper discusses the kind of numerical simulations and other theoretical tools that will be needed to address the these issues and to create mock catalogs that allow end-to-end tests of analysis procedures. Although we have been thinking about these problems in the specific context of DES, many of them are also relevant to other planned dark energy studies.

astro-ph↗

arXiv2004

A new method of measuring the peculiar velocity power spectrum

We show that by directly correlating the cluster kinetic Sunyaev Zeldovich (KSZ) flux, the cluster peculiar velocity power spectrum can be measured to $\sim 10%$ accuracy by future large sky coverage KSZ surveys. This method is almost free of systemics entangled in the usual velocity inversion method. The direct correlation brings extra information of density and velocity clustering. We utilize these information to construct two indicators of the Hubble constant and comoving angular distance and propose a novel method to constrain cosmology.

astro-ph↗

arXiv2003

Isolating Geometry in Weak Lensing Measurements

Given a foreground galaxy-density field or shear field, its cross-correlation with the shear field from a background population of source galaxies scales with the source redshift in a way that is specific to lensing. Such a source-scaling can be exploited to effectively measure geometrical distances as a function of redshift and thereby constrain dark energy properties, free of any assumptions about the galaxy-mass/mass power spectrum (its shape, amplitude or growth). Such a geometrical method can yield a ~ 0.03 - 0.07 f_{sky}^{-1/2} measurement on the dark energy abundance and equation of state, for a photometric redshift accuracy of dz ~ 0.01 - 0.05 and a survey with median redshift of ~ 1. While these constraints are weaker than conventional weak lensing methods, they provide an important consistency check because the geometrical method carries less theoretical baggage: there is no need to assume any structure formation model (e.g. CDM). The geometrical method is at the most conservative end of a whole spectrum of methods which obtain smaller errorbars by making more restrictive assumptions -- we discuss some examples. Our geometrical approach differs from previous investigations along similar lines in three respects. First, the source-scaling we propose to use is less demanding on the photometric redshift accuracy. Second, the scaling works for both galaxy-shear and shear-shear correlations. Third, we find that previous studies underestimate the statistical errors associated with similar geometrical methods, the origin of which is discussed.

astro-ph↗

arXiv2003

Primordial Gravity Waves and Weak Lensing

Inflation produces a primordial spectrum of gravity waves in addition to the density perturbations which seed structure formation. We compute the signature of these gravity waves in the large scale shear field. In particular, the shear can be divided into a gradient mode (G or E) and a curl mode (C or B). The former is produced by both density perturbations and gravity waves, while the latter is produced only by gravity waves, so the observations of a non-zero curl mode could be seen as evidence for inflation. We find that the expected signal from inflation is small, peaking on the largest scales at $l(l+1)C_l/2π< 10^{-11}$ at $l=2$ and falling rapidly there after. Even for an all-sky deep survey, this signal would be below noise at all multipoles. Part of the reason for the smallness of the signal is a cancellation on large scales of the standard line-of-sight effect and the effect of ``metric shear.''

astro-ph↗

arXiv2002

Analysis of Systematic Effects and Statistical Uncertainties in Angular Clustering of Galaxies from Early SDSS Data

The angular distribution of galaxies encodes a wealth of information about large scale structure. Ultimately, the Sloan Digital Sky Survey (SDSS) will record the angular positions of order 10^8 galaxies in five bands, adding significantly to the cosmological constraints. This is the first in a series of papers analyzing a rectangular stripe 2.5x90 degrees from early SDSS data. We present the angular correlation function for galaxies in four separate magnitude bins on angular scales ranging from 0.003 degrees to 15 degrees. Much of the focus of this paper is on potential systematic effects. We show that the final galaxy catalog -- with the mask accounting for regions of poor seeing, reddening, bright stars, etc. -- is free from external and internal systematic effects for galaxies brighter than r* = 22. Our estimator of the angular correlation function includes the effects of the integral constraint and the mask. The full covariance matrix of errors in these estimates is derived using mock catalogs with further estimates using a number of other methods.

astro-ph↗

arXiv2002

The Angular Power Spectrum of Galaxies from Early SDSS Data

We compute the angular power spectrum C_l from 1.5 million galaxies in early SDSS data on large angular scales, l<600. The data set covers about 160 square degrees, with a characteristic depth of order 1 Gpc/h in the faintest (21<r<22) of our four magnitude bins. Cosmological interpretations of these results are presented in a companion paper by Dodelson et al (2001). The data in all four magnitude bins are consistent with a simple flat ``concordance'' model with nonlinear evolution and linear bias factors of order unity. Nonlinear evolution is particularly evident for the brightest galaxies. A series of tests suggest that systematic errors related to seeing, reddening, etc., are negligible, which bodes well for the sixtyfold larger sample that the SDSS is currently collecting. Uncorrelated error bars and well-behaved window functions make our measurements a convenient starting point for cosmological model fitting.

astro-ph↗

arXiv2001

Higher Order Moments of the Angular Distribution of Galaxies from Early SDSS Data

