arXiv subjects

D. N. Spergel

Publications and source records attributed to D. N. Spergel.

72 records · Page 4Linked to original sources

arXiv2003

First Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Data Processing Methods and Systematic Errors Limits

We describe the calibration and data processing methods used to generate full-sky maps of the cosmic microwave background (CMB) from the first year of Wilkinson Microwave Anisotropy Probe (WMAP) observations. Detailed limits on residual systematic errors are assigned based largely on analyses of the flight data supplemented, where necessary, with results from ground tests. The data are calibrated in flight using the dipole modulation of the CMB due to the observatory's motion around the Sun. This constitutes a full-beam calibration source. An iterative algorithm simultaneously fits the time-ordered data to obtain calibration parameters and pixelized sky map temperatures. The noise properties are determined by analyzing the time-ordered data with this sky signal estimate subtracted. Based on this, we apply a pre-whitening filter to the time-ordered data to remove a low level of 1/f noise. We infer and correct for a small ~1% transmission imbalance between the two sky inputs to each differential radiometer, and we subtract a small sidelobe correction from the 23 GHz (K band) map prior to further analysis. No other systematic error corrections are applied to the data. Calibration and baseline artifacts, including the response to environmental perturbations, are negligible. Systematic uncertainties are comparable to statistical uncertainties in the characterization of the beam response. Both are accounted for in the covariance matrix of the window function and are propagated to uncertainties in the final power spectrum. We characterize the combined upper limits to residual systematic uncertainties through the pixel covariance matrix.

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arXiv2003

First Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Angular Power Spectrum

We present the angular power spectrum derived from the first-year Wilkinson Microwave Anisotropy Probe (WMAP) sky maps. We study a variety of power spectrum estimation methods and data combinations and demonstrate that the results are robust. The data are modestly contaminated by diffuse Galactic foreground emission, but we show that a simple Galactic template model is sufficient to remove the signal. Point sources produce a modest contamination in the low frequency data. After masking ~700 known bright sources from the maps, we estimate residual sources contribute ~3500 uK^2 at 41 GHz, and ~130 uK^2 at 94 GHz, to the power spectrum l*(l+1)*C_l/(2*pi) at l=1000. Systematic errors are negligible compared to the (modest) level of foreground emission. Our best estimate of the power spectrum is derived from 28 cross-power spectra of statistically independent channels. The final spectrum is essentially independent of the noise properties of an individual radiometer. The resulting spectrum provides a definitive measurement of the CMB power spectrum, with uncertainties limited by cosmic variance, up to l~350. The spectrum clearly exhibits a first acoustic peak at l=220 and a second acoustic peak at l~540 and it provides strong support for adiabatic initial conditions. Kogut et al. (2003) analyze the C_l^TE power spectrum, and present evidence for a relatively high optical depth, and an early period of cosmic reionization. Among other things, this implies that the temperature power spectrum has been suppressed by \~30% on degree angular scales, due to secondary scattering.

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arXiv2003

Primordial Black Holes as Dark Matter: The Power Spectrum and Evaporation of Early Structures

We consider the possibility that massive primordial black holes are the dominant form of dark matter. Black hole formation generates entropy fluctuations that adds a Poisson noise to the matter power spectrum. We use Lyman-alpha forest observations to constrain this Poisson term in matter power spectrum, then we constrain the mass of black holes to be less than few times 10^4 solar mass. We also find that structures with less than ~ 10^3 primordial black holes evaporate by now.