We present initial results for counts in cells statistics of the angular distribution of galaxies in early data from the Sloan Digital Sky Survey (SDSS). We analyze a rectangular stripe $2.5^\circ$ wide, covering approximately 160 sq. degrees, containing over $10^6$ galaxies in the apparent magnitude range $18 < r^\prime < 22$, with areas of bad seeing, contamination from bright stars, ghosts, and high galactic extinction masked out. This survey region, which forms part of the SDSS Early Data Release, is the same as that for which two-point angular clustering statistics have recently been computed. The third and fourth moments of the cell counts, $s_3$ (skewness) and $s_4$ (kurtosis), constitute the most accurate measurements to date of these quantities (for $r^\prime < 21$) over angular scales $0.015^\circ-0.3^\circ$. They display the approximate hierarchical scaling expected from non-linear structure formation models and are in reasonable agreement with the predictions of $Λ$-dominated cold dark matter models with galaxy biasing that suppresses higher order correlations at small scales. The results are in general consistent with previous measurements in the APM, EDSGC, and Deeprange surveys. These results suggest that the SDSS imaging data are free of systematics to a high degree and will therefore enable determination of the skewness and kurtosis to 1% and less then 10%, as predicted by Colombi, Szapudi, & Szalay (1998).

astro-ph↗

arXiv2001

Galaxy Mass and Luminosity Scaling Laws Determined by Weak Gravitational Lensing

We present new measurements of scaling laws relating the luminosity of galaxies to the amplitude and shape of their dark matter halos. Early imaging and spectroscopic data from the Sloan Digital Sky Survey are used to make weak lensing measurements of the surface mass density contrast Delta Sigma_+ around classes of lens objects. This surface mass density contrast as a function of radius is a measure of the galaxy-mass correlation function (GMCF). Because spectroscopic redshifts are available for all lens objects, the mass and distance scales are well constrained. The GMCF measured around ~31,000 lenses is well fit by a power law of the form Delta Sigma_+ = (2.5+0.7-0.6) (R/1 Mpc)^{-0.8+-0.2} h M_sun pc^-2. We compare this GMCF to galaxy luminosity, type, and environment, and find that it varies strongly with all three. We quantify these variations by comparing the normalization of a fit to the inner 260 h^-1 kpc, M_260, to the galaxy luminosity. While M_260 is not strongly related to luminosity in bluest band u', there is a simple, linear relation between M_260 and luminosity in redder bands (g', r', i', and z'). We test the universality of these mass-to-light scalings by independently measuring them for spiral and elliptical galaxies,and for galaxies in a variety of environments. We find remarkable consistency in these determinations in the red bands, especially i' and z'. This consistency across a wide range of systems suggests that the measured scaling represents an excellent cosmic average, and that the integrated star formation history of galaxies is strongly related to the dark matter environments in which they form.

astro-ph↗

arXiv2001

KL Estimation of the Power Spectrum Parameters from the Angular Distribution of Galaxies in Early SDSS Data

We present measurements of parameters of the 3-dimensional power spectrum of galaxy clustering from 222 square degrees of early imaging data in the Sloan Digital Sky Survey. The projected galaxy distribution on the sky is expanded over a set of Karhunen-Loeve eigenfunctions, which optimize the signal-to-noise ratio in our analysis. A maximum likelihood analysis is used to estimate parameters that set the shape and amplitude of the 3-dimensional power spectrum. Our best estimates are Gamma=0.188 +/- 0.04 and sigma_8L = 0.915 +/- 0.06 (statistical errors only), for a flat Universe with a cosmological constant. We demonstrate that our measurements contain signal from scales at or beyond the peak of the 3D power spectrum. We discuss how the results scale with systematic uncertainties, like the radial selection function. We find that the central values satisfy the analytically estimated scaling relation. We have also explored the effects of evolutionary corrections, various truncations of the KL basis, seeing, sample size and limiting magnitude. We find that the impact of most of these uncertainties stay within the 2-sigma uncertainties of our fiducial result.

astro-ph↗

arXiv2001

The Angular Correlation Function of Galaxies from Early SDSS Data

The Sloan Digital Sky Survey is one of the first multicolor photometric and spectroscopic surveys designed to measure the statistical properties of galaxies within the local Universe. In this Letter we present some of the initial results on the angular 2-point correlation function measured from the early SDSS galaxy data. The form of the correlation function, over the magnitude interval 18<r*<22, is shown to be consistent with results from existing wide-field, photographic-based surveys and narrower CCD galaxy surveys. On scales between 1 arcminute and 1 degree the correlation function is well described by a power-law with an exponent of ~ -0.7. The amplitude of the correlation function, within this angular interval, decreases with fainter magnitudes in good agreement with analyses from existing galaxy surveys. There is a characteristic break in the correlation function on scales of approximately 1-2 degrees. On small scales, < 1', the SDSS correlation function does not appear to be consistent with the power-law form fitted to the 1'< theta <0.5 deg data. With a data set that is less than 2% of the full SDSS survey area, we have obtained high precision measurements of the power-law angular correlation function on angular scales 1' < theta < 1 deg, which are robust to systematic uncertainties. Because of the limited area and the highly correlated nature of the error covariance matrix, these initial results do not yet provide a definitive characterization of departures from the power-law form at smaller and larger angles. In the near future, however, the area of the SDSS imaging survey will be sufficient to allow detailed analysis of the small and large scale regimes, measurements of higher-order correlations, and studies of angular clustering as a function of redshift and galaxy type.

astro-ph↗