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arXiv2003

The Microwave Anisotropy Probe (MAP) Mission

The purpose of the MAP mission is to determine the geometry, content, and evolution of the universe via a 13 arcmin full-width-half-max (FWHM) resolution full sky map of the temperature anisotropy of the cosmic microwave background radiation with uncorrelated pixel noise, minimal systematic errors, multifrequency observations, and accurate calibration. These attributes were key factors in the success of NASA's Cosmic Background Explorer (COBE) mission, which made a 7 degree FWHM resolution full sky map, discovered temperature anisotropy, and characterized the fluctuations with two parameters, a power spectral index and a primordial amplitude. Following COBE considerable progress has been made in higher resolution measurements of the temperature anisotropy. With 45 times the sensitivity and 33 times the angular resolution of the COBE mission, MAP will vastly extend our knowledge of cosmology. MAP will measure the physics of the photon-baryon fluid at recombination. From this, MAP measurements will constrain models of structure formation, the geometry of the universe, and inflation. In this paper we present a pre-launch overview of the design and characteristics of the MAP mission. This information will be necessary for a full understanding of the MAP data and results, and will also be of interest to scientists involved in the design of future cosmic microwave background experiments and/or space science missions.

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arXiv2003

The Optical Design and Characterization of the Microwave Anisotropy Probe

The primary goal of the MAP satellite, now in orbit, is to make high fidelity polarization sensitive maps of the full sky in five frequency bands between 20 and 100 GHz. From these maps we will characterize the properties of the cosmic microwave background (CMB) anisotropy and Galactic and extragalactic emission on angular scales ranging from the effective beam size, <0.23 degree, to the full sky. MAP is a differential microwave radiometer. Two back-to-back shaped offset Gregorian telescopes feed two mirror symmetric arrays of ten corrugated feeds. We describe the prelaunch design and characterization of the optical system, compare the optical models to the measurements, and consider multiple possible sources of systematic error.

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arXiv2002

LSST: a Complementary Probe of Dark Energy

The number of mass clusters and their distribution in redshift are very sensitive to the density of matter Omega_m and the equation of state of dark energy w. Using weak lens gravitational tomography one can detect clusters of dark matter, weigh them, image their projected mass distribution, and determine their 3-D location. The degeneracy curve in the Omega_m - w plane is nearly orthogonal to that from CMB or SN measurements. Thus, a combination of CMB data with weak lens tomography of clusters can yield precision measurements of Omega_m and w, independently of the SN observations. The Large Synoptic Survey Telescope (LSST) will repeatedly survey 30,000 square degrees of the sky in multiple wavelengths. LSST will create a 3-D tomographic assay of mass overdensities back to half the age of the universe by measuring the shear and color-redshift of billions of high redshift galaxies. By simultaneously measuring several functions of cosmic shear and mass cluster abundance, LSST will provide a number of independent constraints on the dark energy density and the equation of state.

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arXiv2001

Gravitational Lens Statistics for Generalized NFW Profiles: Parameter Degeneracy and Implications for Self-Interacting Cold Dark Matter

Strong lensing is a powerful probe of the distribution of matter in the cores of clusters of galaxies. Recent studies suggest that the cold dark matter model predicts cores that are denser than those observed in galaxies, groups and clusters. One possible resolution of the discrepancy is that the dark matter has strong interactions (SIDM), which leads to lower central densities. A generalized form of the Navarro, Frenk and White profile (Zhao profile) may be used to describe these halos. In this paper we examine gravitational lensing statistics for this class of model. The optical depth to multiple imaging is a very sensitive function of the profile parameters in the range of interest for SIDM halos around clusters of galaxies. Less concentrated profiles, which result from larger self-interaction cross-sections, can produce many fewer lensed pairs. Lensing statistics provide a powerful test for SIDM. More realistic and observationally oriented calculations remain to be done, however larger self-interaction cross-sections may well be ruled out by the very existence of strong lenses on galaxy cluster scales. The inclusion of centrally dominant cluster galaxies should boost the cross-section to multiple imaging. However our preliminary calculations suggest that the additional multiple imaging rate is small with respect to the differences in multiple imaging rate for different halo profiles. In future statistical studies, it will be important to properly account for the scatter among halo profiles since the optical depth to multiple imaging is dominated by the most concentrated members of a cluster population.

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arXiv2001

Three Dimensional Structure of the Milky Way Disk

We present a three dimensional model for the Milky Way fit to the far-infrared (FIR) and near-infrared (NIR) data from the COBE/DIRBE instrument for Galactic latitudes |b| < 30 degrees and to within 20 degrees of the Galactic center. Because of the low optical depth at 240 microns, the FIR emission traces the distribution of galactic dust in the Galaxy. We model the dust distribution as due to three components: a warped exponential disk with scale length 0.28 R_sun and a flaring scale height, a spiral arm component with four arms as traced by Galactic HII regions, and the local (Orion) arm which produces prominent emission features at galactic longitude l approx. 80 and -100 degrees. A Cosmic Infrared Background of 1.07 MJy/sr is recovered, consistent with previous determinations. The dust distribution is then used to calculate absorption in J and K, and the stellar emission in these wavebands is modeled with two components: a warped exponential disk with a scale length of 0.28 R_sun and a spiral arm component dominated by two arms. This small scale length is consistent with a maximal disk model for our Galaxy, which is inconsistent with the cuspy dark matter halos predicted in CDM models. We find different amplitudes for the warp in the stars and dust, which starts within the Solar Circle.

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arXiv2001

The Coalescence Rate of Double Neutron Star Systems

We estimate the coalescence rate of close binaries with two neutron stars (NS) and discuss the prospects for the detection of NS-NS inspiral events by ground-based gravitational-wave observatories, such as LIGO. We derive the Galactic coalescence rate using the observed sample of close NS-NS binaries (PSR B1913+16 and PSR B1534+12) and examine in detail each of the sources of uncertainty associated with the estimate. Specifically, we investigate (i) the dynamical evolution of NS-NS binaries in the Galactic potential and the vertical scale height of the population, (ii) the pulsar lifetimes, (iii) the effects of the faint end of the radio pulsar luminosity function and their dependence on the small number of observed objects, (iv) the beaming fraction, and (v) the extrapolation of the Galactic rate to extragalactic distances expected to be reachable by LIGO. We find that the dominant source of uncertainty is the correction factor (up to about 200) for faint (undetectable) pulsars. All other sources are much less important, each with uncertainty factors smaller than 2. Despite the relatively large uncertainty, the derived coalescence rate is approximately consistent with previously derived upper limits, and is more accurate than rates obtained from population studies. We obtain a most conservative lower limit for the LIGO II detection rate of 2 events per year. Our upper limit on the detection rate lies between 300 to more than 1000 events per year.

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arXiv2000

Extragalactic Foregrounds of the Cosmic Microwave Background: Prospects for the MAP Mission

(Abridged) While the major contribution to the Cosmic Microwave Background (CMB) anisotropies are the sought-after primordial fluctuations produced at the surface of last scattering, other effects produce secondary fluctuations at lower redshifts. Here, we study the extragalactic foregrounds of the CMB in the context of the upcoming MAP mission. We first survey the major extragalactic foregrounds and show that discrete sources, the Sunyaev-Zel'dovich (SZ) effect, and gravitational lensing are the most dominant ones for MAP. We then show that MAP will detect (>5 sigma) about 46 discrete sources and 10 SZ clusters directly with 94 GHz fluxes above 2 Jy. The mean SZ fluxes of fainter clusters can be probed by cross-correlating MAP with cluster positions extracted from existing catalogs. For instance, a MAP-XBACs cross-correlation will be sensitive to clusters with S(94GHz)>200mJy, and will thus provide a test of their virialization state and a measurement of their gas fraction. Finally, we consider probing the hot gas on supercluster scales by cross-correlating the CMB with galaxy catalogs. Assuming that galaxies trace the gas, we show that a cross-correlation between MAP and the APM catalog should yield a marginal detection, or at least a four-fold improvement on the COBE upper limits for the rms Compton y-parameter.

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arXiv2000

The Power Spectrum of the Sunyaev-Zel'dovich Effect

(Abridged) The hot gas in the IGM produces anisotropies in the Cosmic Microwave Background (CMB) through the thermal Sunyaev-Zel'dovich (SZ) effect. The SZ effect is a powerful probe of large-scale structure in the universe and must be carefully subtracted from measurements of the primary CMB anisotropies. We use moving-mesh hydrodynamical simulations to study the 3-dimensional statistics of the gas, and compute the mean comptonization parameter and the angular power spectrum of the SZ fluctuations, for different cosmologies. We compare these results with predictions using the Press-Schechter formalism. We find that the two methods agree approximately, but differ in details. We discuss this discrepancy, and show that resolution limits the reliability of our results to the 200<l<2000 range. For cluster- normalized CDM models, the SZ power spectrum is comparable to the primordial power spectrum around l=2000. We show that groups and filaments (kT<5 keV) contribute about 50% of the SZ power spectrum at l=500. About half of the SZ power spectrum on these scales is produced at redshifts z<0.1, and can thus be detected and removed using existing catalogs of galaxies and X-ray clusters. We discuss the implications of these results for the future MAP and Planck Surveyor missions.

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arXiv1999

Using Perturbative Least Action to Run N Body Simulations Back in Time

In this report, we present a new method for reconstructing N body initial conditions from a proscribed final density field. This method, Perturbative Least Action (PLA) is similar to traditional least action approaches, except that orbits of particles are found as expansions around previously determined and physically motivated orbits.

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arXiv1999

Cosmic Microwave Background Observations in the Post-Planck Era

The Microwave Anisotropy Probe and Planck missions will provide low noise maps of the temperature of the cosmic microwave background (CMB). These maps will allow measurement of the power spectrum of the CMB with measurement noise below cosmic variance for l < 1500. It is anticipated that no further all sky CMB temperature observations will be needed after Planck. There are, however, other CMB measurements for which Planck will be not the end but the beginning. Following Planck, precise CMB polarization observations will offer the potential to study physical processes at energies as high as 10^19 GeV. In addition, arcminute scale, multi-frequency observations will allow study of the early phases of the formation of large-scale structure in the universe.

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arXiv1997

The Number Density of Low Surface Brightness Galaxies with 23<mu_0<25 V-mag/sqr-arcsecond

We present results of a large area CCD survey for low surface brightness galaxies (LSBs) that reaches central surface brightnesses of mu_0=25 mag/sqr-" in V. We have analyzed 17.5 degrees of transit scan data, and identified a statistical subset of 7 pure disk LSB's with mu_0 fainter than 23V-mag/sqr-" and with angular exponential scale lengths larger than alpha=2.5". The LSB detection is entirely automated, and the selection efficiency of the survey is well-quantified. After correcting for the selection efficiency, we find a surface density of 4.1^{+2.6}_{-2.1} galaxies/deg^2 for LSBs in the considered range of mu_0 and alpha (90% confidence levels). The final sample of LSBs are at distances comparable to those probed by large galaxy catalogs, and have intrinsic scale lengths of 1.7-3.6 kpc (H=50km/s/Mpc), based upon optical redshifts. We use the redshifts and the selection efficiency to calculate the number density in LSBs with 23<mu_0<25V-mag/sqr-" and find N = 0.01^{+0.006}_{-0.005} galaxies/Mpc^3, with 90% confidence. The measurement of N probably represents a lower limit, due to very strong biases against LSBs with bulges or edge-on LSBs in our sample. Comparing the LSB number density to the number density of normal galaxies with either similar scale lengths or similar luminosities, we find that the number density of LSBs with 23<mu_0<25V-mag/sqr-" is comparable to or greater than the number density of normal galaxies. The luminosity density in LSBs is comparable to the luminosity density of normal galaxies with similar luminosities, but is a factor of 3-10 smaller than the luminosity density of normal galaxies with similar scale lengths (ABRIDGED).

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arXiv1996

The Formation of Disk Galaxies

We present a scenario for the formation of disks which explains not only the properties of normal galaxies, but the properties of the population of low surface brightness galaxies (LSBs) as well. We use a gravitationally self-consistent model for disk collapse to calculate the observable properties of disk galaxies as a function of mass and angular momentum of the initial protogalaxy. The model naturally produces smooth, asymptotically flat rotation curves, exponential surface brightness profiles over many disk scale lengths, the Tully-Fisher relation as a function of surface brightness, the observed distribution of scale lengths and surface brightnesses, and the variation of rotation curve shapes. In the model, low mass and/or high angular momentum halos naturally form low surface brightness disks. Theoretical and numerical calculations suggest galaxy halos should form with a wide range of mass and angular momenta, and thus, the disks which form within these halos should have a wide range of surface brightnesses and scale lengths. We use the formalism to calculate the expected change in the observed luminosity function (LF) and luminosity density as a function of limiting surface brightness. Current measurements of the LF may by off by factors of 2 at L*. [SHORTENED]

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arXiv1995

A Consistent Microlensing Model for the Galactic Bar

We compute a microlensing map for the Galactic bar. The predicted event rate and event duration distribution are consistent with the $55$ events recently reported by the MACHO and OGLE collaborations. Most of the events are due to lensing by stars in the near end of the bar. Lens mass functions with about 30-60\% of lens mass as brown dwarfs are rejected at $2-6σ$ levels. To make our model useful for other workers, we tabulate the bar's optical depth and average event duration (scaled to $1M_\odot$ lenses) on a grid of Galactic coordinates. The distance and the proper motions of the lens and the source are derived from a consistent dynamical model of the stellar bar, which has originally been built to fit data on the stellar light and stellar/gas kinematics of the bar. We explore several alternative models and we find that our standard model best matches observations.

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arXiv1995

THE FORMATION OF LSB GALAXIES

The formation of low surface brightness galaxies is an unavoidable prediction of any hierarchical clustering scenario. In these models, low surface brightness galaxies form at late times from small initial overdensities, and make up most of the faint end of the galaxy luminosity function. Because there are tremendous observational biases against finding low surface brightness galaxies, the observed faint end of the galaxy luminosity function may easily fall short of predictions, if hierarchical structure formation is correct. We calculate the number density and mass density in collapsed objects as a function of baryonic surface density and redshift, and show that the mass in recently formed low surface brightness galaxies can be comparable to the mass bound into ``normal'' high surface brightness galaxies. Because of their low gas surface densities, these galaxies are easily ionized by the UV background and are not expected to appear in HI surveys. Low surface brightness galaxies (LSBs) are not a special case of galaxy formation and are perhaps better viewed as a continuance of the Hubble sequence.

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arXiv1994

Signatures of Bulge Triaxiality from Kinematics in Baade's Window/

We study a sample of 62 Baade's Window (1,-4) K giants that have published proper motions, radial velocity and metallicity. We construct the velocity ellipsoids for the metal rich stars and metal poor stars. We find a vertex deviation characteristic of a non-axisymmetric system. The long axes of the velocity ellipsoids of two samples point to two nearly perpendicular directions (lv,bv)=(-65+-9,+14+-9) and (25+-14,-11+-14) respectively. The vertex deviations of the velocity ellipsoids cannot be consistently explained by ANY oblate model. We populate orbits in a realistic bar potential with a Gaussian velocity distribution, allowing us to simulate and interpret observations. We conclude that the data are consistent with a triaxial bulge pointing towards (l,b) with l<0 and b=0 as suggested by earlier work. We also predict that low latitude abs(b)<4 deg bulge fields should show the vertex deviation more strongly and would therefore be the best locations for future proper motion studies. The metal rich and metal poor stars appear to occupy the direct boxy and retrograde loop orbits. The correlations between the metallicity and the orbit families can develop if the Bulge forms dissipatively on a sufficiently long time scale. However, it is difficult to reconcile the existence of such correlations with a violent history for the potential of the inner Galaxy.

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